Dual, VARIABLE GAIN AMPLIFIER
with Low Noise Preamp
FEATURES
q
LOW NOISE PREAMP:
Low Input Noise: 1.25nV/
Hz
Active Termination Noise Reduction
Switchable Termination Value
80MHz Bandwidth
5dB to 25dB Gain Range
Differential Input /Output
q
LOW NOISE VARIABLE GAIN AMPLIFIER:
Low Noise VCA: 3.3nV/
Hz, Differential
Programming Optimizes Noise Figure
24dB to 45dB Gain
40MHz Bandwidth
Differential Input /Output
q
LOW CROSSTALK: 52dB at Max Gain, 5MHz
q
HIGH-SPEED VARIABLE GAIN ADJUST
q
SWITCHABLE EXTERNAL PROCESSING
APPLICATIONS
q
ULTRASOUND SYSTEMS
q
WIRELESS RECEIVERS
q
TEST EQUIPMENT
VCA2612
VCA2
612
DESCRIPTION
The VCA2612 is a highly integrated, dual receive channel,
signal processing subsystem. Each channel of the product
consists of a low noise preamplifier (LNP) and a Variable Gain
Amplifier (VGA). The LNP circuit provides the necessary
connections to implement Active Termination (AT), a method
of cable termination which results in up to 4.6dB noise figure
improvement. Different cable termination characteristics can
be accommodated by utilizing the VCA2612's switchable
LNA feedback pins. The LNP has the ability to accept both
differential and single-ended inputs, and generates a differen-
tial output signal. The LNP provides strappable gains of 5dB,
17dB, 22dB, and 25dB.
The output of the LNP can be accessed externally for further
signal processing, or fed directly into the VGA. The VCA2612's
VGA section consists of two parts: the Voltage Controlled
Attenuator (VCA) and the Programmable Gain Amplifier
(PGA). The gain and gain range of the PGA can be digitally
programmed. The combination of these two programmable
elements results in a variable gain ranging from 0dB up to a
maximum gain as defined by the user through external connec-
tions. The output of the VGA can be used in either a single-
ended or differential mode to drive high-performance
Analog-to-Digital (A/D) converters.
The VCA2612 also features low crosstalk and outstanding
distortion performance. The combination of low noise, and gain
range programmability make the VCA2612 a versatile building
block in a number of applications where noise performance is
critical. The VCA2612 is available in a TQFP-48 package.
Low Noise
Preamp
5dB to 25dB
Programmable
Gain Amplifier
24 to 45dB
Voltage
Controlled
Attenuator
Analog
Control
Maximum Gain
Select
RF
2
RF
1
FB
SWFB
LNP
IN
P
C
C
C
F
LNP
IN
N
LNP
GS1
LNP
GS2
LNP
GS3
LNP
Gain Set
Input
LNP
OUT
P
SEL
VCA
IN
P
LNP
OUT
N
VCA
IN
N
VCA
CNTL
FBSW
CNTL
VCA
OUT
P
VCA
OUT
N
MGS
1
MGS
2
MGS
3
Maximum Gain Select
VCA2612
(1 of 2 Channels)
www.ti.com
Copyright 2000, Texas Instruments Incorporated
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
SBOS117B OCTOBER 2001
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
VCA2612
2
SBOS117B
www.ti.com
ELECTRICAL CHARACTERISTICS
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Texas Instru-
ments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to
complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes
could cause the device not to meet its published specifications.
ABSOLUTE MAXIMUM RATINGS
Power Supply (+V
S
) ............................................................................. +6V
Analog Input ............................................................. 0.3V to (+V
S
+ 0.3V)
Logic Input ............................................................... 0.3V to (+V
S
+ 0.3V)
Case Temperature ......................................................................... +100
C
Junction Temperature .................................................................... +150
C
Storage Temperature ...................................................... 40
C to +150
C
PACKAGE
PACKAGE
ORDERING
(1)
TRANSPORT
PRODUCT
PACKAGE-LEAD
DESIGNATOR
MARKING
NUMBER
MEDIA, QUANTITY
VCA2612Y
TQFP-48
PFB
VCA2612Y
VCA2612Y/250
Tape and Reel, 250
"
"
"
"
VCA2612Y/2K
Tape and Reel, 2000
PACKAGE/ORDERING INFORMATION
VCA2612Y
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
PREAMPLIFIER
Input Resistance
600
k
Input Capacitance
15
pF
Input Bias Current
1
nA
CMRR
f = 1MHz, VCA
CNTL
= 0.2V
50
dB
Maximum Input Voltage
Preamp Gain = +5dB
1
Vp-p
Preamp Gain = +25dB
112
mVp-p
Input Voltage Noise
(1)
Preamp Gain = +5dB
3.5
nV/
Hz
Preamp Gain = +25dB
1.25
nV/
Hz
Input Current Noise
Independent of Gain
0.35
pA/
Hz
Noise Figure, R
S
= 75
, R
IN
= 75
(1)
R
F
= 550
, Preamp Gain = 22dB,
6.2
dB
PGA Gain = 39dB
Bandwidth
Gain = 22dB
80
MHz
PROGRAMMABLE VARIABLE GAIN AMPLIFIER
Peak Input Voltage
Differential
2
Vp-p
3dB Bandwidth
40
MHz
Slew Rate
300
V/
s
Output Signal Range
R
L
500
Each Side to Ground
2
Vp-p
Output Impedance
f = 5MHz
1
Output Short-Circuit Current
40
mA
Third Harmonic Distortion
f = 5MHz, V
OUT
= 1Vp-p, VCA
CNTL
= 3.0V
45
71
dBc
Second Harmonic Distortion
f = 5MHz, V
OUT
= 1Vp-p, VCA
CNTL
= 3.0V
45
63
dBc
IMD, Two-Tone
V
OUT
= 2Vp-p, f = 1MHz
80
dBc
V
OUT
= 2Vp-p, f = 10MHz
80
dBc
1dB Compression Point
f = 5MHz, Output Referred, Differential
6
Vp-p
Crosstalk
V
OUT
= 1Vp-p, f = 1MHz, Max Gain Both Channels
68
dB
Group Delay Variation
1MHz < f < 10MHz, Full Gain Range
2
ns
DC Output Level, V
IN
= 0
2.5
V
ACCURACY
Gain Slope
10.9
dB/V
Gain Error
1
(2)
dB
Output Offset Voltage
50
mV
GAIN CONTROL INTERFACE
Input Voltage (VCA
CNTL
) Range
0.2 to 3.0
V
Input Resistance
1
M
Response Time
45dB Gain Change, MGS = 111
0.2
s
POWER SUPPLY
Operating Temperature Range
40
+85
C
Specified Operating Range
4.75
5.0
5.25
V
Power Dissipation
Operating, Both Channels
410
495
mW
Thermal Resistance,
JA
TQFP-48
56.5
C/W
NOTE: (1) For preamp driving VGA. (2) Referenced to best fit dB-linear curve.
NOTE: (1) Models with a (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K indicates 2000 devices per reel). Ordering 2000 pieces of
"VCA2612Y/2K" will get a single 2000-piece Tape and Reel.
VCA2612
3
SBOS117B
www.ti.com
PIN CONFIGURATION
1
V
DD
A
Channel A +Supply (+5V)
2
NC
Do Not Connect
3
NC
Do Not Connect
4
VCA
IN
NA
Channel A VCA Negative Input
5
VCA
IN
PA
Channel A VCA Positive Input
6
LNP
OUT
NA
Channel A LNP Negative Output
7
LNP
OUT
PA
Channel A LNP Positive Output
8
SWFBA
Channel A Switched Feedback Output
9
FBA
Channel A Feedback Output
10
COMP1A
Channel A Frequency Compensation 1
11
COMP2A
Channel A Frequency Compensation 2
12
LNP
IN
NA
Channel A LNP Inverting Input
13
LNP
GS3
A
Channel A LNP Gain Strap 3
14
LNP
GS2
A
Channel A LNP Gain Strap 2
15
LNP
GS1
A
Channel A LNP Gain Strap 1
16
LNP
IN
PA
Channel A LNP Noninverting Input
17
V
DD
R
+Supply for Internal Reference (+5V)
18
V
BIAS
0.01
F Bypass to Ground
19
V
CM
0.01
F Bypass to Ground
20
GNDR
Ground for Internal Reference
21
LNP
IN
PB
Channel B LNP Noninverting Input
22
LNP
GS1
B
Channel B LNP Gain Strap 1
23
LNP
GS2
B
Channel B LNP Gain Strap 2
24
LNP
GS3
B
Channel B LNP Gain Strap 3
25
LNP
IN
NB
Channel B LNP Inverting Input
26
COMP2B
Channel B Frequency Compensation 2
27
COMP1B
Channel B Frequency Compensation 1
28
FBB
Channel B Feedback Output
29
SWFBB
Channel B Switched Feedback Output
30
LNP
OUT
PB
Channel B LNP Positive Output
31
LNP
OUT
NB
Channel B LNP Negative Output
32
VCA
IN
PB
Channel B VCA Positive Input
33
VCA
IN
NB
Channel B VCA Negative Input
34
NC
Do Not Connect
35
NC
Do Not Connect
36
V
DD
B
Channel B +Analog Supply (+5V)
37
GNDB
Channel B Analog Ground
38
VCA
OUT
NB
Channel B VCA Negative Output
39
VCA
OUT
PB
Channel B VCA Positive Output
40
MGS
3
Maximum Gain Select 3 (LSB)
41
MGS
2
Maximum Gain Select 2
42
MGS
1
Maximum Gain Select 1 (MSB)
43
VCA
CNTL
VCA Control Voltage
44
VCA
IN
SEL
VCA Input Select, HI = External
45
FBSW
CNTL
Feedback Switch Control: HI = ON
46
VCA
OUT
PA
Channel A VCA Positive Output
47
VCA
OUT
NA
Channel A VCA Negative Output
48
GNDA
Channel A Analog Ground
PIN
DESIGNATOR
DESCRIPTION
PIN
DESIGNATOR
DESCRIPTION
PIN DESCRIPTIONS
36
35
34
33
32
31
30
29
28
27
26
25
V
DD
B
NC
NC
VCA
IN
NB
VCA
IN
PB
LNP
OUT
NB
LNP
OUT
PB
SWFBB
FBB
COMP1B
COMP2B
LNP
IN
NB
GNDA
VCA
OUT
NA
VCA
OUT
PA
FBSW
CNTL
VCA
IN
SEL
VCA
CNTL
MGS
1
MGS
2
MGS
3
VCA
OUT
PB
VCA
OUT
NB
GNDB
LNP
GS3
A
LNP
GS2
A
LNP
GS1
A
LNP
IN
PA
V
DD
R
V
BIAS
V
CM
GNDR
LNP
IN
PB
LNP
GS1
B
LNP
GS2
B
LNP
GS3
B
1
2
3
4
5
6
7
8
9
10
11
12
V
DD
A
NC
NC
VCA
IN
NA
VCA
IN
PA
LNP
OUT
NA
LNP
OUT
PA
SWFBA
FBA
COMP1A
COMP2A
LNP
IN
NA
48
47
46
45
44
43
42
41
40
39
38
13
14
15
16
17
18
19
20
21
22
23
37
24
VCA2612
VCA2612
4
SBOS117B
www.ti.com
TYPICAL CHARACTERISTICS
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
GAIN vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.2
1.0
0.4 0.6 0.8
1.8 2.0 2.2
1.6
1.4
2.4 2.6 2.8 3.0
Gain (dB)
65
60
55
50
45
40
35
30
25
20
15
MGS = 111
MGS = 110
MGS = 101
MGS = 100
MGS = 011
MGS = 010
MGS = 001
MGS = 000
GAIN ERROR vs TEMPERATURE
VCA
CNTL
(V)
0.2
1.0 1.2
0.8
0.4 0.6
2.0 2.2
1.4 1.6 1.8
2.4 2.6 2.8 3.0
Gain Error (dB)
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
+25
C
40
C
+85
C
GAIN ERROR vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.0 1.2
0.8
0.4 0.6
2.0 2.2
1.4 1.6 1.8
2.4 2.6 2.8 3.0
Gain Error (dB)
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
10MHz
1MHz
5MHz
GAIN ERROR vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.0 1.2
0.8
0.4 0.6
2.0 2.2
1.4 1.6 1.8
2.4 2.6 2.8 3.0
Gain Error (dB)
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
MGS = 011
MGS = 000
MGS = 111
GAIN MATCH: CHA to CHB, VCA
CNTL
= 0.2V
Delta Gain (dB)
0.5 0.4 0.3 0.2 0.1 0.0
0.1
0.2
0.3
0.4
0.5
Units
100
90
80
70
60
50
40
30
20
10
0
GAIN MATCH: CHA to CHB, VCA
CNTL
= 3.0V
Delta Gain (dB)
0.5 0.4 0.3 0.2 0.1 0.0
0.1
0.2
0.3
0.4
0.5
Units
100
90
80
70
60
50
40
30
20
10
0
VCA2612
5
SBOS117B
www.ti.com
TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
GAIN vs FREQUENCY
(Pre-Amp)
Frequency (MHz)
0.1
1
10
100
Gain (dB)
30
25
20
15
10
5
0
LNP = 25dB
LNP = 22dB
LNP = 17dB
LNP = 5dB
GAIN vs FREQUENCY
(VCA and PGA, VCA
CNTL
= 0.2V)
Frequency (MHz)
0.1
1
10
100
Gain (dB)
5.0
4.0
3.0
2.0
1.0
0.0
1.0
2.0
3.0
4.0
5.0
MGS = 111
MGS = 100
MGS = 011
MGS = 000
GAIN vs FREQUENCY
(VCA and PGA, VCA
CNTL
= 3.0V)
Frequency (MHz)
0.1
1
10
100
Gain (dB)
45
40
35
30
25
20
15
10
5
0
MGS = 111
MGS = 100
MGS = 011
MGS = 000
GAIN vs FREQUENCY
(VCA
CNTL
= 3.0V)
Frequency (MHz)
0.1
1
10
100
Gain (dB)
60
50
40
30
20
10
0
LNP = 25dB
LNP = 22dB
LNP = 5dB
LNP = 17dB
GAIN vs FREQUENCY
(LNP = 22dB)
Frequency (MHz)
0.1
1
10
100
Gain (dB)
60
50
40
30
20
10
0
VCA
CNTL
= 3.0V
VCA
CNTL
= 1.6V
VCA
CNTL
= 0.2V
OUTPUT REFERRED NOISE vs VCA
CNTL
VCA
CNTL
(V)
0
1.0 1.2
0.4 0.6 0.8
1.8 2.0
1.4 1.6
2.2 2.4 2.6 2.8 3.0
Noise (nV/
Hz)
1800
1600
1400
1200
1000
800
600
400
200
0
R
S
= 50
MGS = 111
MGS = 011
VCA2612
6
SBOS117B
www.ti.com
TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
INPUT REFERRED NOISE vs R
S
R
S
(
)
1
10
100
1000
Noise (nV
Hz
10.0
1.0
0.1
NOISE FIGURE vs R
S
(VCA
CNTL
= 3.0V)
R
S
(
)
10
100
1000
Noise Figure (dB)
11
10
9
8
7
6
5
4
3
2
1
0
NOISE FIGURE vs VCA
CNTL
VCA
CNTL
(V)
Noise Figure (dB)
30
25
20
15
10
5
0
0.2
1.0 1.2
0.4 0.6 0.8
1.8 2.0
1.4 1.6
2.2 2.4 2.6 2.8 3.0
INPUT REFERRED NOISE vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.0 1.2
0.4 0.6 0.8
1.8 2.0
1.4 1.6
2.2 2.4 2.6 2.8 3.0
Noise (nV/
Hz)
24
22
20
18
16
14
12
10
8
6
4
2
0
MGS = 011
MGS = 111
R
S
= 50
LNP vs FREQUENCY
(Differential, 2Vp-p)
Frequency (MHz)
0.1
1
10
100
Harmonic Distortion (dBc)
45
50
55
60
65
70
75
80
3rd Harmonic
2nd Harmonic
LNP vs FREQUENCY
(Single-Ended, 1Vp-p)
Frequency (MHz)
0.1
1
10
100
Harmonic Distortion (dBc)
45
50
55
60
65
70
75
80
2nd Harmonic
3rd Harmonic
VCA2612
7
SBOS117B
www.ti.com
TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 000)
Frequency (MHz)
0.1
1
10
Harmonic Distortion (dBc)
40
45
50
55
60
65
70
75
80
85
90
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 011)
Frequency (Hz)
0.1
1
10
Harmonic Distortion (dBc)
40
45
50
55
60
65
70
75
80
85
90
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 111)
Frequency (MHz)
0.1
1
10
Harmonic Distortion (dBc)
30
35
40
45
50
55
60
65
70
75
80
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 000)
Frequency (MHz)
0.1
1
10
Harmonic Distortion (dBc)
40
45
50
55
60
65
70
75
80
85
90
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 011)
Frequency (MHz)
0.1
1
10
Harmonic Distortion (dBc)
40
45
50
55
60
65
70
75
80
85
90
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 111)
Frequency (MHz)
0.1
1
10
Harmonic Distortion (dBc)
30
35
40
45
50
55
60
65
70
75
80
85
VCA
CNTL
= 0.2V, H2
VCA
CNTL
= 0.2V, H3
VCA
CNTL
= 3.0V, H2
VCA
CNTL
= 3.0V, H3
VCA2612
8
SBOS117B
www.ti.com
TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
1dB COMPRESSION vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.0 1.2
0.4 0.6 0.8
1.6 1.8 2.0 2.2
1.4
2.4 2.6
3.0
2.8
P
IN
(dBm)
0
5
10
15
20
25
30
35
40
3rd-ORDER INTERCEPT vs VCA
CNTL
VCA
CNTL
(V)
0.2
1.0 1.2
0.4 0.6 0.8
1.6 1.8 2.0 2.2
1.4
2.4 2.6
3.0
2.8
IP3 (dBm)
0
5
10
15
20
25
30
35
40
45
50
HARMONIC DISTORTION vs VCA
CNTL
(Differential, 2Vp-p)
VCA
CNTL
(V)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Harmonic Distortion (dBc)
45
50
55
60
65
70
75
80
MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3
HARMONIC DISTORTION vs VCA
CNTL
(Single-Ended, 1Vp-p)
VCA
CNTL
(V)
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
Harmonic Distortion (dBc)
45
50
55
60
65
70
75
80
MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3
INTERMODULATION DISTORTION
(Differential, 2Vp-p, f = 10MHz)
Frequency (MHz)
9.98
9.96
10
10.2
10.4
Power (dBFS)
5
15
25
35
45
55
65
75
85
95
105
INTERMODULATION DISTORTION
(Single-Ended, 1Vp-p, f = 10MHz)
Frequency (MHz)
9.98
9.96
10
10.2
10.4
Power (dBFS)
5
15
25
35
45
55
65
75
85
95
105
VCA2612
9
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TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
CMRR vs FREQUENCY
(LNP only)
Frequency (MHz)
0.1
1
10
100
CMRR (dB)
0
10
20
30
40
50
60
70
80
PULSE RESPONSE (BURSTS)
(Differential, VCA
CNTL
= 3.0V, MGS = 111)
200ns/div
Output
500mV/div
Input
1mV/div
OVERLOAD RECOVERY
(Differential, VCA
CNTL
= 3.0V, MGS = 111)
Input
1mV/div
Output
1V/div
200ns/div
GAIN RESPONSE
(Differential, VCA
CNTL
Pulsed, MGS = 111)
Output
500mV/div
Input
2V/div
100ns/div
CMRR vs FREQUENCY
(VCA only)
Frequency (MHz)
0.1
1
10
100
CMRR (dB)
0
10
20
30
40
50
60
70
80
90
VCA
CNTL
= 0.2V
VCA
CNTL
= 1.4V
VCA
CNTL
= 3.0V
CROSS TALK vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 011)
Frequency (MHz)
1
10
100
Cross Talk (dB)
0
10
20
30
40
50
60
70
80
90
VCA
CNTRL
= 0V
VCA
CNTRL
= 1.5V
VCA
CNTRL
= 3.0V
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TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
DDA
= V
DDB
= V
DDR
= +5V, load resistance = 500
on each output to ground, MGS = 011, LNP = 22dB and f
IN
= 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
80
79.5
79
78.5
78
77.5
77
76.5
76
I
CC
vs TEMPERATURE
Temperature (
C)
40
10
5
25
35
50
20
65
80
95
I
CC
(mA)
GROUP DELAY vs FREQUENCY
Frequency (MHz)
1
10
100
Group Delay (ns)
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
VCA
CNTL
= 3.0V
VCA
CNTL
= 0.2V
45
40
35
30
25
20
15
10
5
0
5
10
PSRR vs FREQUENCY
Frequency (Hz)
10
1k
100
100k
1M
10k
10M
PSRR (dB)
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VCA--OVERVIEW
The magnitude of the differential VCA input signal (from
the LNP or an external source) is reduced by a program-
mable attenuation factor, set by the analog VCA Control
Voltage (VCA
CNTL
) at pin 43. The maximum attenuation
factor is further programmable by using the three MGS bits
(pins 40-42). Figure 3 illustrates this dual-adjustable charac-
teristic. Internally, the signal is attenuated by having the
analog VCA
CNTL
vary the channel resistance of a set of
shunt-connected FET transistors. The MGS bits effectively
adjust the overall size of the shunt FET by switching parallel
components in or out under logic control. At any given
maximum gain setting, the analog variable gain characteris-
tic is linear in dB as a function of the control voltage, and
is created as a piecewise approximation of an ideal dB-linear
transfer function. The VCA gain control circuitry is com-
mon to both channels of the VCA2612.
FIGURE 1. Simplified Block Diagram of the VCA2612.
FIGURE 2. Recommended Circuit for Coupling an External
Signal into the VCA Inputs.
0
24
VCA Attenuation (dB)
45
Control Voltage
0
Maximum Attenuation
Minimum Attenuation
3.0V
FIGURE 3. Swept Attenuator Characteristic.
THEORY OF OPERATION
The VCA2612 is a dual-channel system consisting of three
primary blocks: a Low Noise Preamplifier (LNP), a Voltage
Controlled Attenuator (VCA), and a Programmable Gain
Amplifier (PGA). For greater system flexibility, an onboard
multiplexer is provided for the VCA inputs, selecting either
the LNP outputs or external signal inputs. Figure 1 shows a
simplified block diagram of the dual-channel system.
LNP--OVERVIEW
The LNP input may be connected to provide active-feedback
signal termination, achieving lower system noise perfor-
mance than conventional passive shunt termination. Even
lower noise performance is obtained if signal termination is
not required. The unterminated LNP input impedance is
600k
. The LNP can process fully differential or single-
ended signals in each channel. Differential signal processing
results in significantly reduced 2nd-harmonic distortion and
improved rejection of common-mode and power supply
noise. The first gain stage of the LNP is AC coupled into its
output buffer with a 44
s time constant (3.6kHz high-pass
characteristic). The buffered LNP outputs are designed to
drive the succeeding VCA directly or, if desired, external
loads as low as 135
with minimal impact on signal distor-
tion. The LNP employs very low impedance local feedback
to achieve stable gain with the lowest possible noise and
distortion. Four pin-programmable gain settings are avail-
able: 5dB, 17dB, 22dB, and 25dB. Additional intermediate
gains can be programmed by adding trim resistors between
the Gain Strap programming pins.
The common-mode DC level at the LNP output is nominally
2.5V, matching the input common-mode requirement of the
VCA for simple direct coupling. When external signals are
fed to the VCA, they should also be set up with a 2.5VDC
common-mode level. Figure 2 shows a circuit that demon-
strates the recommended coupling method using an external
op amp. The "V
CM
" node shown in the drawing is the V
CM
output (pin 19). Typical R and C values are shown, yielding
a high-pass time constant similar to that of the LNP. If a
different common-mode referencing method is used, it is
important that the common-mode level be within 10mV of
the V
CM
output for proper operation.
V
CM
(+2.5V)
1k
1k
47nF
To VCA
IN
Input
Signal
VCA
LNP
Channel A
Input
VCA
Control
PGA
Channel A
Output
External
In
A
Maximum
Gain
Select
MGS
Analog
Control
VCA
LNP
Channel B
Input
PGA
Channel B
Output
External
In
B
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The VCA2612 includes a built-in reference, common to
both channels, to supply a regulated voltage for critical areas
of the circuit. This reduces the susceptibility to power supply
variation, ripple, and noise. In addition, separate power
supply and ground connections are provided for each chan-
nel and for the reference circuitry, further reducing
interchannel cross-talk.
Further details regarding the design, operation and use of
each circuit block are provided in the following sections.
LOW NOISE PREAMPLIFIER (LNP)--DETAIL
The LNP is designed to achieve a low noise figure, espe-
cially when employing active termination. Figure 4 is a
simplified schematic of the LNP, illustrating the differential
input and output capability. The input stage employs low
resistance local feedback to achieve stable low noise, low
distortion performance with very high input impedance.
Normally, low noise circuits exhibit high power consump-
tion due to the large bias currents required in both input and
output stages. The LNP uses a patented technique that
combines the input and output stages such that they share the
same bias current. Transistors Q
4
and Q
5
amplify the signal
at the gate-source input of Q
4
, the +IN side of the LNP. The
signal is further amplified by the Q
1
and Q
2
stage, and then
by the final Q
3
and R
L
gain stage, which uses the same bias
current as the input devices Q
4
and Q
5
. Devices Q
6
through
Q
10
play the same role for signals on the IN side.
The differential gain of the LNP is given in Equation (1):
R
L
93
R
S1
105
Q
3
Q
4
Q
5
Q
2
Q
1
R
S2
34
R
S3
17
LNP
GS2
LNP
IN
P
LNP
IN
N
To Bias
Circuitry
LNP
GS1
LNP
GS3
R
L
93
COMP2A
V
DD
COMP1A
LNP
OUT
N
LNP
OUT
P
Buffer
Buffer
Q
8
Q
7
To Bias
Circuitry
Q
6
Q
9
Q
10
R
W
R
W
C
COMP
(External
Capacitor)
FIGURE 4. Schematic of the Low Noise Preamplifier (LNP).
Gain
R
R
L
S
=
2
(1)
PGA OVERVIEW AND OVERALL DEVICE
CHARACTERISTICS
The differential output of the VCA attenuator is then ampli-
fied by the PGA circuit block. This post-amplifier is pro-
grammed by the same MGS bits that control the VCA
attenuator, yielding an overall swept-gain amplifier charac-
teristic in which the VCA PGA gain varies from 0dB
(unity) to a programmable peak gain of 24dB, 27dB, 30dB,
33dB, 36dB, 39dB, 42dB, or 45dB.
The "GAIN vs VCA
CNTL
" curve on page 4 shows the
composite gain control characteristic of the entire VCA2612.
Setting VCA
CNTL
to 3.0V causes the digital MGS gain
control to step in 3dB increments. Setting VCA
CNTL
to 0V
causes all the MGS-controlled gain curves to converge at
one point. The gain at the convergence point is the LNP gain
less 6dB, because the measurement setup looks at only one
side of the differential PGA output, resulting in 6dB lower
signal amplitude.
ADDITIONAL FEATURES--OVERVIEW
Overload protection stages are placed between the attenuator
and the PGA, providing a symmetrically clipped output
whenever the input becomes large enough to overload the
PGA. A comparator senses the overload signal amplitude
and substitutes a fixed DC level to prevent undesirable
overload recovery effects. As with the previous stages, the
VCA is AC coupled into the PGA. In this case, the coupling
time constant varies from 5
s at the highest gain (45dB) to
59
s at the lowest gain (25dB).
VCA2612
13
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LNP GAIN (dB)
Input-Referred
Output-Referred
25
1.54
2260
22
1.59
1650
17
1.82
1060
5
4.07
597
The LNP is capable of generating a 2Vp-p differential
signal. The maximum signal at the LNP input is therefore
2Vp-p divided by the LNP gain. An input signal greater than
this would exceed the linear range of the LNP, an especially
important consideration at low LNP gain settings.
ACTIVE FEEDBACK WITH THE LNP
One of the key features of the LNP architecture is the ability
to employ active-feedback termination to achieve superior
noise performance. Active feedback termination achieves a
lower noise figure than conventional shunt termination,
essentially because no signal current is wasted in the termi-
nation resistor itself. Another way to understand this is as
follows: Consider first that the input source, at the far end of
the signal cable has a cable-matching source resistance of
R
S
. Using conventional shunt termination at the LNP input,
a second terminating resistor of value R
S
is connected to
ground. Therefore, the signal loss is 6dB due to the voltage
divider action of the series and shunt R
S
resistors. The
effective source resistance has been reduced by the same
factor of 2, but the noise contribution has been reduced by
only the
2, only a 3dB reduction. Therefore, the net
theoretical SNR degradation is 3dB, assuming a noise-free
amplifier input. (In practice, the amplifier noise contribution
will degrade both the unterminated and the terminated noise
figures, somewhat reducing the distinction between them.)
See Figure 5 for an amplifier using active feedback. This
diagram appears very similar to a traditional inverting am-
plifier. However, the analysis is somewhat different because
the gain "A" in this case is not a very large open-loop op
amp gain; rather it is the relatively low and controlled gain
of the LNP itself. Thus, the impedance at the inverting
amplifier terminal will be reduced by a finite amount, as
given in the familiar relationship of Equation (3):
where R
F
is the feedback resistor (supplied externally be-
tween the LNP
IN
P and FB terminals for each channel), A is
the user-selected gain of the LNP, and R
IN
is the resulting
amplifier input impedance with active feedback. In this case,
unlike the conventional termination above, both the signal
voltage and the R
S
noise are attenuated by the same factor of
It is also possible to create other gain settings by connecting
an external resistor between LNPG
S1
on one side, and
LNPG
S2
and/or LNPG
S3
on the other. In that case, the
internal resistor values shown in Figure 4 should be com-
bined with the external resistor to calculate the effective
value of R
S
for use in Equation (1). The resulting expression
for external resistor value is given in Equation (2).
where R
EXT
is the externally selected resistor value needed
to achieve the desired gain setting, R
S1
is the fixed parallel
resistor in Figure 4, and R
FIX
is the effective fixed value of
the remaining internal resistors: R
S2
, R
S3
, or (R
S2
|| R
S3
)
depending on the pin connections.
Note that the best process and temperature stability will be
achieved by using the pre-programmed fixed gain options of
Table I, since the gain is then set entirely by internal resistor
ratios, which are typically accurate to
0.5%, and track quite
well over process and temperature. When combining exter-
nal resistors with the internal values to create an effective R
S
value, note that the internal resistors have a typical tempera-
ture coefficient of +700ppm/
C and an absolute value toler-
ance of approximately
5%, yielding somewhat less predict-
able and stable gain settings. With or without external
resistors, the board layout should use short Gain Strap
connections to minimize parasitic resistance and inductance
effects.
The overall noise performance of the VCA2612 will vary as
a function of gain. Table II shows the typical input-and
output-referred noise densities of the entire VCA2612 for
maximum VCA and PGA gain; i.e., VCA
CNTL
set to 3.0V
and all MGS bits set to "1". Note that the input-referred
noise values include the contribution of a 50
fixed source
impedance, and are therefore somewhat larger than the
intrinsic input noise. As the LNP gain is reduced, the noise
contribution from the VCA/PGA portion becomes more
significant, resulting in higher input-referred noise. How-
ever, the output-referred noise, which is indicative of the
overall SNR at that gain setting, is reduced.
NOISE (nV/
Hz)
TABLE II. Noise Performance for MGS = 111 and VCA
CNTL
= 3.0V.
LNP PIN STRAPPING
LNP GAIN (dB)
LNPG
S1
, LNPG
S2
, LNPG
S3
Connected Together
25
LNPG
S1
Connected to LNPG
S3
22
LNPG
S1
Connected to LNPG
S2
17
All Pins Open
5
TABLE I. Pin Strappings of the LNP for Various Gains.
(3)
(2)
R
R
A
IN
F
=
+
(
)
1
where R
L
is the load resistor in the drains of Q
3
and Q
8
, and
R
S
is the resistor connected between the sources of the input
transistors Q
4
and Q
7
. The connections for various R
S
combinations are brought out to device pins LNPG
S1
,
LNPG
S2,
and LNPG
S3
(pins 13-15 for channel A, 22-24 for
channel B). These Gain Strap pins allow the user to establish
one of four fixed LNP gain options as shown in Table I.
R
R R
R
R
Gain R R
Gain R
R
EXT
S
L
FIX
L
S
FIX
S
L
=
+
2
2
2
1
1
1
To preserve the low noise performance of the LNP, the user
should take care to minimize resistance in the input lead. A
parasitic resistance of only 10
will contribute 0.4nV/
Hz.
VCA2612
14
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R
F
A
R
IN
R
IN
=
R
S
R
S
R
S
= R
S
LNP
IN
R
F
1 + A
Active Feedback
A
Conventional Cable Termination
FIGURE 5. Configurations for Active Feedback and Con-
ventional Cable Termination.
VCA NOISE = 3.8nV
Hz, LNP GAIN = 20dB
Source Impedance (
)
0
300
100
200
500
400
600
700
800
900 1000
Noise Figure (dB)
9
8
7
6
5
4
3
2
1
0
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
LNP Noise
nV/
Hz
Source Impedance (
)
0
300
100
200
500
400
600
700
900 1000
800
Noise Figure (dB)
14
12
10
8
6
4
2
0
VCA NOISE = 3.8nV
Hz, LNP GAIN = 20dB
LNP Noise
nV/
Hz
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
FIGURE 6. Noise Figure for Active Termination.
FIGURE 7. Noise Figure for Conventional Termination.
FIGURE 8. Low Frequency LNP Time Constants.
two (6dB) before being re-amplified by the "A" gain setting.
This avoids the extra 3dB degradation due to the square-root
effect described above, the key advantage of the active
termination technique.
As mentioned above, the previous explanation ignored the
input noise contribution of the LNP itself. Also, the noise
contribution of the feedback resistor must be included for a
completely correct analysis. The curves given in Figures 6
and 7 allow the VCA2612 user to compare the achievable
noise figure for active and conventional termination meth-
ods. The left-most set of data points in each graph give the
results for typical 50
cable termination, showing the worst
noise figure but also the greatest advantage of the active
feedback method.
A switch, controlled by the FBSW
CNTL
signal on pin 45,
enables the user to reduce the feedback resistance by adding
an additional parallel component, connected between the
LNP
IN
P and SWFB terminals. The two different values of
feedback resistance will result in two different values of
active-feedback input resistance. Thus, the active-feedback
impedance can be optimized at two different LNP gain
settings. The switch is connected at the buffered output of
the LNP and has an "ON" resistance of approximately 1
.
When employing active feedback, the user should be careful
to avoid low-frequency instability or overload problems.
Figure 8 illustrates the various low-frequency time con-
stants. Referring again to the input resistance calculation of
Equation (3), and considering that the gain term "A" falls
off below 3.6kHz, it is evident that the effective LNP input
impedance will rise below 3.6kHz, with a DC limit of
approximately R
F
. To avoid interaction with the feedback
pole/zero at low frequencies, and to avoid the higher signal
levels resulting from the rising impedance characteristic, it
is recommended that the external R
F
C
C
time constant be set
to about 5
s.
R
S
1M
C
C
C
F
0.001
F
V
CM
R
F
44pF
Buffer
1M
V
CM
LNP
OUT
N
LNP
OUT
P
44pF
Gain
Stage
(VCA) LNP
Buffer
VCA2612
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Achieving the best active feedback architecture is difficult
with conventional op amp circuit structures. The overall
gain "A" must be negative in order to close the feedback
loop, the input impedance must be high to maintain low
current noise and good gain accuracy, but the gain ratio must
be set with very low value resistors to maintain good voltage
noise. Using a two-amplifier configuration (noninverting for
high impedance plus inverting for negative feedback rea-
sons) results in excessive phase lag and stability problems
when the loop is closed. The VCA2612 uses a patented
architecture that achieves these requirements, with the addi-
tional benefits of low power dissipation and differential
signal handling at both input and output.
For greatest flexibility and lowest noise, the user may wish
to shape the frequency response of the LNP. The COMP1
and COMP2 pins for each channel (pins 10 and 11 for
channel A, pins 26 and 27 for channel B) correspond to the
drains of Q
3
and Q
8
in Figure 4. A capacitor placed between
these pins will create a single-pole low-pass response, in
which the effective "R" of the "RC" time constant is ap-
proximately 186
.
COMPENSATION WHEN USING ACTIVE
FEEDBACK
The typical open-loop gain versus frequency characteristic
for the LNP is shown in Figure 9. The 3dB bandwidth is
approximately 180MHz and the phase response is such that
when feedback is applied the LNP will exhibit a peaked
response or might even oscillate. One method for compen-
sating for this undesirable behavior is to place a compensa-
tion capacitor at the input to the LNP, as shown in Figure 10.
This method is effective when the desired 3dB bandwidth
is much less than the open-loop bandwidth of the LNP. This
compensation technique also allows the total compensation
capacitor to include any stray or cable capacitance that is
25dB
Gain
3dB Bandwidth
180MHz
Output
Input
R
F
R
I
C
A
associated with the input connection. Equation 4 relates the
bandwidth to the various impedances that are connected to
the LNP.
BW
A 1 R
R
2pC(R )(R )
I
F
I
F
=
+
(
)
+
(4)
LNP OUTPUT BUFFER
The differential LNP output is buffered by wideband class
AB voltage followers which are designed to drive low
impedance loads. This is necessary to maintain LNP gain
accuracy, since the VCA input exhibits gain-dependent
input impedance. The buffers are also useful when the LNP
output is brought out to drive external filters or other signal
processing circuitry. Good distortion performance is main-
tained with buffer loads as low as 135
. As mentioned
previously, the buffer inputs are AC coupled to the LNP
outputs with a 3.6kHz high-pass characteristic, and the DC
common mode level is maintained at the correct V
CM
for
compatibility with the VCA input.
VOLTAGE-CONTROLLED ATTENUATOR (VCA)--DETAIL
The VCA is designed to have a "dB-linear" attenuation
characteristic, i.e. the gain loss in dB is constant for each
equal increment of the VCA
CNTL
control voltage. See
Figure 11 for a diagram of the VCA. The attenuator is
essentially a variable voltage divider consisting of one series
input resistor, R
S
, and ten identical shunt FETs, placed in
parallel and controlled by sequentially activated clipping
amplifiers. Each clipping amplifier can be thought of as a
specialized voltage comparator with a "soft" transfer charac-
teristic and well-controlled output limit voltages. The refer-
ence voltages V1 through V10 are equally spaced over the
0V to 3.0V control voltage range. As the control voltage
rises through the input range of each clipping amplifier, the
amplifier output will rise from 0V (FET completely "ON")
to V
CM
V
T
(FET nearly "OFF "), where V
CM
is the common
source voltage and V
T
is the threshold voltage of the FET.
As each FET approaches its "OFF" state and the control
voltage continues to rise, the next clipping amplifier/FET
combination takes over for the next portion of the piecewise-
linear attenuation characteristic. Thus, low control voltages
have most of the FETs turned "ON", while high control
voltages have most turned "OFF". Each FET acts to de-
crease the shunt resistance of the voltage divider formed by
R
S
and the parallel FET network.
The attenuator is comprised of two sections, with five
parallel clipping amplifier/FET combinations in each. Spe-
cial reference circuitry is provided so that the (V
CM
V
T
)
limit voltage will track temperature and IC process varia-
tions, minimizing the effects on the attenuator control char-
acteristic.
In addition to the analog VCA
CNTL
gain setting input, the
attenuator architecture provides digitally programmable ad-
justment in eight steps, via the three Maximum Gain Setting
(MGS) bits. These adjust the maximum achievable gain
FIGURE 9. Open-Loop Gain Characteristic of LNP.
FIGURE 10. LNP with Compensation Capacitor.
VCA2612
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SBOS117B
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R
S
Attenuator
Input
Attenuator
Output
A1 - A10 Attenuator Stages
Control
Input
Q
1
V
CM
0dB
4.5dB
Q
2
Q
3
C
1
V1
Q
4
Q
5
Q
S
C1 - C10 Clipping Amplifiers
Attenuation Characteristic of Individual FETs
Q
6
Q
7
Q
8
Q
9
Q
10
C
2
V2
V
CM
-V
T
0
V1
V2
V3
V4
V5
V6
V7
V8
V9
V10
Characteristic of Attenuator Control Stage Output
OVERALL CONTROL CHARACTERISTICS OF ATTENUATOR
4.5dB
0dB
0.3V
3V
Control Signal
C
3
V3
C
4
V4
C
5
V5
C
6
V6
C
7
V7
C
8
V8
C
9
V9
C
10
V10
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
FIGURE 11. Piecewise Approximation to Logarithmic Control Characteristics.
(corresponding to minimum attenuation in the VCA, with
VCA
CNTL
= 3.0V) in 3dB increments. This function is
accomplished by providing multiple FET sub-elements for
each of the Q
1
to Q
10
FET shunt elements shown in
Figure 11. In the simplified diagram of Figure 12, each shunt
FET is shown as two sub-elements, Q
NA
and Q
NB
. Selector
switches, driven by the MGS bits, activate either or both of
the sub-element FETs to adjust the maximum R
ON
and thus
achieve the stepped attenuation options.
The VCA can be used to process either differential or single-
ended signals. Fully differential operation will reduce 2nd-
harmonic distortion by about 10dB for full-scale signals.
VCA2612
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SBOS117B
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R
S
Q
1A
A1
B1
VCM
INPUT
OUTPUT
PROGRAMMABLE ATTENUATOR SECTION
B2
Q
1B
Q
2A
A2
Q
2B
Q
3A
A3
Q
3B
Q
4A
A4
Q
4B
Q
5A
A5
Q
5B
FIGURE 12. Programmable Attenuator Section.
Input impedance of the VCA will vary with gain setting, due
to the changing resistances of the programmable voltage
divider structure. At large attenuation factors (i.e., low gain
settings), the impedance will approach the series resistor
value of approximately 135
.
As with the LNP stage, the VCA output is AC coupled into
the PGA. This means that the attenuation-dependent DC
common-mode voltage will not propagate into the PGA, and
so the PGA's DC output level will remain constant.
Finally, note that the VCA
CNTL
input consists of FET gate
inputs. This provides very high impedance and ensures that
multiple VCA2612 devices may be connected in parallel
with no significant loading effects. The nominal voltage
range for the VCA
CNTL
input spans from 0V to 3V. Over
driving this input (
5V) does not affect the performance.
OVERLOAD RECOVERY CIRCUITRY--DETAIL
With a maximum overall gain of 70dB, the VCA2612 is
prone to signal overloading. Such a condition may occur in
either the LNP or the PGA depending on the various gain
and attenuation settings available. The LNP is designed to
produce low-distortion outputs as large as 1Vp-p single-
ended (2Vp-p differential). Therefore the maximum input
signal for linear operation is 2Vp-p divided by the LNP
differential gain setting. Clamping circuits in the LNP en-
sure that larger input amplitudes will exhibit symmetrical
clipping and short recovery times. The VCA itself, being
basically a voltage divider, is intrinsically free of overload
conditions. However, the PGA post-amplifier is vulnerable
to sudden overload, particularly at high gain settings. Rapid
overload recovery is essential in many signal processing
applications such as ultrasound imaging. A special compara-
tor circuit is provided at the PGA input which detects
overrange signals (detection level dependent on PGA gain
setting). When the signal exceeds the comparator input
threshold, the VCA output is blocked and an appropriate
fixed DC level is substituted, providing fast and clean
overload recovery. The basic architecture is shown in
Figure 13. Both high and low overrange conditions are
sensed and corrected by this circuit.
Figures 14 and 15 show typical overload recovery wave-
forms with MGS = 100, for VCA + PGA minimum gain
(0dB) and maximum gain (36dB), respectively. LNP gain is
set to 25dB in both cases.
FIGURE 14. Overload Recovery Response For Minimum Gain.
FIGURE 15. Overload Recovery Response For Maximum Gain.
VCA
CNTL
= 0.2V, DIFFERENTIAL, MGS = 100, (0dB)
200ns/div
1V/div
Output
Input
VCA
CNTL
= 3.0V, DIFFERENTIAL, MGS = 100, (36dB)
200ns/div
1V/div
Output
Input
FIGURE 13. Overload Protection Circuitry.
Comparators
E = Maximum Peak Amplitude
E
A
From VCA
Selection
Logic
PGA
Gain = A
Output
E
A
VCA2612
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SBOS117B
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INPUT OVERLOAD RECOVERY
One of the most important applications for the VCA2612 is
processing signals in an ultrasound system. The ultrasound
signal flow begins when a large signal is applied to a
transducer, which converts electrical energy to acoustic
energy. It is not uncommon for the amplitude of the electri-
cal signal that is applied to the transducer to be
50V or
greater. To prevent damage, it is necessary to place a
protection circuit between the transducer and the VCA2612,
as shown in Figure 16. Care must be taken to prevent any
signal from turning the ESD diodes on. Turning on the ESD
diodes inside the VCA2612 could cause the input coupling
capacitor (C
C
) to charge to the wrong value.
MGS
ATTENUATOR GAIN DIFFERENTIAL
ATTENUATOR +
SETTING
VCA
CNTL
= 0V to 3V
PGA GAIN
DIFF. PGA GAIN
000
24dB to 0dB
24dB
0dB to 24dB
001
27dB to 0dB
27dB
0dB to 27dB
010
30dB to 0dB
30dB
0dB to 30dB
011
33dB to 0dB
33dB
0dB to 33dB
100
36dB to 0dB
36dB
0dB to 36dB
101
39dB to 0dB
39dB
0dB to 39dB
110
42dB to 0dB
42dB
0dB to 42dB
111
45dB to 0dB
45dB
0dB to 45dB
TABLE III. MGS Settings.
attenuation at that setting. Therefore, the VCA + PGA overall
gain will always be 0dB (unity) when the analog VCA
CNTL
input is set to 0V (= maximum attenuation). For VCA
CNTL
= 3V
(no attenuation), the VCA + PGA gain will be controlled by the
programmed PGA gain (24dB to 45dB in 3dB steps).
For clarity, the gain and attenuation factors are detailed in
Table III.
FIGURE 17. Simplified Block Diagram of the PGA Section Within the VCA2612.
FIGURE 16. VCA2612 Diode Bridge Protection Circuit.
R
S1
R
L
R
S2
VCA
OUT
P
+In
Q
11
Q
3
Q
4
Q
5
Q
1
V
CM
Q
2
VCA
OUT
N
Q
9
Q
8
Q
13
Q
14
Q
7
Q
6
Q
12
V
DD
V
CM
R
L
Q
10
In
To Bias
Circuitry
To Bias
Circuitry
The PGA architecture consists of a differential, program-
mable-gain voltage to current converter stage followed by
transimpedance amplifiers to create and buffer each side of
the differential output. The circuitry associated with the volt-
age to current converter is similar to that previously described
for the LNP, with the addition of eight selectable PGA gain-
setting resistor combinations (controlled by the MGS bits) in
place of the fixed resistor network used in the LNP. Low input
noise is also a requirement of the PGA design due to the large
amount of signal attenuation which can be inserted between
the LNP and the PGA. At minimum VCA attenuation (used
for small input signals) the LNP noise dominates; at maxi-
mum VCA attenuation (large input signals) the PGA noise
dominates. Note that if the PGA output is used single-ended,
the apparent gain will be 6dB lower.
R
F
C
F
V
DD
LNP
IN
P
ESD Diode
Protection
Network
LNP
OUT
N
LNP
PGA POST-AMPLIFIER--DETAIL
Figure 17 shows a simplified circuit diagram of the PGA
block. As described previously, the PGA gain is programmed
with the same MGS bits which control the VCA maximum
attenuation factor. Specifically, the PGA gain at each MGS
setting is the inverse (reciprocal) of the maximum VCA
VCA2612
19
SBOS117B
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PACKAGE DRAWING
MTQF019A JANUARY 1995 REVISED JANUARY 1998
PFB (S-PQFP-G48)
PLASTIC QUAD FLATPACK
4073176 / B 10/96
Gage Plane
0,13 NOM
0,25
0,45
0,75
Seating Plane
0,05 MIN
0,17
0,27
24
25
13
12
SQ
36
37
7,20
6,80
48
1
5,50 TYP
SQ
8,80
9,20
1,05
0,95
1,20 MAX
0,08
0,50
M
0,08
0
7
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-026
PACKAGING INFORMATION
ORDERABLE DEVICE
STATUS(1)
PACKAGE TYPE
PACKAGE DRAWING
PINS
PACKAGE QTY
VCA2612Y/250
ACTIVE
TQFP
PFB
48
250
VCA2612Y/2K
ACTIVE
TQFP
PFB
48
2000
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
PACKAGE OPTION ADDENDUM
www.ti.com
3-Oct-2003
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