C2719 is a current-to-voltage conversion amplifier used to amplify weak photocurrent from a photodiode with very low noise. Three ranges of
photocurrent detection sensitivity level (H, M, L) are selectable to match the input signal. A 10-turn potentiometer is used to zero the amplifier
level so fine adjustments are possible with high resolution.
C2719 operates on the built-in dry batteries so it can be easily used anywhere. An external power input connector is also provided at the rear
panel for a long, continuous operation or for use as part of a measurement system. In such applications, use the dedicated plug to connect to a
stabilized DC power supply.
C2719 was developed specifically for use with Si photodiodes, but it can be used to detect photocurrent from phototubes.
Features
l Three sensitivity ranges
H : 10
9
V/A
M: 10
7
V/A
L : 10
5
V/A
l Zero adjustment with high resolution
l Operates on either dry batteries or external power supply
l Compact and lightweight
Applications
l Precision photometry
l Laser monitors
l Optical power meters
l Colorimeters
l Low signal current preamplifiers
M O D U L E
Photosensor amplifier
Current-to-voltage conversion amplifier for amplifying weak photocurrent with low noise
C2719
I General ratings (Ta=25 C)
Parameter
Condition
Min.
Typ.
Max.
Unit
Output voltage amplitude
R
L
=2 kW
5 (10)
-
-
V
Offset drift
-
-
0.1
mV/hr
Offset temperature
characteristic
-
-
25
V/C
Input voltage
-
-
100
V
Recommended input
capacitance
-
-
2 10
-9
F
Supply voltage
*1
-
9
-
V
External power supply
6
-
15
V
Battery lifetime
R
L
>10 kW
200
-
-
hr
Weight
Including batteries
-
420
-
g
Value in parentheses is measured when external power supply is used.
*1: Batteries: 006 P (9 V) 2 pieces
I Electrical and optical characteristics (Ta=25 C)
Internal batteries (Vs=9 V)
External power supply (Vs=15 V)
Parameter
H
M
L
H
M
L
Unit
Conversion impedance
10
9
10
7
10
5
10
9
10
7
10
5
V/A
Input current range
to 5 10
-9
to 5 10
-7
to 510
-5
to 1010
-9
to 1010
-7
to 1010
-5
A
Frequency bandwidth
DC to 16
DC to 1600 DC to 1600
DC to 16
DC to 1600 DC to 1600
Hz
0.2
0.2
0.2
0.2
0.2
0.2
mVp-p
Output noise
*2
(all bandwidth)
0.02
0.02
0.02
0.02
0.02
0.02
mVrms
Equivalent noise
input current
2 10
-14
2 10
-12
2 10
-10
2 10
-14
2 10
-12
2 10
-10
A
*2: Measured with a 1.6 kHz lowpass filter added to the output of C2719.
1
Photosensor amplifier
C2719
LIGHT
330
Cf
1
+Vs
-Vs
ZERO ADJ
(CASE)
PHOTOCURRENT SIGNAL
i
A
-
+
Rf
1
Rf
2
Rf
3
Cf
2
L
M
H
Vout = -(i Rf)
PROTECTION CIRCUIT
A
K
+Vs
0 V
-Vs
+
006P (9 V) 2
EXTERNAL POWER
SUPPLY TERMINAL
HR10-7R-4P (HIROSE)
+
Figure 1 Equivalent circuit (when photodiode cathode is grounded)
<Amplifier circuit>
KACCC0030EC
KACCC0031EA
I Equivalent circuit
In the equivalent circuit diagram (Figure 1), the photocurrent sig-
nal i is connected to the input terminal of the negative feedback
amplifier A. The input resistance of A is designed to be much
larger than feedback resistance Rf, so all of i is flows through Rf.
The result is that the output voltage of A is converted to -i Rf (V),
the reverse phase of i. Rf is switched to the High, Medium and Low
ranges by the conversion impedance, with Medium being 1/100
of High and Low being 1/10000 of High.
Feedback capacitance Cf is 10 pF (1 nF + 10 pF at Low range) to
prevent the circuit from becoming unstable at high frequencies.
In this case, the cut-off frequency is calculated as follows:
Rf
Cf
2
1
=
f
(Hz)
At this frequency, the output voltage is 0.7 times that at the low
frequency.
I Pulsed light measurement
G Output response waveform
If the photodiode terminal capacitance is approximately 2 nF or
more, ringing may appear in the output waveform of C2719
when pulsed light is detected, resulting in inaccurate measure-
ments. In this case, use a photodiode with a smaller terminal
capacitance, or add a reverse voltage to the photodiode to re-
duce the terminal capacitance.
Figures 2 (a) and (b) show pulse response waveforms when
used with S2281 photodiode which is designed to easily con-
nect to C2719. The terminal capacitance of S2281 is 1.28 nF.
G Frequency response
Figure 3 shows the frequency response characteristics when
used with S2281. As stated above, if the photodiode terminal
capacitance is too large, a noticeable peak is formed in the fre-
quency characteristics. This situation is shown in Figure 4. A
frequency at which this kind of peak exists equals the reciprocal
of the ringing cycle generated in the pulse response output
waveform.
G S/N
As explained above, when the terminal capacitance of a photo-
diode is too large, peaking exists in the noise characteristics
even when there is no light input, making it impossible to obtain
an accurate S/N during light measurement.
Figure 5 shows the frequency characteristics of the signal and
noise when S2281 is used. Here, peaking cannot be observed.
LED drive pulse waveform
(2 V/div.)
Output waveform
(2 V/div.)
LED drive pulse waveform
(2 V/div.)
Output waveform
(2 V/div.)
Figure 2 Output response waveform (S2281 + E2573)
(a) H range
(b) L, M range
2
<Power supply circuit>
Photosensor amplifier
C2719
10
-1
FREQUENCY (Hz)
NOISE GAIN, OPEN LOOP GAIN
f
1'
f
1
f
2
f
3
10
8
10
0
10
7
10
1
10
6
10
2
10
5
10
3
10
4
10
-3
10
6
10
5
10
4
10
3
10
2
10
1
10
0
10
-1
10
-2
OP-AMP OPEN LOOP GAIN
10
5
10
4
10
3
10
2
FREQUENCY (Hz)
OUTPUT
VOLTAGE (10 dB/div.)
TERMINAL CAPACITANCE:
Ct=12 nF
TERMINAL CAPACITANCE:
Ct=1.2 nF
5
0
FREQUENCY (kHz)
OUTPUT VOLTAGE (10 dB/div.)
SIGNAL FREQUENCY
RESPONSE
NOISE FREQUENCY
RESPONSE
REF -17.0 dBm ATT10 dB
TG LEVEL -10.0 dBm
RBW 100 Hz
VBW 300 Hz
1
2
3
4
KACCB0033EA
Figure 5 Frequency response of output signal and noise (S2281 + E2573)
[Light source: Directly modulated laser diode (DC biased)]
I Selecting photodiodes
As described previously, stable measurement may become impossible depending on the photodiode type. The diagrams are
used here to explain how to select a correct photodiode which will not generate gain peaking when used with C2719.
Figure 6 shows an example in which a photodiode with a terminal capacitance Ct of 1.28 nF and a shunt resistance*
2
Rsh of 130
M
(See note below). In the low-frequency range , the op-amp noise gain of C2719 is determined only by the ratio between Rsh
and the feedback resistance Rf of C2719. Starting from frequency f1 at 1/
Ct > Rsh, the frequency increase is accompanied by
an increase in gain over the interval indicated by
. Next, as the frequency increases from f2 at 1/Cf > Rf, the circuit noise gain
is determined by the ratio of Ct to Cf, during the interval indicated by
. Then, in the range covered by interval , the noise gain
decreases, with a slope identical to that of the op-amp open loop gain starting from the point where it contacts frequency f3, which
normally decreases at 6 dB/oct.
At this point, if Ct is multiplied by 10, f1 moves to f1' and the setting of the noise gain rise at
exceeds the curve of the op-amp
open loop gain. This causes area
to disappear. If this happens, C2719 can no longer maintain stable operation, and gain
peaking appears. Consequently, it is necessary to make sure that area
always exists. With C2719, Rf and Cf are determined
by the sensitivity range. Thus, whether gain peaking occurs or not depends on the Ct value of the photodiode, so selecting the
appropriate photodiode is essential. Figure 7 shows what happens to each of the sensitivity ranges of C2719 when used with
S2281.
*2: Shunt resistance Rsh=
mV)
10
(V
I
mV
10
R
D
=
Figure 6 Schematic pursuit of gain peaking
KACCB0034EA
KACCB0031EA
KACCB0032EA
Figure 3 Frequency response (S2281 + E2573)
Figure 4 Gain peaking
10
3
10
0
FREQUENCY (Hz)
10
6
10
1
10
5
10
2
10
4
10
3
OUTPUT
(V/A)
10
10
10
9
10
8
10
7
10
6
10
5
10
4
H RANGE
M RANGE
L RANGE
3
Photosensor amplifier
C2719
10
-1
FREQUENCY (Hz)
NOISE GAIN, OPEN LOOP GAIN
10
-3
10
-4
10
6
10
5
10
4
10
3
10
2
10
1
10
0
10
-1
10
-2
10
8
10
9
10
0
10
7
10
1
10
6
10
2
10
5
10
3
10
4
PHOTODIODE: S2281
M RANGE
L RANGE
H RANGE
I Operation example
Figure 8 Typical application using S2281 photodiode
C2719
VOLTMETER
A/D CONVERTER
, etc.
PEN RECORDER
S2281 *
* The output of C2719 is positive going because the anode of S2281 is grounded.
COAXIAL CABLE
E2573
LIGHT
C2719
VOLTMETER
, etc.
COAXIAL CABLE
E2573
PHOTODIODE *
* The output of C2719 is negative going when the photodiode cathode is
grounded, and is positive going when the anode is grounded.
DARK BOX
LIGHT
C2719
VOLTMETER
, etc.
COAXIAL CABLE
E2573
DARK BOX
PHOTO-
TUBE
K
A
LIGHT
Figure 7 Example of ranges
KACCB0035EA
Figure 9 Typical application using an ordinary photodiode
Figure 10 Typical application using a phototube
114
39
90
PILOT LAMP
POWER SUPPLY
SWITCH
ZERO ADJUSTMENT
CONTROL
Rf CHANGE SWITCH
INPUT
BNC-R
OUTPUT
BNC-R
EXTERNAL
POWER SUPPLY
<UPPER SURFACE>
<FRONT>
KACCA0022EA
I Accessories
Plug for external power supply input
HR10-7P-4S (HIROSE)
Lead : AWG#24 (length: approx. 500 mm)
Color: red (+Vs), blue (-Vs), black (GND)
I Option
E2573
BNC-BNC coaxial cable
(coaxial cable: 1.5D-2V, cable length: 1 m)
Figure 11 Dimensional outline (unit: mm)
KACCC0032EB
KACCC0033EB
KACCC0034EA
4
Photosensor amplifier
C2719
G Avoid using C2719 in locations subjected to excessive
vibrations. If unavoidable, use a low-noise coaxial cable
as the signal input cable.
G In low-light-level detection circuits, insulation resistance
of the circuit board and other insulation materials can
affect measurement accuracy. Avoid humidity and soiling.
G Noise increases when the input capacitance becomes
larger, so keep the input coaxial cable as short as pos-
sible.
G Remove the batteries when C2719 is not to be used for a
long period.
G An input protection circuit is incorporated, but even so, do
not apply more than 100 V.
G For external power supply, use a regulated DC power
supply with a ripple of less than 3 mVp-p. Make sure that
the supply voltage does not exceed 15 V even momen-
tarily. Avoid using a switching power supply, as this can
cause noise problems during measurement.
I Precaution for use
G When an external power supply is used for C2719, volt-
age is supplied regardless of whether the power switch
of C2719 is on or off. Use the power switch of the external
power supply to turn voltage on and off.
G There are photodiodes whose cathode is connected to
the metal package. To connect the cathode of such a
photodiode to C2719, be sure to hold the photodiode
with insulator as shown in Figure 12. The signal current
cannot be obtained if the photodiode package is
grounded.
Figure 12 Connection to photodiode whose cathode is
connected to metal package
TO C2719
METAL CASE
K
A
LIGHT
PHOTODIODE
PACKAGE
INSULATOR
KACCC0035EA
Hamamatsu also provides C6386 high-speed photosensor amplifier that incorporates a photodiode at the installation port of
the optical fiber (1 meter long).
Photosensor amplifier
C6386
l Easy handling
Built-in photodiode allows easy detection of light just by
connecting to a voltmeter.
l Optical fiber light input
Measures light at a narrow detection point. Separating the
amplifier from the detection point allows measurement in
unusual environments and improves S/N.
l High-speed response
l Three sensitivity ranges
Features
Range
Photocurrent
detection sensitivity Cut-off frequency
H
10
#
V/A
1 MHz
M
10
"
V/A
3 MHz
L10
!
V/A
10 MHz
5
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