Datasheet
7
®
IVC102
60pF
30pF
10pF
0.1µF
0.1µF
1
I
Photodiode
Sensor
RC
2
3
4
5
6
11
I: Signal - Dependent Current
R: Sensor Resistance
C: Sensor Capacitance
12 13
10 V
O
14
V+
+15V
S
1
S
1
C
1
C
2
C
3
S
2
S
2
9
–15V
V–
Digital
Data
A/D
Converter
See timing
signals below
3a
3b
3c
Charge transferred
from sensor C
to C
INT
.
A
A
B
B
Transfer Function
Offset Voltage
Ramp due to
input bias current
(exaggerated).
Effective
Signal Integration
Period, T
S
V
O
waveform with
approx. half-scale input current.
V
O
waveform with
zero input current.
∆Q
S
1
Opening
0V
0V
0V
V
O
S
2
S
1
V
O
0V
+10mV
–10mV
10µs
Hold
10µs
Reset
10µs
Hold
10µs
Reset
10µs
Pre-Int.
Hold
∆Q
S
2
Opening
∆Q
S
1
Closing
Op Amp
V
OS
(S
1
Open) (S
1
Closed)
(S
2
Open)
FIGURE 3. Switched-Input Measurement Technique.
Input connections and timing are shown in Figure 3.
The timing diagram, Figure 3b, shows that S
1
is closed only
when S
2
is open. During the short period that S
1
is open
(30µs in this timing example), any signal current produced
by the sensor will charge the sensor’s source capacitance.
This charge is then transferred to C
INT
when S
1
is closed. As
a result, no charge produced by the sensor is lost and the
input signal is continuously integrated. Even fast input
pulses are accurately integrated.
SWITCHED-INPUT MEASUREMENT TECHNIQUE
While the basic reset-and-integrate measurement arrange-
ment in Figure 1 is satisfactory for many applications, the
switched-input timing technique shown in Figure 3 has
important advantages. This method can provide continuous
integration of the input signal. Furthermore, it can hold the
output voltage constant after integration for stable conver-
sion (desirable for a/d converter without a sample/hold).