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

A

A

B

B

Transfer Function

Offset Voltage

Ramp due to

input bias current

(exaggerated).

Effective

Signal Integration

Period, T

S

V

approx. half-scale input current.

V

zero input current.

∆Q

S

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

∆Q

S

Op Amp

V

OS

(S

(S

(S

FIGURE 3. Switched-Input Measurement Technique.

Input connections and timing are shown in Figure 3.

The timing diagram, Figure 3b, shows that S

when S

(30

µs in this timing example), any signal current produced

by the sensor will charge the sensor’s source capacitance.

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).