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 S1 is open

(30

µs in this timing example), any signal current produced

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

This charge is then transferred to CINT when S1 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).