AD8541/AD8542/AD8544

Rev. E | Page 14 of 20

PHOTODIODE APPLICATION

The AD854x family has very high impedance with an input bias

current typically around 4 pA. This characteristic allows the

AD854x op amps to be used in photodiode applications and

other applications that require high input impedance. Note that

the AD854x has significant voltage offset that can be removed

by capacitive coupling or software calibration.

Figure 39 illustrates a photodiode or current measurement

application. The feedback resistor is limited to 10 MΩ to avoid

excessive output offset. Also, note that a resistor is not needed

on the noninverting input to cancel bias current offset because

the bias current-related output offset is not significant when

compared to the voltage offset contribution. For best

performance, follow the standard high impedance layout

techniques, which include:

•

Shielding the circuit.

•

Cleaning the circuit board.

•

Putting a trace connected to the noninverting input around

the inverting input.

•

Using separate analog and digital power supplies.

AD8541

4

6

7

3

2

D

OR

V+

C

100pF

R

10MΩ

Figure 39. High Input Impedance Application—Photodiode Amplifier