AD8541/AD8542/AD8544

Rev. F | Page 15 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 40 illustrates a photodiode or current measurement

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

excessive output offset. In addition, 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 the

following:

•

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 40. High Input Impedance Application—Photodiode Amplifier