ADA4625-1ARDZ-R7 datasheet(26/30 Pages) AD

26 / 30 page

ADA4625-1

Data Sheet

Rev. 0 | Page 26 of 30

Design Example

As a design example, Figure 81 shows the ADA4625-1 configured

as a TIA amplifier in a photodiode preamp application. Assuming

the photodiode has a CD of 5 pF and an ID of 200 μA, and the

desired full-scale VOUT is 10 V, and using Equation 1, RF is 50 kΩ.

0.1µF

+15V

49.9kΩ

VOUT

1kΩ

2.2pF

ADA4625-1

Figure 81. Single-Supply TIA Circuit Using the ADA4625-1

The ADA4625-1 input capacitance (CCM + CDM) is 19.9 pF;

therefore, the total input capacitance (CIN) is 24.9 pF. By

substituting CIN = 24.9 pF, RF = 50 kΩ, and fGBP = 18 MHz into

Equation 7 and Equation 9, the resulting feedback capacitor

value (CF) and the −3 dB signal bandwidth (fP) are 2.2 pF and

1.45 MHz, respectively.

Figure 82 and Figure 83 show the compensations of the TIA circuit.

The system has a bandwidth of 1.45 MHz when it is maximized

for a signal bandwidth with CF = 2.2 pF. Increasing CF to 3.9 pF

reduces the bandwidth to 0.82 MHz; however, it greatly reduces the

overshoot (see Figure 84). In practice, an optimum CF value is

determined experimentally by varying it slightly to optimize the

output pulse response.

100

10k

100k

–20

100

10k

100k

10M

100M

10G

FREQUENCY (Hz)

OPEN-LOOP

GAIN

NOISE GAIN

I-V GAIN

Figure 82. Compensating the TIA, CF = 2.2pF

100

10k

100k

–20

100

10k

100k

10M

100M

10G

FREQUENCY (Hz)

OPEN-LOOP

GAIN

NOISE GAIN

I-V GAIN

Figure 83. Compensating the TIA, CF = 3.9 pF

–50

100

150

TIME(1µS/DIV)

CF =3.9pF

CF =2.2pF

Figure 84. Pulse Response vs. CF

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