ADA4625-2ARDZ-R7 datasheet(29/35 Pages) AD

29 / 35 page

Data Sheet

ADA4625-1/ADA4625-2

Rev. A | Page 29 of 35

Design Example

As a design example, Figure 101 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 101. 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 102 and Figure 103 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 104). 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 102. 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 103. Compensating the TIA, CF = 3.9 pF

–50

100

150

TIME(1µS/DIV)

CF = 3.9pF

CF = 2.2pF

Figure 104. Pulse Response vs. CF

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