ADA4620-2ARZ-R7 datasheet(35/49 Pages) AD

35 / 49 page

Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

35 of 49

APPLICATIONS INFORMATION

Photodiode Preamplifier/Transimpedance Amplifier

The ADA4620 devices are an excellent choice for photodiode preamplifier applications. The low input bias current

minimizes the DC error at the output of the preamplifier. In addition, the high gain bandwidth product and low

input capacitance maximize the signal bandwidth of the photodiode preamplifier. Figure 115 shows the ADA4620-

1/ADA4620-2 as a current to voltage (I to V) converter with an electrical model of a photodiode.

Figure 115. Equivalent TIA Circuit

The following basic transfer function describes the transimpedance gain of the photodiode preamplifier:

�� =

�� × ��

1 + ��

(1)

where, IPHOTO is the output current of the photodiode. The parallel combination of RF and CF sets the signal

bandwidth (see the I to V gain trace in Figure 116). Note that RF must be set so the maximum attainable output

voltage corresponds to the maximum diode output current, IPHOTO, which allows use of the full output swing. The

attainable signal bandwidth with this photodiode preamplifier is a function of RF, the gain bandwidth product (fGBP)

of the amplifier, and the total capacitance at the amplifier summing junction, including CSH and the amplifier input

capacitance, CD and CCM.

�� = �� + �� + ��

(2)

RF and the total capacitance produce a pole with loop frequency (fP).

��

�� =

2��

(3)

With the additional pole from the amplifier open-loop response, the two-pole system results in peaking and

instability due to an insufficient phase margin. See Figure 116.

CCM

VOUT

RSH = 1011Ω

CSH

IPD

PHOTODIODE MODEL

ADA4620-1/

ADA4620-2

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