ADA4622-2ACPZ-R7 datasheet(28/34 Pages) AD

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ADA4622-1/ADA4622-2

Data Sheet

Rev. B | Page 28 of 34

The following basic transfer function describes the transimpedance

gain of the photodiode preamplifier:

PHOTO

OUT







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 curve in Figure 95).

s refers to the s-plane.

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

CS and the amplifier input capacitance, CD and CM. RF and the

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





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 94).

log f

G = 1

G = R2C1s

OPEN-LOOP GAIN

–180°

–135°

–90°

–45°

0°

fGBP

Figure 94. Gain and Phase Plot of the Transimpedance Amplifier Design,

Without Compensation

OPEN-LOOP GAIN

G = 1

fGBP

G = 1 + CS/CF

I TO V GAIN

–135°

–90°

–45°

0°

45°

90°

G = RFCS(s)

Figure 95. Gain and Phase Plot of the Transimpedance Amplifier Design with

Compensation

Adding CF creates a zero in the loop transmission that compensates

for the effect of the input pole, which stabilizes the photodiode

preamplifier design because of the increased phase margin. Adding

CF also sets the signal bandwidth (see Figure 95). The signal

bandwidth and the zero frequency are determined by



where fZ is the zero frequency.

Setting the zero at the fX frequency maximizes the signal bandwidth

with a 45° phase margin. Because fX is the geometric mean of fP

and fGBP, it can be calculated by

GBP





Combining these equations, the CF value that produces fX is

GBP







The frequency response in this case shows about 2 dB of peaking

and 15% overshoot. Doubling CF and halving the bandwidth results

in a flat frequency response with about 5% transient overshoot.

The dominant sources of output noise in the wideband photodiode

preamp design are the input voltage noise of the amplifier, VNOISE,

and the resistor noise due to RF. The gray curve in Figure 95 shows

the noise gain over frequencies for the photodiode preamp.

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