ADA4622-2ACPZ-R7 datasheet(30/36 Pages) AD

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Data Sheet

ADA4622-1/ADA4622-2/ADA4622-4

APPLICATIONS INFORMATION

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Rev. F | 30 of 36

The following basic transfer function describes the transimpedance

gain of the photodiode preamplifier:

VOUT = IPHOTO×RF

1+sCFRF

(4)

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 96).

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 band-

width 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).

fP= 12πRFCS

(5)

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 95).

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

Without Compensation

Figure 96. Gain and Phase Plot of the Transimpedance Amplifier Design with

Compensation

Adding CF creates a zero in the loop transmission that compen-

sates 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 96). The signal

bandwidth and the zero frequency are determined by

fZ = 12πRFCF

(6)

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

fX= fP×fGBP

(7)

Combining these equations, the CF value that produces fX is

CF= CS

2π×RF×fGBP

(8)

The frequency response in this case shows approximately 2 dB

of peaking and 15% overshoot. Doubling CF and halving the band-

width results in a flat frequency response with approximately 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 trace in Figure 96 shows

the noise gain over frequencies for the photodiode preamp.

Calculate the noise bandwidth at the fN frequency by

fN= fGBP

(CS+CF)/CF

(9)

Figure 97 shows the ADA4622-1/ADA4622-2/ADA4622-4 config-

ured as a transimpedance photodiode amplifier. The amplifiers

are used in conjunction with a photodiode detector with an input

capacitance of 5 pF. Figure 98 shows the transimpedance response

of the ADA4622-1/ADA4622-2/ADA4622-4 when IPHOTO is 1 µA

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