ADA4350ARUZ-R7 datasheet(33/38 Pages) AD

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ADA4350

Data Sheet

Rev. B | Page 32 of 37

TRANSIMPEDANCE AMPLIFIER DESIGN THEORY

Because its low input bias current minimizes the dc error at the

preamp output, the ADA4350 works well in photodiode preamp

applications. In addition, its high gain bandwidth product and

low input capacitance maximizes the signal bandwidth of the

photodiode preamp. Figure 58 shows the transimpedance

amplifier model of the ADA4350.

–

VOUT

ADA4350

RSH = 1011Ω

IPHOTO

Figure 58. Transimpedance Amplifier Model of the ADA4350

The basic transfer function in Equation 4 describes the

transimpedance gain of the photodiode preamp.

PHOTO

OUT







(4)

where:

IPHOTO is the output current of the photodiode.

RF is the feedback resistor.

CF is the feedback capacitance.

The signal bandwidth is 1/(RF × CF), as determined by Equation 4.

In general, set RF such that the maximum attainable output

voltage corresponds to the maximum diode current, IPHOTO,

allowing the use of the full output swing.

The signal bandwidth attainable with this preamp is a function

of RF, the gain bandwidth product (fGBW) of the amplifier, and

the total capacitance at the amplifier summing junction, including

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

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

fP = 1/2πRFCS

(5)

With the additional pole from the open-loop response of the

amplifier, the two-pole system results in peaking and instability

due to an insufficient phase margin (see gray lines for the noise

gain and phase in Figure 59).

Adding CF to the feedback loop creates a zero in the loop

transmission that compensates for the effect of the input pole,

which stabilizes the photodiode preamp design because of the

increased phase margin (see the gray lines for the noise gain and

phase in Figure 60). It also sets the signal bandwidth, fZ (see the

I to V gain line for the signal gain in Figure 60). The signal

bandwidth and the zero frequency, fZ, are determined by

2π



(6)

Equating the zero frequency, fZ, with the fX frequency maximizes

the signal bandwidth with a 45° phase margin. Calculate fX as

follows because fX is the geometric mean of fP and fGBW:

GBW





(7)

By combining Equation 5, Equation 6, and Equation 7, the CF

value that produces fX is defined by

GBW





(8)

The frequency response in this case shows approximately 2 dB

peaking and 15% overshoot. Doubling CF and cutting the

bandwidth in half results in a flat frequency response with

approximately 5% transient overshoot.

log f

G = 1

G = R2C1s

fGBW

OPEN-LOOP GAIN

–180°

–135°

–90°

–45°

0°

Figure 59. Noise Gain and Phase Bode Plot of the Transimpedance Amplifier

Design Without Compensation

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