ADA4351-2ACPZ-R7 datasheet(29/36 Pages) AD

29 / 36 page

Data Sheet

ADA4351-2

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 29 of 36

Figure 88. Loop Gain Elements and Closed-Loop Transimpedance Frequency

Response for the Overcompensated Response in Figure 86

The noise gain here intersects the ADA4351-2 AOL curve at about

1.1 MHz, where the loop gain is at the 0 dB crossover with the

loop-gain phase curve showing a stable 71° phase margin in Figure

89. Higher frequency poles in the ADA4351-2 open-loop response

reduce this phase margin slightly but still yield a stable design.

Figure 89. Loop Gain Magnitude and Phase for the Design of Figure 88

Generally, a good starting point for a design is to set the feedback

pole as shown in Equation 2, which is in Hertz. Setting the feedback

pole, P1, at 0.707 × f0 yields an approximate Butterworth response,

giving a maximally flat closed-loop response with only 4% step

response overshoot.

P1= 12πRFCF= GBP4πRFCS=f02

(2)

If P1 is set as shown in Equation 2, the closed-loop transimpedance

response has a f−3dB ≈ f0.

Example Transimpedance Design for Higher

Gain, Lower Bandwidth

Re-executing the design shown in Figure 86 for a higher gain, and

setting P1 to approximate a Butterworth closed-loop design, results

in the design shown in Figure 90.

Figure 90. Higher Gain, Lower Bandwidth Butterworth Design Example with

CD = 50 pF

This simplified design equation is effective when CS > 5 × CF, as it

is here. Under those conditions, the approximate zero, Z1, is 1/(2π

× 200 kΩ × 55.4 pF) = 14.4 kHz (note that this is neglecting CF in

the Z1 equation shown in Figure 87).

The characteristic frequency is approximately the following:

f0= GBP×Z1= 8.5 MHz × 14.4 kHz

=348 kHz

(3)

The feedback pole is placed at 0.707 × f0 = 246 kHz, and the

resulting f−3 dB must be near 350 kHz = f0. Rerunning the loop

gain and response shape curves for the updated design of Figure

90 gives a close fit as shown in Figure 91 with an f−3 dB = 340

kHz showing a flat Butterworth response. The required feedback

capacitor in this example is largely the internal 3 pF (CF, INT), where

that 0.2 pF externally across the 200 kΩ feedback shown in Figure

90 is approximately the parasitic capacitance for a surface-mount

resistor.

Figure 91. Redesigned TIA Design for a RF = 200 kΩ Butterworth Response

with CD = 50 pF

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