AD9632AR datasheet(13/20 Pages) AD

13 / 20 page

REV. C

AD9631/AD9632

–13–

THEORY OF OPERATION

General

The AD9631 and AD9632 are wide bandwidth, voltage feedback

amplifiers. Since their open-loop frequency response follows the

conventional 6 dB/octave roll-off, their gain bandwidth product

is basically constant. Increasing their closed-loop gain results in

a corresponding decrease in small signal bandwidth. This can

be observed by noting the bandwidth specification between the

AD9631 (gain of +1) and AD9632 (gain of +2). The AD9631/

AD9632 typically maintain 65 degrees of phase margin. This

high margin minimizes the effects of signal and noise peaking.

Feedback Resistor Choice

The value of the feedback resistor is critical for optimum perfor-

mance on the AD9631 (gain of +1) and less critical as the gain

increases. Therefore, this section is specifically targeted at the

AD9631.

At minimum stable gain (+1), the AD9631 provides optimum

dynamic performance with RF = 140

W. This resistor acts as a

parasitic suppressor only against damped RF oscillations that

can occur due to lead (input, feedback) inductance and parasitic

capacitance. This value of RF provides the best combination of

wide bandwidth, low parasitic peaking, and fast settling time.

In fact, for the same reasons, a 100

W–130 W resistor should be

placed in series with the positive input for other AD9631 noninver-

ting and all AD9631 inverting configurations. The correct

connection is shown in Figures 3 and 4.

AD9631/

AD9632

+VS

–VS

100 –130

RTERM

RIN

VIN

0.1 F

10 F

0.1 F

10 F

VOUT

G = 1 +

Figure 3. Noninverting Operation

AD9631/

AD9632

+VS

–VS

100 –130

RTERM

RIN

0.1 F

10 F

0.1 F

10 F

VOUT

VIN

G = –

Figure 4. Inverting Operation

When the AD9631 is used in the transimpedance (I to V) mode,

such as in photodiode detection, the value of RF and diode capaci-

tance (CI) are usually known. Generally, the value of RF selected

will be in the k

W range, and a shunt capacitor (CF) across RF will

be required to maintain good amplifier stability. The value of

CF required to maintain optimal flatness (<1 dB peaking) and

settling time can be estimated as

CC R

()

[]

–/

where

wO is equal to the unity gain bandwidth product of the

amplifier in rad/sec, and CI is the equivalent total input

capacitance at the inverting input. Typically

wO = 800 106

rad/sec (see TPC 15).

As an example, choosing RF = 10 k

W and CI = 5 pF requires CF

to be 1.1 pF (Note: CI includes both source and parasitic circuit

capacitance). The bandwidth of the amplifier can be estimated

using the CF calculated as

AD9631

VOUT

Figure 5. Transimpedance Configuration

For general voltage gain applications, the amplifier bandwidth

can be closely estimated as

R /R

3dB

()

This estimation loses accuracy for gains of +2/–1 or lower due

to the amplifier’s damping factor. For these “low gain” cases,

the bandwidth will actually extend beyond the calculated value

(see TPCs 13 and 25).

As a general rule, capacitor CF will not be required if

() ¥£

where NG is the noise gain (1 + RF/RG) of the circuit. For most

voltage gain applications, this should be the case.

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