AD9632 datasheet(15/20 Pages) AD

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Data Sheet

AD9631/AD9632

Rev. D | Page 15 of 20

THEORY OF OPERATION

GENERAL

The AD9631/AD9632 are wide bandwidth, voltage feedback

amplifiers. Because 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° 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 per-

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

gain increases. Therefore, this section is specifically targeted

at the AD9631.

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

mum dynamic performance with RF = 140 Ω. 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 combi-

nation of wide bandwidth, low parasitic peaking, and fast

settling time.

In fact, for the same reasons, place a 100 Ω to 130 Ω resistor in

series with the positive input for other AD9631 noninverting

and all AD9631 inverting configurations. The correct connec-

tion is shown in Figure 59 and Figure 60.

Figure 59. Noninverting Operation

Figure 60. 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 capacitance (CI) are usually known. Generally, the value

of RF selected will be in the kΩ 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 by

(

)

[

−

≅

where:

ωO is equal to the unity gain bandwidth product of the amplifier

in rad/sec.

CI is the equivalent total input capacitance at the inverting input.

Typically ωO = 800 × 106 rad/sec (see Figure 19).

As an example, choosing RF = 10 kΩ and CI = 5 pF requires CF

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

circuit capacitance). The bandwidth of the amplifier can be

estimated using CF:

3dB

≅

Figure 61. Transimpedance Configuration

For general voltage gain applications, the amplifier bandwidth

can be closely estimated as

(

)

3dB

≅

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

to the damping factor of the amplifier. For these low gain cases,

the bandwidth will actually extend beyond the calculated value

(see Figure 17 and Figure 29).

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.

+VS

0.1µF

10µF

–VS

100Ω TO

130Ω

RIN

RTERM

VIN

VOUT

G = 1 +

AD9631/

AD9632

+VS

0.1µF

10µF

–VS

RTERM

VIN

VOUT

G = 1 –

AD9631/

AD9632

100Ω TO

130Ω

RIN

VOUT

AD9631

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