ADA4941-1YRZ-R7 datasheet(19/23 Pages) AD

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Data Sheet

ADA4941-1

Rev. D | Page 19 of 23

FREQUENCY RESPONSE VS. CLOSED-LOOP GAIN

The operational amplifiers used in the ADA4941-1 are voltage

feedback with an open-loop frequency response that can be

approximated with the integrator response, as shown in Figure 54.

100

0.001

100

FREQUENCY (MHz)

0.01

0.1

fcr = 50MHz

Figure 54. ADA4941-1 Op Amp Open-Loop Gain vs. Frequency

For each amplifier, the frequency response can be approximated

by the following equations:

















































fcr

VIN

_A1

(15)

(Noninverting Response)









































 





fcr

VIN

_A2

(16)

(Inverting Response)

fCR is the gain-bandwidth frequency of the amplifier (where the

open-loop gain shown in Figure 54 equals 1). fCR for both

amplifiers is about 50 MHz.

The inverting amplifier A2 has a fixed feedback network. The

transfer function is approximately















































MHz

VOP

VIN

(17)

The frequency response of A1 depends on the external feedback

network as indicated by Equation 15. The overall differential

output voltage is therefore

VO, dm = VOP − VON = VOP + VOP ×















MHz

(18)

































































MHz

VIN

, dm

(19)

Multiplying the terms and neglecting negligible terms leads to

the following approximation:











































































MHz

VIN

(20)

There are two poles in this transfer function, and the lower

frequency pole limits the bandwidth of the differential

amplifier. If VOP is shorted to IN− (A1 is a unity gain follower),

the 25 MHz closed-loop bandwidth of the inverting channel

limits the overall bandwidth. When A1 is operating with higher

noise gains, the bandwidth is limited by the closed-loop

bandwidth of A1, which is inversely proportional to the noise

gain (1 + RF/RG). For instance, if the external feedback network

provides a noise gain of 10, the bandwidth drops to 5 MHz.

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