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Data Sheet

ADA4939-1/ADA4939-2

Rev. A | Page 17 of 24

THEORY OF OPERATION

The ADA4939-1/ADA4939-2 differ from conventional op amps

in that they have two outputs whose voltages move in opposite

directions and an additional input, VOCM. Like op amps, they

rely on high open-loop gain and negative feedback to force

these outputs to the desired voltages. The ADA4939-1/

ADA4939-2 behave much like standard voltage feedback op

amps and facilitate single-ended-to-differential conversions,

common-mode level shifting, and amplifications of differential

signals. Like op amps, the ADA4939-1/ADA4939-2 have high

input impedance and low output impedance. Because they use

voltage feedback, the ADA4939-1/ADA4939-2 manifest a

nominally constant gain-bandwidth product.

Two feedback loops are employed to control the differential and

common-mode output voltages. The differential feedback, set

with external resistors, controls only the differential output voltage.

The common-mode feedback controls only the common-mode

output voltage. This architecture makes it easy to set the output

common-mode level to any arbitrary value within the specified

limits. The output common-mode voltage is forced by the internal

common-mode feedback loop to be equal to the voltage applied

to the VOCM input.

The internal common-mode feedback loop produces outputs

that are highly balanced over a wide frequency range without

requiring tightly matched external components. This results in

differential outputs that are very close to the ideal of being

identical in amplitude and are exactly 180° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The ADA4939-1/ADA4939-2 use high open-loop gain and

negative feedback to force their differential and common-mode

output voltages in such a way as to minimize the differential

and common-mode error voltages. The differential error

voltage is defined as the voltage between the differential inputs

labeled +IN and −IN (see Figure 42). For most purposes, this

voltage is zero. Similarly, the difference between the actual

output common-mode voltage and the voltage applied to VOCM

is also zero. Starting from these two assumptions, any

application circuit can be analyzed.

SETTING THE CLOSED-LOOP GAIN

The differential-mode gain of the circuit in Figure 42 can be

determined by

OUT



This presumes that the input resistors (RG) and feedback resistors

(RF) on each side are equal.

STABLE FOR GAINS ≥2

The ADA4939-1/ADA4939-2 frequency response exhibits

excessive peaking for differential gains <2; therefore, operate the

devioce with differential gains ≥2.

ESTIMATING THE OUTPUT NOISE VOLTAGE

To estimate the differential output noise of the ADA4939-1/

ADA4939-2 use the noise model shown in Figure 43. The input-

referred noise voltage density, vnIN, is modeled as a differential

input, and the noise currents, inIN− and inIN+, appear between

each input and ground. The output voltage due to vnIN is obtained

by multiplying vnIN by the noise gain, GN (defined in the GN

equation that follows). The noise currents are uncorrelated with

the same mean-square value, and each produces an output voltage

that is equal to the noise current multiplied by the associated

feedback resistance. The noise voltage density at the VOCM/VOCMx

pin is vnCM. When the feedback networks have the same feedback

factor, as in most cases, the output noise due to vnCM is common-

mode. Each of the four resistors contributes (4kTRxx)1/2. The noise

from the feedback resistors appears directly at the output, and

the noise from the gain resistors appears at the output multiplied

by RFx/RGx. Table 11 summarizes the input noise sources, the

multiplication factors, and the output-referred noise density

terms.

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

ADA4939-1/

ADA4939-2

Figure 43. Noise Model

ADA4939-1YCPZ-R7 Datasheet(PDF) 17 Page - Analog Devices

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