ADA4939-1YCPZ-R2 datasheet(17/24 Pages) AD

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ADA4939-1/ADA4939-2

Rev. 0 | Page 17 of 24

THEORY OF OPERATION

The ADA4939 differs from conventional op amps in that it has

two outputs whose voltages move in opposite directions and an

additional input, VOCM. Like an op amp, it relies on high open-

loop gain and negative feedback to force these outputs to the

desired voltages. The ADA4939 behaves much like a standard

voltage feedback op amp and facilitates single-ended-to-differential

conversions, common-mode level shifting, and amplifications of

differential signals. Like an op amp, the ADA4939 has high input

impedance and low output impedance. Because it uses voltage

feedback, the ADA4939 manifests 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 uses high open-loop gain and negative feedback

to force its 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 can be assumed

to be zero. Similarly, the difference between the actual output

common-mode voltage and the voltage applied to VOCM can also

be assumed to be 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 frequency response exhibits excessive peaking

for differential gains <2; therefore, the part should be operated

with differential gains ≥2.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4939 can be estimated

using the noise model 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 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 RF/RG. Table 11 summarizes the input noise sources, the

multiplication factors, and the output-referred noise density terms.

ADA4939

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 43. Noise Model

ADA4939-1YCPZ-R2 Datasheet(PDF) 17 Page - Analog Devices

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