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Data Sheet

ADA4930-1/ADA4930-2

Rev. B | Page 17 of 25

THEORY OF OPERATION

The ADA4930-1/ADA4930-2 differ from conventional op amps

in that they have two outputs whose voltages move in opposite

directions and an additional input, VOCM. Like an op amp, they rely

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

outputs to the desired voltages. The ADA4930-1/ADA4930-2

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 ADA4930-1/ADA4930-2 have high input impedance and low

output impedance.

Two feedback loops control the differential and common-mode

output voltages. The differential feedback, set with external

resistors, controls the differential output voltage. The common-

mode feedback controls 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 to be equal to the voltage applied

to the VOCM input by the internal common-mode feedback loop.

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 ADA4930-1/ADA4930-2 use high open-loop gain and

negative feedback to force their differential and common-mode

output voltages 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 is

determined by

OUT



where the gain and feedback resistors,

RG and RF, on each side

are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4930-1/ADA4930-2 can

be estimated using the noise model in Figure 43. The input-referred

noise voltage density, vnIN, is modeled as differential. The noise

currents, inIN− and inIN+, appear between each input and ground.

ADA4930

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 43. Noise Model

Similar to the case of conventional op amps, the output noise

voltage densities can be estimated by multiplying the input-

referred terms at +IN and −IN by an appropriate output factor.

The output voltage due to vnIN is obtained by multiplying vnIN by

the noise gain, GN.

The circuit noise gain is







where the feedback factors are





and





When the feedback factors are matched, RF1/RG1 = RF2/RG2,

β1 = β2 = β, and the noise gain becomes





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 and the

output noise from VOCM is zero.

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.

The total differential output noise density, vnOD, is the root-sum-

square of the individual output noise terms.





)

(

nODi

nOD

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