ADA4930-1YCPZ-R2 datasheet(17/25 Pages) AD

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Data Sheet

ADA4930-1/ADA4930-2

Rev. D | Page 17 of 25

THEORY OF OPERATION

The ADA4930-1/ADA4930-2 differ fromconventional op amps

in that they have two outputs whose voltagesmove in opposite

directionsand anadditional input, VOCM. Likean op amp, theyrely

on high open-loop gain and negative feedbackto force these

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

behave much like standardvoltagefeedbackopamps and facilitate

single-ended-to-differential conversions, common-mode level

shifting, and amplifications of differential signals.Like op amps,

the ADA4930-1/ADA4930-2 havehigh inputimpedance and low

output impedance.

Two feedbackloops control the differential and common-mode

output voltages. The differential feedback, set with external

resistors, controls the differential output voltage. The common-

mode feedbackcontrolsthecommon-modeoutputvoltage.This

architecture makesit easy to set the output common-modelevel

to any arbitrary value within the specified limits. The output

common-modevoltageis forced tobeequal tothevoltageapplied

to the VOCM input by the internal common-modefeedbackloop.

The internal common-mode feedbackloop produces outputs

that are highly balanced over a wide frequency rangewithout

requiring tightly matched external components. This results

in differential outputs that arevery close to the ideal of being

identical in amplitude and areexactly 180°apart in phase.

ANALYZING ANAPPLICATIONCIRCUIT

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

negative feedbackto 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 voltagecan be assumed

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

common-modevoltage and the voltageappliedto 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-modegain of the circuit in Figure 42 is

determined by

OUT

where the gain and feedbackresistors,RG and RF, on each side

are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noiseof theADA4930-1/ADA4930-2 can

be estimatedusingthenoisemodelin Figure43. Theinput-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-

referredtermsat +IN and−INbyan appropriateoutputfactor.

The output voltageduetovnIN is obtained by multiplying vnIN by

the noise gain, GN.

The circuit noise gain is

(

)

where the feedbackfactorsare

and

When the feedbackfactors arematched, RF1/RG1= RF2/RG2,

β1 = β2 = β, and the noise gain becomes

The noise currents are uncorrelated with thesamemean-square

value, and eachproducesan output voltagethat is equal to the

noise current multiplied by theassociated feedbackresistance.

The noise voltage density at the VOCM pin is vnCM. When the

feedbacknetworks havethe samefeedbackfactor, as in most

cases, the output noise dueto vnCM is common-modeand the

output noise fromVOCM is zero.

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

from the feedbackresistorsappears directly at the output, and

the noise from the gain resistorsappears at theoutputmultiplied

by RF/RG.

The total differential output noisedensity, vnOD, is the root-sum-

square of the individual output noise terms.

∑

)

( nODi

nOD

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