ADA4937-1YCPZ-R2 datasheet(18/28 Pages) AD

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ADA4937-1

Rev. 0 | Page 18 of 28

THEORY OF OPERATION

The ADA4937-1 differs from conventional op amps in that it

has two outputs whose voltages move in opposite directions.

Like an op amp, it relies on open-loop gain and negative

feedback to force these outputs to the desired voltages. The

ADA4937-1 behaves much like a standard voltage feedback op

amp and makes it easier to perform single-ended-to-differential

conversions, common-mode level shifting, and amplifications

of differential signals. Also like an op amp, the ADA4937-1 has

high input impedance and low output impedance.

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. It is forced,

by internal common-mode feedback, to be equal to the voltage

applied to the VOCM input, without affecting the differential

output voltage.

The ADA4937-1 architecture results in outputs that are highly

balanced over a wide frequency range without requiring tightly

matched external components. The common-mode feedback

loop forces the signal component of the output common-

mode voltage to zero. This results in nearly perfectly balanced

differential outputs that are identical in amplitude and are

exactly 180° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The ADA4937-1 uses 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 46). 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 46 can be

determined by

OUT

This assumes the input resistors (

RG) and feedback resistors (RF)

on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4937-1 can be

estimated using the noise model in Figure 47. 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 noise currents are assumed to be

equal and produce a voltage across the parallel combination of

the gain and feedback resistances. vnCM is the noise voltage

density at the VOCM pin. Each of the four resistors contributes

(4kTRx)1/2. Table 8 summarizes the input noise sources, the

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

ADA4937-1

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 47. ADA4937-1 Noise Model

Table 8. Output Noise Voltage Density Calculations

Input Noise Contribution

Input Noise Term

Input Noise

Voltage Density

Output

Multiplication Factor

Output Noise

Voltage Density Term

Differential Input

vnIN

vnO1 = GN(vnIN)

Inverting Input

inIN−

inIN− × (RG2||RF2)

vnO2 = GN[inIN− × (RG2||RF2)]

Noninverting Input

inIN+

inIN+ × (RG1||RF1)

vnO3 = GN[inIN+ × (RG1||RF1)]

VOCM Input

vnCM

GN(β1 − β2)

vnO4 = GN(β1 − β2)(vnCM)

Gain Resistor RG1

vnRG1

(4kTRG1)1/2

GN(1 − β2)

vnO5 = GN(1 − β2)(4kTRG1)1/2

Gain Resistor RG2

vnRG2

(4kTRG2)1/2

GN(1 − β1)

vnO6 = GN(1 − β1)(4kTRG2)1/2

Feedback Resistor RF1

vnRF1

(4kTRF1)1/2

vnO7 = (4kTRF1)1/2

Feedback Resistor RF2

vnRF2

(4kTRF2)1/2

vnO8 = (4kTRF2)1/2

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