ADA4937-2YCPZ-R2 datasheet(18/28 Pages) AD

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ADA4937-1/ADA4937-2

Data Sheet

Rev. F | Page 18 of 28

THEORY OF OPERATION

The ADA4937-1/ADA4937-2 differ from conventional

operational amplifiers in that they have two outputs whose voltages

move in opposite directions. Like an operational amplifier, they

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

outputs to the desired voltages. The ADA4937-1/ADA4937-2

behave much like standard voltage feedback operational amplifiers,

which makes it easier to perform single-ended-to-differential

conversions, common-mode level shifting, and amplifications

of differential signals. Also like an operational amplifier, the

ADA4937-1/ADA4937-2 have high input impedance and low

output impedance.

Two feedback loops control the differential and common-mode

output voltages. The differential feedback loop, set with external

resistors, controls only the differential output voltage. The

common-mode feedback loop 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/ADA4937-2 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/ADA4937-2 use 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 52). 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 52 can be

determined by

OUT



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

(RF) on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

To estimate the differential output noise of the ADA4937-1/

ADA4937-2 use the noise model in Figure 53. 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. vn, cm is the noise voltage density at the VOCM pin. Each

of the four resistors contributes (4kTRx)1/2. Table 9 summarizes

the input noise sources, the multiplication factors, and the output-

referred noise density terms.

ADA4937

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 53. ADA4937-1/ADA4937-2 Noise Model

Table 9. 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

vn, cm

GN(β1 − β2)

vnO4 = GN(β1 − β2)(vn, cm)

Gain Resistor RG1

vnRG1

(4kTRG1)1/2

GN(1 − β1)

vnO5 = GN(1 − β1)(4kTRG1)1/2

Gain Resistor RG2

vnRG2

(4kTRG2)1/2

GN(1 − β2)

vnO6 = GN(1 − β2)(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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