Data Sheet

ADA4932-1/ADA4932-2

APPLICATIONS INFORMATION

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Rev. F | 20 of 27

ANALYZING AN APPLICATION CIRCUIT

The ADA4932-1/ADA4932-2 use high open-loop gain and negative

feedback to force their differential and common-mode output voltag-

es in such a way as to minimize the differential and common-mode

error voltages. The differential error voltage is defined as the volt-

age between the differential inputs labeled +IN and −IN (see Figure

54). For most purposes, this voltage is zero. Similarly, the difference

between the actual output common-mode voltage and the voltage

applied to VOCM is also zero. Starting from these principles, any

application circuit can be analyzed.

SETTING THE CLOSED-LOOP GAIN

Using the approach described in the Analyzing an Application

Circuit section, the differential gain of the circuit in Figure 54 can be

determined by

VOUT,dm

VIN,dm

=RFRG

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

(RF) on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4932-1/ADA4932-2 can be

estimated using the noise model in Figure 55. 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/VOCMx pin is vnCM. When the

feedback networks have the same feedback factor, as is true 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.

Figure 55. Noise Model

Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks

Input Noise Contribution

Input Noise Term

Input Noise Voltage Density

Output Multiplication Factor

Differential Output Noise Voltage

Density Term

Differential Input

vnIN

vnIN

GN

vnO1 = GN(vnIN)

Inverting Input

inIN−

inIN− × (RF2)

1

vnO2 = (inIN−)(RF2)

Noninverting Input

inIN+

inIN+ × (RF1)

1

vnO3 = (inIN+)(RF1)

VOCM Input

vnCM

vnCM

0

vnO4 = 0 V

Gain Resistor, RG1

vnRG1

(4kTRG1)1/2

RF1/RG1

vnO5 = (RF1/RG1)(4kTRG1)1/2

Gain Resistor, RG2

vnRG2

(4kTRG2)1/2

RF2/RG2

vnO6 = (RF2/RG2)(4kTRG2)1/2

Feedback Resistor, RF1

vnRF1

(4kTRF1)1/2

1

vnO7 = (4kTRF1)1/2

Feedback Resistor, RF2

vnRF2

(4kTRF2)1/2

1

vnO8 = (4kTRF2)1/2

Table 12. Differential Input, DC-Coupled

Nominal Gain (dB)

RF (Ω)

RG (Ω)

RIN, dm (Ω)

Differential Output Noise Density (nV/√Hz)

0

499

499

998

9.25

6

499

249

498

12.9

10

768

243

486

18.2

Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, RS = 50 Ω

Nominal Gain (dB)

RF (Ω)

RG1 (Ω)

RT (Ω) (Std 1%)

RIN, cm (Ω)

RG2 (Ω)1

Differential Output Noise Density (nV/√Hz)

0

511

499

53.6

665

525

9.19

6

523

249

57.6

374

276

12.6

10

806

243

57.6

392

270

17.7

1 RG2 = RG1 + (RS||RT).