ADA4932-2YCPZ-R21 datasheet(20/28 Pages) AD

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ADA4932-1/ADA4932-2

Rev. A | Page 20 of 28

APPLICATIONS INFORMATION

ANALYZING AN APPLICATION CIRCUIT

The ADA4932-x uses high 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 55). 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 principles, any applica-

tion 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 55

can be determined by

OUT



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-x can be

estimated using the noise model in Figure 56. 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 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 multip-

lied by RF/RG. Table 11 summarizes the input noise sources, the

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

ADA4932-x

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 56. 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

vnO1 = GN(vnIN)

Inverting Input

inIN−

inIN− × (RF2)

vnO2 = (inIN−)(RF2)

Noninverting Input

inIN+

inIN+ × (RF1)

vnO3 = (inIN+)(RF1)

VOCM Input

vnCM

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

vnO7 = (4kTRF1)1/2

Feedback Resistor, RF2

vnRF2

(4kTRF2)1/2

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)

499

998

9.25

499

249

498

12.9

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)

511

499

53.6

665

525

9.19

523

249

57.6

374

276

12.6

806

243

57.6

392

270

17.7

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

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