ADA4932-1YCPZ-R7 datasheet(20/27 Pages) AD

20 / 27 page

ADA4932-1/ADA4932-2

Data Sheet

Rev. E | Page 20 of 27

APPLICATIONS INFORMATION

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 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 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

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 ADA4932-1/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 multip-

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

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

ADA4932-1/

ADA4932-2

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

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

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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