ADA4945-1ACPZ-R7 datasheet(46/57 Pages) AD

46 / 57 page

Data Sheet

ADA4945-1

analog.com

Rev. A

46 of 57

APPLICATIONS INFORMATION

Analyzing an Application Circuit

The ADA4945-1 uses open-loop gain and negative feedback to force the differential and common-mode output

voltages to minimize the differential and common-mode error voltages. The differential error voltage is the voltage

between the differential inputs labeled +IN and −IN (see Figure 98). For most purposes, this voltage is 0 V. Similarly,

the difference between the actual output common-mode voltage and the voltage applied to VOCM is also 0 V. Starting

from these two assumptions, any application circuit can be analyzed.

Setting the Closed-Loop Gain

Determine the differential mode gain of the circuit in Figure 98 by using the following equation:

VOUT,dm

VIN,dm

| =

This calculation assumes 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 ADA4945-1 can be estimated by using the noise model in Figure 102. 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 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 13 summarizes the input noise sources, the multiplication factors, and the

output referred noise density terms. For more noise calculation information, go to the Analog Devices Differential

Amplifier Calculator (DiffAmpCalc™), click ADIDiffAmpCalculator.zip, and follow the on-screen prompts.

Figure 102. ADA4945-1 Noise Model

As with conventional op amps, the output noise voltage densities can be estimated by multiplying the input

referred terms at +IN and −IN by the appropriate output factor, where:

GN =

(β1+β2)

is the circuit noise gain.

β1 =

RG1

RF1 + RG1

and β2 =

RG2

RF2 + RG2

are the feedback factors. When RF1/RG1 = RF2/RG2, then β1 = β2 = β, and the noise gain becomes

GN =

= 1 +

ADA4945-1

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

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