ADA4940-1ACPZ-R7 datasheet(23/30 Pages) AD

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Data Sheet

ADA4940-1/ADA4940-2

Rev. D | Page 23 of 30

APPLICATIONS INFORMATION

ANALYZING AN APPLICATION CIRCUIT

The ADA4940-1/ADA4940-2 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 61). 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 two

assumptions, any application circuit can be analyzed.

SETTING THE CLOSED-LOOP GAIN

Determine the differential mode gain of the circuit in Figure 61

by using the following equation:

OUT



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

(RF) on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

Estimate the differential output noise of the ADA4940-1/

ADA4940-2 by using the noise model in Figure 63. 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 14

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.

ADA4940-1/

ADA4940-2

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

Figure 63. ADA4940-1/ADA4940-2 Noise Model

As with conventional op amp, the output noise voltage densities

can be estimated by multiplying the input-referred terms at +IN

and −IN by the appropriate output factor,

where:







is the circuit noise gain.





and





are the feedback factors.

When RF1/RG1 = RF2/RG2, then β1 = β2 = β, and the noise gain

becomes





Note that the output noise from VOCM goes to zero in this case.

The total differential output noise density, vnOD, is the root-sum-

square of the individual output noise terms.





nOi

nOD

Table 14. Output Noise Voltage Density Calculations

Input Noise Contribution

Input Noise Term

Input Noise

Voltage Density

Output

Multiplication Factor

Output-Referred 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

vnCM

GN (β1 − β2)

vnO4 = GN (β1 − β2)(vnCM)

Gain Resistor RG1

vnRG1

(4kTRG1)1/2

GN (1 − β2)

vnO5 = GN (1 − β2)(4kTRG1)1/2

Gain Resistor RG2

vnRG2

(4kTRG2)1/2

GN (1 − β1)

vnO6 = GN (1 − β1)(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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