ADA4940-1ACPZ-R7 datasheet(22/32 Pages) AD

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ADA4940-1/ADA4940-2

Data Sheet

Rev. B | Page 22 of 32

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 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 two assumptions, any application

circuit can be analyzed.

SETTING THE CLOSED-LOOP GAIN

The differential mode gain of the circuit in Figure 61 can be

determined by

OUT

This 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 ADA4940-1/ADA4940-2 can

be estimated 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 to be 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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