ADA4945-1ACPZ-R2 datasheet(37/44 Pages) AD

37 / 44 page

Data Sheet

ADA4945-1

Rev. 0 | Page 37 of 44

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:

OUT

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.

ADA4945-1

RF2

VnOD

VnCM

VOCM

VnIN

RF1

RG2

RG1

VnRF1

VnRF2

VnRG1

VnRG2

inIN+

inIN–

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:

(

)

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 13. 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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