ADA4927-1/ADA4927-2

Data Sheet

Rev. B | Page 18 of 25

APPLICATIONS INFORMATION

ANALYZING AN APPLICATION CIRCUIT

The ADA4927 uses high open-loop transimpedance and negative

current feedback to control the differential output voltage in

such a way as to minimize the differential error currents. The

differential error currents are defined as the currents that flow

in and out of the differential inputs labeled +IN and −IN (see

Figure 46). For most purposes, these currents can be assumed

to be zero. The voltage between the +IN and −IN inputs is

internally bootstrapped to 0 V; therefore, the voltages at the

amplifier inputs are equal, and external analysis can be carried

out in a similar fashion to that of voltage feedback amplifiers.

Similarly, the difference between the actual output common-

to be zero. Starting from these principles, any application circuit

can be analyzed.

SETTING THE CLOSED-LOOP GAIN

Using the approach previously described, the differential gain of

the circuit in Figure 46 can be determined by

G

F

dm

IN

dm

OUT

R

R

V

V

=

,

,

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4927 can be estimated

using the noise model in Figure 47. The input-referred noise

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

networks have the same feedback factor, as in most cases, the

resistors appears directly at the output, and the noise from each

summarizes the input noise sources, the multiplication factors,

and the output-referred noise density terms.

ADA4927

+

Figure 47. 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

Inverting Input

1

Noninverting Input

1

0

1

1