ADA4938-1

Preliminary Technical Data

Rev. PrD | Page 10 of 14

THEORY OF OPERATION

The ADA4938-1 differs from conventional op amps in that it

has two outputs whose voltages move in opposite directions.

Like an op amp, it relies on open-loop gain and negative

feedback to force these outputs to the desired voltages. The

ADA4938-1 behaves much like a standard voltage feedback op

amp and makes it easier to perform single-ended-to-differential

conversions, common-mode level shifting, and amplifications

of differential signals. Also like an op amp, the ADA4938-1 has

high input impedance and low output impedance.

Two feedback loops are employed to control the differential and

common-mode output voltages. The differential feedback, set

with external resistors, controls only the differential output

voltage. The common-mode feedback controls only the common-

mode output voltage. This architecture makes it easy to set the

output common-mode level to any arbitrary value. It is forced,

by internal common-mode feedback, to be equal to the voltage

output voltage.

The ADA4938-1 architecture results in outputs that are highly

balanced over a wide frequency range without requiring tightly

matched external components. The common-mode feedback

loop forces the signal component of the output common-

mode voltage to zero. This results in nearly perfectly balanced

differential outputs that are identical in amplitude and are

exactly 180° apart in phase.

ANALYZING AN APPLICATION CIRCUIT

The ADA4938-1 uses open-loop gain and negative feedback to

force its 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 4). For most purposes, this voltage can be assumed to be

zero. Similarly, the difference between the actual output

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 4 can be

determined by

G

F

dm

IN

dm

OUT

R

R

V

V

=

,

,

This assumes the input resistors (

on each side are equal.

ESTIMATING THE OUTPUT NOISE VOLTAGE

The differential output noise of the ADA4938-1 can be

estimated using the noise model in Figure 5. The input-referred

ground. The noise currents are assumed to be equal and

produce a voltage across the parallel combination of the gain

6 summarizes the input noise sources, the multiplication factors,

and the output-referred noise density terms.

ADA4938-1

+

Figure 5. ADA4938-1 Noise Model

Table 6. Output Noise Voltage Density Calculations

Input Noise Contribution

Input Noise Term

Input Noise

Voltage Density

Output

Multiplication Factor

Output Noise

Voltage Density Term

Differential Input

Inverting Input

Noninverting Input

1

1