Data Sheet

AD8390A

Rev. B | Page 9 of 12

THEORY OF OPERATION

56kΩ

56kΩ

VCC

VCC

VCOM

OUTP

AD8390A

OUTN

INP

INN

VEE

VEE

C

A

B

56kΩ

56kΩ

Figure 15. Functional Block Diagram

The AD8390A is a true differential amplifier with common-

mode feedback. The AD8390A is functionally equivalent to three

amplifiers, as shown in Figure 15. Amplifier A and Amplifier B

form a standard dual amplifier in an inverting configuration.

Amplifier C maintains the common-mode voltage VCOM at

the output.

With VCOM left unconnected, the outputs are internally biased

to midsupply. VCOM can be driven externally to set the dc

output common-mode voltage.

OUTN

OUTP

INP

INN

VCOM

+

–

–

+

Figure 16. Basic Application Circuit

The high open-loop gain of the AD8390A and the negative

feedback minimize the differential and common-mode error

voltages.

With the differential and common-mode error voltages assumed

to be 0, the differential-mode gain and input impedance of the

basic application circuit shown in Figure 16 are as follows:

G

F

DM

IN

DM

OUT

R

R

V

V



,

,

G

DM

IN

R

R



 2

,