Data Sheet
THEORY OF OPERATION
VCC
INP
AD8390A
A
VCC
56kΩ
56kΩ
C
56kΩ
VEE
56kΩ
B
INN
OUTN
VCOM
OUTP
VEE
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.
AD8390A
+
RG INP
VIN, DM
–
VCOM
RG
INN
RF
OUTN
–
OUTP
RL, DM VOUT, DM
+
RF
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:
VOUT ,DM R F
VIN ,DM
RG
RIN ,DM 2 RG
Rev. B | Page 9 of 12