Data Sheet AD8479
Rev. A | Page 11 of 16
THEORY OF OPERATION
The AD8479 is a unity-gain, differential-to-single-ended
amplifier that can reject extremely high common-mode signals
in excess of 600 V with 15 V supplies. The AD8479 consists of
an operational amplifier (op amp) and a resistor network (see
Figure 33).
2
3
4
1
7
6
8
REF(–)
–IN
1MΩ
AD8479
1MΩ
+IN
–V
S
NC
+V
S
OUTPUT
111
18-033
5
REF(+)
NOTES
1. NC = NO CONNEC
T. DO NOT CONNECT TO THIS PIN.
Figure 33. Functional Block Diagram
To achieve the high common-mode voltage range, an internal
resistor divider—connected to Pin 3 and Pin 5—attenuates the
noninverting signal by a factor of 60. The internal resistors at
Pin 1 and Pin 2, as well as the feedback resistor, restore the gain
to provide a differential gain of unity.
The complete transfer function is
V
OUT
= V (+IN) − V (−IN)
Laser wafer-trimming provides resistor matching so that
common-mode signals are rejected and differential input
signals are amplified.
To reduce output voltage drift, the op amp uses super beta tran-
sistors in its input stage. The input offset current and its associated
temperature coefficient contribute no appreciable output voltage
offset or drift, which has the added benefit of reducing voltage
noise because the corner where 1/f noise becomes dominant is
below 5 Hz. To reduce the dependence of gain accuracy on the
op amp, the open-loop voltage gain of the op amp exceeds
20 million V/V, and the PSRR exceeds 90 dB.
