Datasheet

AD8220
Rev. B | Page 23 of 28
APPLICATIONS INFORMATION
AC-COUPLED INSTRUMENTATION AMPLIFIER
Measuring small signals that are in the noise or offset of the
amplifier can be a challenge. Figure 64 shows a circuit that
can improve the resolution of small ac signals. The large gain
reduces the referred input noise of the amplifier to 14 nV/√Hz.
Therefore, smaller signals can be measured because the noise
floor is lower. DC offsets that would have been gained by 100
are eliminated from the AD8220 output by the integrator
feedback network.
At low frequencies, the OP1177 forces the AD8220 output to
0 V. Once a signal exceeds f
HIGH-PASS
, the AD8220 outputs the
amplified input signal.
AD8220
OP1177
R
15.8k
+
V
S
+IN
–IN
0.1µF
0.1µF
0.1µF
0.1µF
10µF10µF
REF
C
1µF
–V
S
–V
S
+V
S
+V
S
–V
S
R
499
1
2RC
f
HIGH-PASS
=
V
REF
03579-004
Figure 64. AC-Coupled Circuit
DIFFERENTIAL OUTPUT
In certain applications, it is necessary to create a differential
signal. New high resolution ADCs often require a differential
input. In other cases, transmission over a long distance can
require differential processing for better immunity to
interference.
Figure 65 shows how to configure the AD8220 to output a
differential signal. An OP1177 op amp is used to create a
differential voltage. Errors from the op amp are common to
both outputs and are thus common mode. Likewise, errors from
using mismatched resistors cause a common-mode dc offset
error. Such errors are rejected in differential signal processing
by differential input ADCs or instrumentation amplifiers.
When using this circuit to drive a differential ADC, V
REF
can be
set using a resistor divider from the reference of the ADC to
make the output ratiometric with the ADC as shown in Figure 66.