AD8221
Rev. C | Page 21 of 24
In this topology, an
OP27
sets the reference voltage of the
AD8221. The output signal of the instrumentation amplifier is
taken across the OUT pin and the REF pin. Two 1 kΩ resistors
and a 499 Ω resistor attenuate the ±10 V signal to +4 V. An
optional capacitor, C1, can serve as an antialiasing filter. An
AD8022 is used to drive the ADC.
This topology has five benefits. In addition to level-shifting and
attenuation, very little noise is contributed to the system. Noise
from R1 and R2 is common to both of the inputs of the ADC
and is easily rejected. R5 adds a third of the dominant noise and
therefore makes a negligible contribution to the noise of the
system. The attenuator divides the noise from R3 and R4. Likewise,
its noise contribution is negligible. The fourth benefit of this
interface circuit is that the acquisition time of the AD8221 is
reduced by a factor of 2. With the help of the OP27, the AD8221
only needs to deliver one-half of the full swing; therefore, signals
can settle more quickly. Lastly, the AD8022 settles quickly,
which is helpful because the shorter the settling time, the
more bits that can be resolved when the ADC acquires data.
This configuration provides attenuation, a level-shift, and a
convenient interface with a differential input ADC while
maintaining performance.
AC-COUPLED INSTRUMENTATION AMPLIFIER
Measuring small signals that are in the noise or offset of the
amplifier can be a challenge. Figure 52 shows a circuit that can
improve the resolution of small ac signals. The large gain
reduces the referred input noise of the amplifier to 8 nV/√Hz.
Thus, smaller signals can be measured because the noise floor is
lower. DC offsets that would have been gained by 100 are
eliminated from the output of the AD8221 by the integrator
feedback network.
At low frequencies, the
OP1177
forces the output of the AD8221 to
0 V. Once a signal exceeds f
HIGH-PASS
, the AD8221 outputs the
amplified input signal.
AD8221
OP1177
R
15.8k
+V
S
+IN
–IN
0.1µF
0.1µF
0.1µF
0.1µF
10µF
10µF
REF
C
1µF
–V
S
–V
S
+V
S
+V
S
–V
S
R
499
1
ʌ5&
f
HIGH-PASS
=
0
3
1
4
9
-0
5
1
Figure 52. AC-Coupled Circuit
