AD8221
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 ꢀ signal to +4 ꢀ. An
optional capacitor, C1, can serve as an antialiasing filter. An
AD8022 is used to drive the ADC.
reduces the referred input noise of the amplifier to 8 nꢀ/√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 ꢀ. Once a signal exceeds fHIGH-PASS, the AD8221 outputs the
amplified input signal.
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.
+V
S
0.1µF
+IN
R
1
fHIGH-PASS
=
2πRC
AD8221
499Ω
R
REF
C
1µF
15.8kΩ
–IN
+V
S
0.1µF
0.1µF
–V
S
OP1177
AC-COUPLED INSTRUMENTATION AMPLIFIER
+V
–V
S
S
Measuring small signals that are in the noise or offset of the
amplifier can be a challenge. Figure 51 shows a circuit that can
improve the resolution of small ac signals. The large gain
10µF
10µF
0.1µF
–V
S
Figure 51. AC-Coupled Circuit
Rev. B | Page 20 of 24
ADI [ ADI ]
