AD8221ARMZ-R7 datasheet(20/24 Pages) AD

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AD8221

Rev. B | Page 20 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 51 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 fHIGH-PASS, the AD8221 outputs the

amplified input signal.

AD8221

OP1177

15.8kΩ

+VS

+IN

–IN

0.1µF

10µF

REF

1µF

–VS

+VS

–VS

499Ω

2πRC

fHIGH-PASS =

Figure 51. AC-Coupled Circuit

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