AD8221ARM datasheet(17/20 Pages) AD

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AD8221

Rev. A | Page 17 of 20

+12V

+IN

–IN

0.1

µF

0.1

µF

–12V

R3 1k

Ω

+2.5V

R4 1k

Ω

REF

499

Ω

10k

Ω

10k

Ω

470pF

+12V

0.1

µF

0.1

µF

–12V

+12V

+5V

0.1

µF

10nF

0.1

µF

–12V

+12V

0.1

µF

0.1

µF

–12V

R6 27.4

Ω

R7 27.4

Ω

220

µF

0.1

µF22µF

+5V

2.5V

220nF

10nF

AD8221

AD8022

OP27

AD8022

AD7723

VIN+

AVDD

AGND VGND REF1 REF2

DVDD

VIN–

AD780

GND

VIN

VOUT

(½)

Figure 51. Interfacing to a Differential Input ADC

CONDITIONING ±10 V SIGNALS FOR A +5 V

DIFFERENTIAL INPUT ADC

There is a need in many applications to condition ±10 V signals.

However, many of today’s ADCs and digital ICs operate on

much lower, single-supply voltages. Furthermore, new ADCs

have differential inputs because they provide better common-

mode rejection, noise immunity, and performance at low supply

voltages. Interfacing a ±10 V, single-ended instrumentation

amplifier to a +5 V, differential ADC may be a challenge.

Interfacing the in-amp to the ADC requires attenuation and a

level shift. A solution is shown in Figure 51.

In this topology, an OP27 sets the AD8221’s reference voltage.

The in-amp’s output signal 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, may serve as an

ant aliasing 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 ADC’s inputs and is

easily rejected. R5 adds a third of the dominant noise and there-

fore 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 inter-

face circuit is that the AD8221’s acquisition time 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 pro-

vides 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 amplifier’s noise or offset

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 since the noise floor is lower. DC

offsets that would have been gained by 100 are eliminated from

the AD8221’s output by the integrator feedback network.

At low frequencies, the OP1177 forces the AD8221’s output to

0 V. Once a signal exceeds fHIGH-PASS, the AD8221 outputs the

amplified input signal.

AD8221

OP1177

15.8k

Ω

+VS

+IN

–IN

0.1

µF

0.1

µF

0.1

µF

0.1

µF

REF

C 1

µF

–VS

+VS

–VS

499

Ω

πRC

fHIGH-PASS =

Figure 52. AC-Coupled Circuit

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