AD8221BRZ-R7 datasheet(21/25 Pages) AD

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AD8221

Rev. C | Page 20 of 24

RF INTERFERENCE

RF rectification is often a problem when amplifiers are used in

applications where there are strong RF signals. The disturbance

can appear as a small dc offset voltage. High frequency signals

can be filtered with a low-pass RC network placed at the input

of the instrumentation amplifier, as shown in Figure 49. The

filter limits the input signal bandwidth according to the following

relationship:

)

(

Diff

FilterFreq

where C

10C

AD8221

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

499

10nF

1nF

4.02k

Figure 49. RFI Suppression

D affects the difference signal, and CC affects the common-

mode signal. Values of R and C

C should be chosen to minimize

RFI. Mismatch between the R × C

C at the positive input and the

R × C

C at the negative input degrades the CMRR of the AD8221.

By using a value of C

D one magnitude larger than CC, the effect

of the mismatch is reduced, and therefore, performance is

improved.

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8221

make it an excellent candidate for bridge measurements. As

shown in Figure 50, the bridge can be directly connected to

the inputs of the amplifier.

+5V

+2.5V

10µF

0.1µF

AD8221

+IN

–IN

350

–

Figure 50. Precision Strain Gage

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 can be a challenge.

Interfacing the instrumentation amplifier to the ADC requires

attenuation and a level shift. A solution is shown in Figure 51.

+12V

+IN

–IN

0.1µF

10µF

0.1µF

10µF

–12V

+2.5V

REF

499

10k

470pF

+12V

0.1µF

–12V

+12V

+5V

0.1µF

10nF

0.1µF

–12V

+12V

0.1µF

–12V

27.4

220µF

10µF

0.1µF

22µF

+5V

2.5V

220nF

10nF

AD8221

AD8022

OP27

AD8022

AD7723

VIN(+)

AVDD

AGND DGND REF1 REF2

DVDD

VIN(–)

AD780

GND

+VIN

VOUT

(½)

Figure 51. Interfacing to a Differential Input ADC

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