AD8221 查看數據表（PDF） - Analog Devices

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AD8221

Precision Instrumentation Amplifier

Analog Devices 

AD8221

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:

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

mode signal. Values of R and CC should be chosen to minimize

RFI. Mismatch between the R × CC at the positive input and the

R × CC at the negative input degrades the CMRR of the AD8221.

By using a value of CD one magnitude larger than CC, the effect

of the mismatch is reduced, and therefore, performance is

improved.

PRECISION STRAIN GAGE

FilterFreqDiff

FilterFreqCM

where CD t 10CC.

1

2πR(2CD  CC)

1

2πRCC

+15V

0.1µF

10µF

CC

R

4.02kŸ

CD

R

4.02kŸ

CC

1nF

+IN

10nF

R1

499Ÿ

–IN

AD8221

REF

VOUT

1nF

0.1µF

10µF

–15V

Figure 49. RFI Suppression

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

10µF 0.1µF

350Ÿ

350Ÿ

350Ÿ

350Ÿ

+IN +

R AD8221

–

–IN

+2.5V

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

+2.5V

10µF 0.1µF

+IN

10µF

–IN

0.1µF

AD8221

R1

REF 10kŸ

R2

10kŸ

–12V

+12V

0.1µF

OP27

0.1µF

–12V

R3

1kŸ

C1

470pF

R5

499Ÿ

R4

1kŸ

+12V

0.1µF

AD8022

(½)

0.1µF

–12V

+12V

0.1µF

AD8022

(½)

R6

27.4Ÿ

C2

220µF

R7

27.4Ÿ

+5V

+5V

10nF

AVDD

VIN(+)

DVDD

AD7723

VIN(–)

AGND DGND REF1 REF2

220nF 10nF

0.1µF

–12V

+5V

10µF

0.1µF

+VIN

2.5V

VOUT

AD780 22µF

GND

Figure 51. Interfacing to a Differential Input ADC

Rev. C | Page 20 of 24

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