AD8222ACPZ-R7 datasheet(20/24 Pages) AD

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AD8222

Rev. A | Page 20 of 24

AD8222

(DIFF OUT)

100pF

NPO

100pF

NPO

1000pF

–IN

+IN

0.1µF

10µF

0.1µF

10µF

–12V

+12V

1kΩ

IN+

VDD

GND

REF

10µF

X5R

AD7688

IN–

0.1µF

+5V

ADR435

GND

VIN

VOUT

0.1µF

+12V

0.1µF

+5V REF

+IN2

REF2

+5V REF

+OUT

–OUT

1kΩ

2200pF

1kΩ

Figure 51. Driving a Differential ADC

DRIVING A DIFFERENTIAL INPUT ADC

The AD8222 can be configured in differential output mode

to drive a differential analog-to-digital converter. Figure 51

illustrates several of the concepts.

RFI and Antialiasing Filter

The 1 kΩ resistors, 1000 pF capacitor, and 100 pF capacitors in

front of the in-amp form filter circuitry that performs many

functions. The 1 kΩ and 100 pF capacitors form common-mode

filters that protect the amplifier from incoming radio frequency

signals. Without the filtering, these RFI signals can be rectified

in the in-amp. The 1 kΩ resistors provide some overvoltage

protection. The 1 kΩ resistors and 1000 pF capacitor form a

76 kHz antialiasing filter for the ADC.

Note that the 100 pF capacitors are 5% COG/NPO types. These

capacitors match well over time and temperature, which keeps

the system CMRR high over frequency.

Second Antialiasing Filter

A 1 kΩ resistor and 2200 pF capacitor are placed between each

AD8222 output and ADC input. They create a 72 kHz low-pass

filter for another stage of antialiasing protection.

These four elements also improve distortion performance. The

2200 pF capacitor provides charge to the switched capacitor

front end of the ADC, and the 1 kΩ resistor shields the AD8222

from driving any sharp current changes. If the application

requires a lower frequency antialiasing filter and is distortion

sensitive, increase the value of the capacitor rather than the

resistor.

The 1 kΩ resistors can also protect an ADC from overvoltages.

Because the AD8222 runs on wider supply voltages than a

typical ADC, there is a possibility of overdriving the ADC. This

is not an issue with a PulSAR® converter, such as the AD7688.

Its input can handle a 130 mA overdrive, which is much higher

than the short-circuit limit of the AD8222. However, other conver-

ters have less robust inputs and may need the added protection.

Reference

The ADR435 supplies a reference voltage to both the ADC and

the AD8222. Because REF2 on the AD8222 is grounded, the

common-mode output voltage is precisely half the reference

voltage, exactly where it needs to be for the ADC.

PRECISION STRAIN GAGE

The low offset and high CMRR over frequency of the AD8222

make it an excellent candidate for both ac and dc bridge measure-

ments. As shown in Figure 52, the bridge can be connected to

the inputs of the amplifier directly.

2.5V

10µF

0.1µF

AD8222

+IN

–IN

350Ω

–

Figure 52. Precision Strain Gauge

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