AD8222

Rev. 0 | Page 20 of 24

AD8222

(DIFF OUT)

100pF

NPO

5%

100pF

NPO

5%

1000pF

–IN

+IN

0.1µF

10µF

0.1µF

10µF

–12V

+12V

1kΩ

1kΩ

+

+

IN+

VDD

GND

REF

10µF

X5R

AD7688

IN–

0.1µF

+5V

ADR435

GND

0.1µF

+12V

0.1µF

+5V REF

+IN2

REF2

+5V REF

+OUT

–OUT

1kΩ

2200pF

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.

First Antialiasing Filter

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

front of the in-amp form a 76 kHz filter. This is the first of two

antialiasing filters in the circuit and helps to reduce the noise of

the system. The 100 pF capacitors protect against common-

mode RFI signals. Note that they are 5% COG/NPO types.

These capacitors match well over time and temperature, which

keeps the system’s CMRR high over frequency.

Second Antialiasing Filter

A 1 kΩ resistor and 2200 pF capacitor are located 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 help distortion performance. The

2200 pF capacitor provides charge to the switched capacitor

front end of the ADC, while 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

converters 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 GAUGE

The low offset and high CMRR over frequency of the

AD8222 make it an excellent candidate for both ac and dc

bridge measurements. As shown in Figure 52, the bridge can

be connected to the inputs of the amplifier directly.

5V

2.5V

10µF

0.1µF

AD8222

+IN

–IN

350Ω

350Ω

350Ω

350Ω

+

–

Figure 52. Precision Strain Gauge