AD8224ACPZ-R7 datasheet(25/28 Pages) AD

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AD8224

Rev. B | Page 25 of 28

AD8224

(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

806Ω

2.7nF

806Ω

Figure 65. Driving a Differential ADC

DRIVING A DIFFERENTIAL INPUT ADC

The AD8224 can be configured in differential output mode

to drive a differential ADC. Figure 65 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 CMRR of the system high over frequency.

Second Antialiasing Filter

An 806 Ω resistor and a 2.7 nF capacitor are located between

each AD8224 output and ADC input. These components

create a 73 kHz low-pass filter for another stage of antialiasing

protection.

These four elements also isolate the ADC from loading the

AD8224. The 806 Ω resistor shields the AD8224 from the

switched capacitor input of the ADC, which looks like a time-

varying load. The 2.7 nF capacitor provides a charge to the

switched capacitor front end of the ADC. If the application

requires a lower frequency antialiasing filter, increase the value

of the capacitor rather than the resistor.

The 806 Ω resistors can also protect an ADC from overvoltages.

Because the AD8224 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 AD8224.

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 AD8224. Because REF2 on the AD8224 is grounded, the

common-mode output voltage is precisely half the reference

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

DRIVING CABLING

All cables have a certain capacitance per unit length, which

varies widely with cable type. The capacitive load from the cable

may cause peaking in the AD8224 output response. To reduce

peaking, use a resistor between the AD8224 and the cable.

Because cable capacitance and desired output response vary

widely, this resistor is best determined empirically. A good

starting point is 50 Ω.

The AD8224 operates at a low enough frequency that

transmission line effects are rarely an issue; therefore, the

resistor need not match the characteristic impedance of

the cable.

AD8224

(DIFF OUT)

AD8224

(SINGLE OUT)

Figure 66. Driving a Cable

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