AD8251ARMZ-R7 datasheet(20/24 Pages) AD

20 / 24 page

AD8251

Rev. 0 | Page 20 of 24

AD8251

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

In this example, a 1 nF capacitor and a 49.9 Ω resistor create an

antialiasing filter for the AD7612. The 1 nF capacitor also serves

to store and deliver necessary charge to the switched capacitor

input of the ADC. The 49.9 Ω series resistor reduces the burden

of the 1 nF load from the amplifier and isolates it from the

kickback current injected from the switched capacitor input of

the AD7612. Selecting too small a resistor improves the

correlation between the voltage at the output of the AD8251

and the voltage at the input of the AD7612 but may destabilize

the AD8251. A trade-off must be made between selecting a

resistor small enough to maintain accuracy and large enough to

maintain stability.

0.1μF

1nF

49.9Ω

AD7612

ADR435

+12V

–12V

+5V

+15V

–15V

+IN

–IN

10μF0.1µF

REF

AD8251

DGND

Figure 56. RFI Suppression

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 negative input degrades the CMRR of the AD8251.

By using a value of CD that is 10 times larger than the value of

CC, the effect of the mismatch is reduced and performance is

improved.

DRIVING AN ANALOG-TO-DIGITAL CONVERTER

An instrumentation amplifier is often used in front of an

analog-to-digital converter to provide CMRR. Usually,

instrumentation amplifiers require a buffer to drive an ADC.

However, the low output noise, low distortion, and low settle

time of the AD8251 make it an excellent ADC driver.

Figure 57. Driving an ADC

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