AD8251ARMZ-R7 datasheet(21/25 Pages) AD

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AD8251

Rev. B | Page 20 of 24

RF INTERFERENCE

DRIVING AN ADC

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 58. The

filter limits the input signal bandwidth according to the following

relationship:

An instrumentation amplifier is often used in front of an ADC

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.

In Figure 59, a 1 nF capacitor and a 49.9 Ω resistor create an

antialiasing filter for the AD7612. The 1 nF capacitor stores and

delivers the 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.

)

(

FilterFreq

DIFF

FilterFreq

where CD ≥ 10 CC.

AD8251

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

0.1

μF

0.1

μF

1nF

49.9

Ω

AD7612

ADR435

+12V

–12V

+5V

+15V

–15V

+IN

–IN

μF0.1µF

REF

AD8251

DGND

Figure 58. RFI Suppression

Values of R and CC should be chosen to minimize RFI. A

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

Figure 59. Driving an ADC

, the effect of the mismatch is reduced and performance is

improved.

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