AD8250ARMZ-R7 datasheet(20/25 Pages) AD

20 / 25 page

Data Sheet

AD8250

Rev. C | Page 19 of 24

RF INTERFERENCE

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 54. The filter

limits the input signal bandwidth according to the following

relationship:

)

DIFF

FilterFreq







FilterFreq





where CD ≥ 10 CC.

AD8250

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

–15V

OUT

Figure 54. 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 the negative input degrades the CMRR of the AD8250.

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 ADC

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 AD8250 make it an excellent

ADC driver.

In this example, a 1 nF capacitor and a 100 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 100 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 AD8250 and the voltage at the

input of the AD7612 but may destabilize the AD8250. A trade-

off must be made between selecting a resistor small enough to

maintain accuracy and large enough to maintain stability.

0.1μF

1nF

100Ω

AD7612

ADR435

+12V

–12V

+5V

+15V

–15V

+IN

–IN

10μF0.1µF

REF

AD8250

DGND

Figure 55. Driving an ADC

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