AD8220BRMZ-R7 datasheet(23/29 Pages) AD

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AD8220

Rev. B | Page 22 of 28

RF INTERFERENCE

RF rectification is often a problem in applications where there are

large RF signals. The problem appears as a small dc offset voltage.

The AD8220 by its nature has a 5 pF gate capacitance, CG, at its

inputs. Matched series resistors form a natural low-pass filter that

reduces rectification at high frequency (see Figure 61). The

relationship between external, matched series resistors and the

internal gate capacitance is expressed as follows:

DIFF

FilterFreq



FilterFreq



AD8220

VOUT

–VS

REF

–VS

+IN

–IN

+15V

–15V

0.1µF

10µF

0.1µF

10µF

Figure 61. RFI Filtering Without External Capacitors

To eliminate high frequency common-mode signals while using

smaller source resistors, a low-pass RC network can be placed at

the input of the instrumentation amplifier (see Figure 62). The

filter limits the input signal bandwidth according to the following

relationship:

)

(

DIFF

FilterFreq





)

(

FilterFreq





Mismatched CC capacitors result in mismatched low-pass filters.

The imbalance causes the AD8220 to treat what would have

been a common-mode signal as a differential signal. To reduce

the effect of mismatched external CC capacitors, select a value of

CD greater than 10 times CC. This sets the differential filter

frequency lower than the common-mode frequency.

AD8220

+15V

+IN

–IN

0.1µF

10µF

0.1µF

REF

VOUT

–15V

10nF

1nF

4.02kΩ

Figure 62. RFI Suppression

COMMON-MODE INPUT VOLTAGE RANGE

The common-mode input voltage range is a function of the

input range and the outputs of Internal Amplifier A1, Internal

Amplifier A2, and Internal Amplifier A3, the reference voltage,

and the gain. Figure 27 to Figure 30 show common-mode

voltage ranges for various supply voltages and gains.

DRIVING AN ADC

An instrumentation amplifier is often used in front of an ADC

to provide CMRR and additional conditioning, such as a voltage

level shift and gain (see Figure 63). In this example, a 2.7 nF

capacitor and a 1 kΩ resistor create an antialiasing filter for the

AD7685. The 2.7 nF capacitor also serves to store and deliver

the necessary charge to the switched capacitor input of the

ADC. The 1 kΩ series resistor reduces the burden of the 2.7 nF

load from the amplifier. However, large source impedance in

front of the ADC can degrade THD.

The example shown in Figure 63 is for sub-60 kHz applications.

For higher bandwidth applications where THD is important,

the series resistor needs to be small. At worst, a small series

resistor can load the AD8220, potentially causing the output to

overshoot or ring. In such cases, a buffer amplifier, such as the

AD8615, should be used after the AD8220 to drive the ADC.

AD8220

AD7685

4.7µF

ADR435

+5V

2.7nF

REF

1kΩ

1.07kΩ

+2.5V

+IN

–IN

±50mV

+5V

0.1µF

10µF

Figure 63. Driving an ADC in a Low Frequency Application

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