Datasheet

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, C
G
, 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:
G
DIFF
RC
FilterFreq
π2
1

G
CM
RC
FilterFreq
π2
1

AD8220
V
OUT
C
G
C
G
–V
S
REF
–V
S
R
R
+IN
–IN
+15
V
–15V
0.1µF 10µF
0.1µF 10µF
03579-030
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:
)2(π2
1
G
CD
DIFF
CCCR
FilterFreq


)(π2
1
G
C
CM
CCR
FilterFreq


Mismatched C
C
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 C
C
capacitors, select a value of
C
D
greater than 10 times C
C
. This sets the differential filter
frequency lower than the common-mode frequency.
R
R
AD8220
+15
V
+IN
–IN
0.1µF
10µF
10µF
0.1µF
REF
V
OUT
–15V
C
D
C
C
C
C
10nF
1nF
1nF
4.02k
4.02k
03579-003
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
1k
1.07k
+2.5V
+IN
–IN
±50mV
+5
V
0.1µF10µF
0
3579-033
Figure 63. Driving an ADC in a Low Frequency Application