Datasheet

AD8220
Rev. B | Page 25 of 28
ELECTROCARDIOGRAM SIGNAL CONDITIONING
The AD8220 makes an excellent input amplifier for next
generation ECGs. Its small size, high CMRR over frequency,
rail-to-rail output, and JFET inputs are well suited for this
application. Potentials measured on the skin range from 0.2 mV
to 2 mV. The AD8220 solves many of the typical challenges of
measuring these body surface potentials. The high CMRR of
the AD8220 helps reject common-mode signals that come in
the form of line noise or high frequency EMI from equipment
in the operating room. Its rail-to-rail output offers a wide
dynamic range allowing for higher gains than would be possible
using other instrumentation amplifiers. JFET inputs offer a
large input capacitance of 5 pF. A natural RC filter is formed
reducing high frequency noise when series input resistors are
used in front of the AD8220 (see the RF Interference section).
In addition, the AD8220 JFET inputs have ultralow input
bias current and no current noise, making it useful for ECG
applications where there are often large impedances. The MSOP
and the optimal pinout of the AD8220 allow smaller footprints
and more efficient layout, paving the way for next-generation
portable ECGs.
Figure 67 shows an example ECG schematic. Following the
AD8220 is a 0.033 Hz high-pass filter, formed by the 4.7 μF
capacitor and the 1 MΩ resistor, which removes the dc offset
that develops between the electrodes. An additional gain of 50,
provided by the AD8618, makes use of the 0 V to 5 V input
range of the ADC. An active, fifth-order, low-pass Bessel filter
removes signals greater than approximately 160 Hz. An OP2177
buffers, inverts, and gains the common-mode voltage taken at
the midpoint of the AD8220 gain setting resistors. This right-
leg drive circuit helps cancel common-mode signals by
inverting the common-mode signal and driving it back into the
body. A 499 kΩ series resistor at the output of the OP2177
limits the current driven into the body.
ADC
AD7685
AD8618
+5V
2.7nF
500
AD8618
14.5k19.3k
2.5V
22nF
4.7µF
REF
+5V
REFERENCE
ADR435
68nF
+5V
AD8618
+5V
1.15k
4.99k
2.5V2.5V2.5V
33nF
19.3k 14.5k
33nF
LOW-P
A
SS FIFTH ORDER FILTER AT 157Hz
AD8618
+5V
–5V
–5V
+5V
47nF
14k
14k57.6k1.18k
G=+50
4.7µF
2.5V
1M
12.7k
499k
220pF
HIGH-PASS FILTER 0.033Hz
INSTRUMENTATION
AMPLIFIER
G=+14
AD8220
+5V
4.12k
24.9k
24.9k
–5V
+5V
OP AMPS
OP2177
OP2177
866k
68pF
AB
C
03579-032
15k
–5V
+5V
–5V
+5V
2.2pF
2.2pF
10pF
10k
10k
Figure 67. Example ECG Schematic