AD8220ARMZ-R7 datasheet(26/29 Pages) AD

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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

500Ω

AD8618

14.5kΩ

19.3kΩ

2.5V

22nF

4.7µF

REF

+5V

REFERENCE

ADR435

68nF

+5V

AD8618

+5V

1.15kΩ

4.99kΩ

2.5V

33nF

19.3kΩ

14.5kΩ

33nF

LOW-PASS FIFTH ORDER FILTER AT 157Hz

AD8618

+5V

–5V

+5V

47nF

14kΩ

57.6kΩ

1.18kΩ

G= +50

4.7µF

2.5V

1MΩ

12.7kΩ

499kΩ

220pF

HIGH-PASS FILTER 0.033Hz

INSTRUMENTATION

AMPLIFIER

G=+14

AD8220

+5V

4.12kΩ

24.9kΩ

–5V

+5V

OP AMPS

OP2177

866kΩ

68pF

15kΩ

–5V

+5V

–5V

+5V

2.2pF

10pF

10kΩ

Figure 67. Example ECG Schematic

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