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AD8420

Data Sheet

Rev. 0 | Page 24 of 28

APPLICATIONS INFORMATION

AD8420 IN ELECTROCARDIOGRAPHY (ECG)

A high-pass filter is commonly used in ECG signal conditioning

circuitry to remove electrode offset and motion artifacts. To avoid

degrading the input impedance and CMRR of the system, this

filtering is typically implemented after the instrumentation

amplifier, which limits the gain that can be applied with the

instrumentation amplifier.

With a 3-op-amp instrumentation amplifier, gain is applied in the

first stage. Because of this, the electrode offset is gained and then

must be removed afterward with a high-pass filter. In the AD8420

architecture, the offset can be accounted for in the input stage

by unbalancing the transconductance amplifier at the REF and

FB pins. In the steady state, the offset at the input is not gained

to the output, and higher frequency signals can be gained and

passed through. Using the AD8420 in this way, the offset tolerance

is nearly the differential input range of the part (±1 V).

Figure 69 shows an ECG front end that applies a gain of 100 to

the signal while rejecting dc and high frequencies. This circuit

combines the AD8420 with the AD8657, which is a low power,

low cost, dual, precision CMOS op amp.

–5V

8200pF

10MΩ

INSTRUMENTATION

AMPLIFIER

G = +100

AD8420

+5V

–5V

+5V

3.3μF

402kΩ

200pF

2000pF

100kΩ

+5V

–5V

AD8657-2

AD8657-1

REF

100kΩ

1kΩ

200kΩ

0.015μF

THREE-POLE LPF,

BESSEL RESPONSE

FC = 50Hz

INTEGRATOR PROVIDES

HIGH-PASS POLE AT 0.5Hz

110kΩ

0.022μF

500kΩ

Figure 69. AD8420 in an ECG Front End

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