AD8237ARMZ-R7 datasheet(27/29 Pages) AD

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AD8237

Data Sheet

Rev. 0 | Page 26 of 28

AD8237 IN AN ECG FRONT END

Electrocardiogram (ECG) circuits must operate with a differential

dc offset due to the half-cell potential of the electrodes. The tolerance

for this over potential is typically ±300 mV; however, it can be a volt

or more in some situations. As ECG circuits move to lower supply

voltages, the half-cell potential problem becomes more difficult,

strictly limiting the gain that can be applied in the first stage. The

AD8237 architecture provides a unique solution to this problem.

If the REF pin is left unconnected to the gain setting network, a

low frequency inverting integrator can be connected from the output

to the REF pin. Because the AD8237 applies gain to the integrator

output, the integrator only has to swing as far as the dc offset to

compensate for it, rather than the dc offset multiplied by the gain.

With this system architecture, large gains can be applied at the in-amp

stage, and the requirements of the rest of the system can be greatly

reduced. This also reduces noise and offset error contributions from

devices after the in-amp in the signal path. The circuit in Figure 77

illustrates the core concept. Additional op amps can be added for

improved performance, such as input buffering, filtering, and driven

lead, if it is required by the system. Proper decoupling is not shown.

INSTRUMENTATION

AMPLIFIER

G = +100

+5V

3.3μF

+5V

AD8607

REF

100kΩ

1kΩ

110kΩ

22nF

ECG OUT

100kΩ

PATIENT

PROTECTION

VMID

2MΩ

AD8237

47nF

Figure 77. AD8237 in ECG

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