Datasheet

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
+5V
+5V
3.3μF
A B
C
+5V
AD8607
AD8607
REF
FB
100kΩ
1kΩ
110kΩ
22nF
10289-079
ECG OUT
100kΩ
100kΩ
PATIENT
PROTECTION
V
MID
V
MID
2MΩ
AD8237
47nF
Figure 77. AD8237 in ECG