Datasheet

AD8236
Rev. 0 | Page 19 of 20
LOW POWER HEART RATE MONITOR
The low power and small size of the AD8236 make it an
excellent choice for heart rate monitors. As shown in Figure 42,
the AD8236 measures the biopotential signals from the body.
It rejects common-mode signals and serves as the primary gain
stage set at G = 5. The 4.7 µF capacitor and the 100 k resistor
set the −3 dB cutoff of the high-pass filter that follows the
instrumentation amplifier. It rejects any differential dc offsets
that may develop from the half-cell overpotential of the electrode.
A secondary gain stage, set at G = 403, amplifies the ECG signal,
which is then sent into a second-order, low-pass, Bessel filter
with −3 dB cutoff at 48 Hz. The 324  resistor and 1 F capacitor
serve as an antialiasing filter. The 1 µF capacitor also serves as a
charge reservoir for the ADC’s switched capacitor input stage.
This circuit was designed and tested using the AD8609, low
power, quad op amp. The fourth op amp is configured as a Schmitt
trigger to indicate if the right arm or left arm electrodes fall off
the body. Used in conjunction with the 953 k resistors at the
inputs of the AD8236, the resistors pull the inputs apart when
the electrodes fall off the body. The Schmitt trigger sends an
active low signal to indicate a leads off condition.
The reference electrode (right leg) is set tied to ground. Likewise,
the shield of the electrode cable is also tied to ground. Some
portable heart rate monitors do not have a third electrode. In
such cases, the negative input of the AD8236 can be tied to GND.
Note that this circuit is shown, solely, to demonstrate the capability
of the AD8236. Additional effort must be made to ensure
compliance with medical safety guidelines.
+2.5V
+2.5V
–2.5V
+2.5V
–2.5V
–2.5V
–2.5V
0.1µF
0.1µF
0.1µF
+2.5V
AD8609
AD8236
IN-AMP
AD8609
AD8609
AD8609
+2.5
V
–
2.5
V
LEADS OFF DETECTION
INTERRUPT
LEADS OFF
402k
1k
1k
24.9k
324
1µF
220nF
4.7µF
100k
953k
RA LA
RL
953k
1k
20k
5k
680nF
4.02k
MCU + ADC
10-BIT ADC
08000-143
0.1µF
Figure 42. Example Low Power Heart Rate Monitor Schematic