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 kΩ 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 Ω resistor and 1 μF 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 kΩ 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

+2.5V

–2.5V

LEADS OFF DETECTION

INTERRUPT

LEADS OFF

402kΩ

1kΩ

1kΩ

24.9kΩ

324Ω

1µF

220nF

4.7µF

100kΩ

953kΩ

RA

LA

RL

953kΩ

1kΩ

20kΩ

5kΩ

680nF

4.02kΩ

MCU + ADC

10-BIT ADC

0.1µF

Figure 42. Example Low Power Heart Rate Monitor Schematic