AD8235

Rev. 0 | Page 19 of 20

APPLICATIONS INFORMATION

AC-COUPLED INSTRUMENTATION AMPLIFIER

An integrator can be tied to the AD8235 in feedback to create a

high-pass filter, as shown in Figure 43. This circuit can be used

to reject dc voltages and offsets. At low frequencies, the impedance

of the capacitor, C, is high. Therefore, the gain of the integrator

is high. DC voltage at the output of the AD8235 is inverted and

gained by the integrator. The inverted signal is injected back into

the REF pin, nulling the output. In contrast, at high frequencies,

the integrator has low gain because the impedance of C is low.

Voltage changes at high frequencies are inverted but at a low

gain. The signal is injected into the REF pins, but it is not enough to

null the output. At very high frequencies, the capacitor appears as

a short. The op amp is at unity gain. High frequency signals are,

therefore, allowed to pass.

pass filtered input signal.

AD8235

AD8603

R

+IN

–IN

0.1µF

0.1µF

10µF

REF

C

1

2πRC

SDN

Figure 43. AC-Coupled Circuit

LOW POWER HEART RATE MONITOR

The low power and small size of the AD8235 make it an excellent

choice for heart rate monitors. As shown in Figure 44, the

AD8235 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 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 AD8235, 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 AD8235 can be tied to GND.

Note that this circuit is shown, solely, to demonstrate the capability

of the AD8235. 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

–2.5V

–2.5V

0.1µF

0.1µF

0.1µF

+2.5V

AD8609

AD8235

IN-AMP

AD8609

AD8609

AD8609

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

SDN

Figure 44. Example Low Power Heart Rate Monitor Schematic