Datasheet

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.
When a signal exceeds f
HIGH-PASS
, the AD8235 outputs the high-
pass filtered input signal.
AD8235
AD8603
R
+
V
S
+IN
–IN
0.1µF
0.1µF
10µF
REF
C
+V
S
+V
S
V
REF
1
2RC
f
HIGH-PASS
=
08211-041
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 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 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 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.
+2.5
V
–
2.5
V
+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
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
0.1µF
SDN
08211-042
Figure 44. Example Low Power Heart Rate Monitor Schematic