Datasheet

AD8232 Data Sheet
Rev. A | Page 24 of 28
APPLICATION CIRCUITS
HEART RATE MEASUREMENT NEXT TO THE HEART
For wearable exercise devices, the AD8232 is typically placed in
a pod near the heart. The two sense electrodes are placed under-
neath the pectoral muscles; no driven electrode is used. Because
the distance from the heart to the AD8232 is small, the heart
signal is strong and there is less muscle artifact interference.
In this configuration, space is at a premium. By using as few
external components as possible, the circuit in Figure 62 is
optimized for size.
Figure 62. Circuit for Heart Rate Measurement Next to Heart
A shorter distance from the AD8232 to the heart makes this
application less vulnerable to common-mode interference.
However, since RLD is not used to drive an electrode, it can be
used to improve the common-mode rejection by maintaining
the midscale voltage through the 10 MΩ bias resistors.
A single-pole high-pass filter is set at 7 Hz, and there is no low-
pass filter. No gain is used on the output op amp thereby
reducing the number of resistors for a total system gain of 100.
Figure 63. Frequency Response for HRM Next to Heart Circuit
The input terminals in this configuration use two 180 kΩ
resistors, to protect the user from fault conditions. Two 10 MΩ
resistors provide input bias. Use higher values for electrodes
with high output impedance, such as cloth electrodes.
The schematic also shows two 10 MΩ resistors to set the
midscale reference voltage. If there is already a reference voltage
available, it can be driven into the REFIN input to eliminate
these two 10 MΩ resistors.
EXERCISE APPLICATION: HEART RATE MEASURED
AT THE HANDS
In this application, the heart rate signal is measured at the
hands with stainless steel electrodes. The user’s arm and upper
body movement create large motion artifacts and the long lead
length makes the system susceptible to common-mode inter-
ference. A very narrow band-pass characteristic is required to
separate the heart signal from the interferers.
Figure 64. Circuit for Heart Rate Measurement at Hands
The circuit in Figure 64 uses a two-pole high-pass filter set at
7 Hz. A two-pole low-pass filter at 24 Hz follows the high-pass
filters to eliminate any other artifacts and line noise.
Figure 65. Frequency Response for HRM Circuit Taken at the Hands
10866-161
+V
S
+V
S
+IN
–IN
HPDRIVE
+V
S
HPSENSE
IAOUT
REFIN
GND
FR
AC/DC
SDN
LO+
LO–
RLD
RLDFB
OUT
OPAMP+
OPAMP–
REFOUT
SW
AD8232
180kΩ
180kΩ
10MΩ
10MΩ 0.1µF
10MΩ
1nF
10MΩ
ELECTRODE
INTERFACE
0.1µF
0.22µF
TO DIGITAL
INTERFACE
SIGNAL
OUTPUT
10MΩ
70
0
0.1 10k
MAGNITUDE (dB)
FREQUENCY (Hz)
10866-057
10
20
30
40
50
60
1 10 100 1k
10866-262
RL
RA
LA
+V
S
+V
S
+V
S
+IN
–IN
HPDRIVE
+V
S
HPSENSE
IAOUT
REFIN
GND
FR
AC/DC
SDN
LO+
LO–
RLD
RLDFB
OUT
OPAMP+
OPAMP–
REFOUT
SW
AD8232
22nF
1MΩ
1MΩ
100kΩ
3.3nF
100kΩ
180kΩ
180kΩ
1MΩ
10MΩ
10MΩ 0.1µF
10MΩ
1nF
10MΩ
10MΩ
0.1µF
360kΩ
0.22µF
TO DIGITAL
INTERFACE
SIGNAL OUTPUT
0.22µF
70
0
0.1 1k
MAGNITUDE (dB)
FREQUENCY (Hz)
10866-059
10
20
30
40
50
60
1 10 100