AD8232ACPZ Datasheet, PDF (24/28 Pages) Analog Devices – Single-Lead, Heart Rate Monitor Front End

AD8232
Data Sheet
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.
0.22µF
ELECTRODE
INTERFACE
10MΩ
10MΩ
180kΩ
180kΩ
HPDRIVE
+IN
–IN
HPSENSE
IAOUT
REFIN
RLDFB
+V_S
1nF
RLD
GND
AD8232
SW
FR
10MΩ
+V_S
10MΩ
0.1µF
0.1µF
10MΩ
OPAMP+
REFOUT
OPAMP–
OUT
AC/DC
SDN
LO+
LO–
+V_S
TO DIGITAL
INTERFACE
SIGNAL
OUTPUT
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.
70
60
50
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.
+V_S
0.22µF
10MΩ
LA
10MΩ _180kΩ
180kΩ
RA
0.22µF
360kΩ
1nF
RL
1MΩ
1MΩ
100kΩ 22nF
100kΩ
3.3nF
1MΩ
HPDRIVE
HPSENSE
+IN
IAOUT
–IN
RLDFB
REFIN
+V_S
RLD
GND
AD8232
SW
FR
OPAMP+
AC/DC
REFOUT
SDN
OPAMP–
LO+
OUT
LO–
10MΩ
+V_S
10MΩ
0.1µF
0.1µF
10MΩ
+V_S
TO DIGITAL
INTERFACE
SIGNAL OUTPUT
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.
70
60
40
50
30
40
20
30
10
20
0
0.1
1
10
100
1k
10k
FREQUENCY (Hz)
Figure 63. Frequency Response for HRM Next to Heart Circuit
10
0
0.1
1
10
100
1k
FREQUENCY (Hz)
Figure 65. Frequency Response for HRM Circuit Taken at the Hands
Rev. A | Page 24 of 28