Datasheet

Data Sheet AD8232
Rev. A | Page 25 of 28
The overall narrow-band nature of this filter combination
distorts the ECG waveform significantly. Therefore, it is only
suitable to determine the heart rate, and not to analyze the ECG
signal characteristics.
The low-pass filter stage also includes a gain of 11, to bring the
total system gain close to 1100 (note that the filter roll off
prevents the maximum gain from reaching this value). Because
the ECG signal is measured at the hands, it is weaker than when
measured closer to the heart.
The RLD circuit drives to the third electrode, which can also be
located at the hands, to cancel common-mode interference.
CARDIAC MONITOR CONFIGURATION
This configuration is designed for monitoring the shape of the
ECG waveform. It assumes that the patient remains relatively
still during the measurement, and therefore, motion artifacts
are less of an issue.
Figure 66. Circuit for ECG Waveform Monitoring
To obtain an ECG waveform with minimal distortion, the
AD8232 is configured with a 0.5 Hz two-pole high-pass filter
followed by a two-pole, 40 Hz, low-pass filter. A third electrode
is driven for optimum common-mode rejection.
Figure 67. Frequency Response of Cardiac Monitor Circuit
In addition to 40 Hz filtering, the op amp stage is configured for
a gain of 11, resulting in a total system gain of 1100. To
optimize the dynamic range of the system, the gain level is
adjustable, depending on the input signal amplitude (which
may vary with electrode placement) and ADC input range.
PORTABLE CARDIAC MONITOR WITH ELIMINA-
TION OF MOTION ARTIFACTS
The circuit in Figure 68 shows an implementation of a battery-
powered embedded system for monitoring heart rate in
applications where the patient engages in moderate activity,
such as with a Holter monitor. The AD8232 uses a three-
electrode patient interface and implements a two-pole high-
pass filter with a cutoff at 0.3 Hz, and a two-pole low-pass filter
with a cutoff frequency of 37 Hz. The total signal gain in the
pass band is 400. The fully conditioned signal is sampled by the
sigma-delta ADC integrated on the low power microcontroller,
ADuCM360. The wide dynamic range of this ADC provides
flexibility to reduce the signal gain to avoid saturation, depending
on electrode placement.
Because the pass band is relatively wide for ambulatory applica-
tions, the ADXL346 accelerometer signal can be used to further
minimize the noise introduced by the motion of the patient.
Moreover, the microcontroller can use the motion information
to monitor inactivity and to issue a system shutdown to save
battery power.
The low dropout regulator ensures that the maximum of 3 V is
not exceeded, especially during charge cycles of the battery,
which can be a lithium-ion cell.
In this application, the ADuCM360 uses its Port 0 to perform
DMA transfers to the host communication interface or to an
on-board memory, if recording the waveform for later transfer.
However, in any particular application, this port should be used
for the busiest interface to minimize CPU cycles and maintain
low power operation.
Note that this circuit is shown to demonstrate the capabilities of
AD8232 and other system components. It is not a complete
system design and additional effort must be made to ensure
compliance with medical safety guidelines from regulatory
agencies.
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
10nF
1MΩ
1MΩ
100kΩ
1.5nF
180kΩ
180kΩ
1MΩ
10MΩ
10MΩ 0.1µF
1nF
10MΩ
10MΩ
0.1µF
360kΩ
0.33µF
0.33µF
REFOUT
10MΩ
1.4MΩ
TO DIGITAL
INTERFACE
SIGNAL OUTPUT
10866-266
10MΩ
70
0
0.01 1k
MAGNITUDE (dB)
FREQUENCY (Hz)
10866-061
10
20
30
40
50
60
0.1 1 10 100