AD8232ACPZ-R7 datasheet(26/29 Pages) AD

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Data Sheet

AD8232

Rev. B | 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.

+VS

+IN

–IN

HPDRIVE

+VS

HPSENSE

IAOUT

REFIN

GND

AC/DC

SDN

LO+

LO–

RLD

RLDFB

OUT

OPAMP+

OPAMP–

REFOUT

AD8232

10nF

1MΩ

100kΩ

1.5nF

180kΩ

1MΩ

10MΩ

0.1µF

1nF

10MΩ

0.1µF

360kΩ

0.33µF

REFOUT

10M

Ω 1.4MΩ

TO DIGITAL

INTERFACE

SIGNAL OUTPUT

10MΩ

0.01

FREQUENCY (Hz)

0.1

100

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