Medical Applications User Guide (pdf) - Freescale Semiconductor

10.4
Filtering ECG
The ECG has three common noise sources:
• Baseline wander
• Power line interference
• Muscle noise
The baseline wander is caused by electrode
impedance, respiration, body movements and
low- and high-frequency noise. This makes
it necessary to use a band-pass filter as
described in Chapter 5, Heart Rate Monitor. To
eliminate the low-frequency noise, a high-pass
filter with a cut-off frequency of 0.67 Hz is used,
because this corresponds to the slowest heart
rate of around 40 beats per minute. However,
because this is not an absolute data point, it
is better to use a cut-off frequency of 0.5 Hz.
Figure 10-5 shows a basic implementation
circuit that detects the electrical currents
through the electrodes.
10.5
Electrodes Interface
The amplitude of the signals detected by
the electrodes is too small. The signals are
connected to operational amplifier inputs
through series limiter resistors (typically 100K),
and amplified a little. The feedback network
helps to stabilize the system at the beginning
of the capture time, reducing fluctuations.
Finally, the signal is sent to an active low-pass
filter. The filter eliminates the high-frequency
noise which might be induced by the AC line.
Other noise sources such as respiration and
muscular movement (low-frequency noise) are
filtered using a high-pass filter. These noise
sources require a band-pass filter and not just
a low-pass filter.
AN4323: Freescale’s Solutions for
Electrocardiograph and Heart Rate
Monitor Applications
This application note describes how to use
the MED-EKG development board, a highly
efficient board that can be connected to
the Freescale Tower System to obtain an
electrocardiogram signal and measure
heart rate.
Figure 10-4: Einthoven Triangle
Figure 9-4: Einthoven Triangle
Right
Arm
Diagnostic and Therapy Devices
freescale .com/medical 57
–
-
Y
II III
Figure 10-5: Electrodes Connection Circuit and Signal Conditioning
Figure 9-5: Electrodes Connection Circuit and Signal Conditioning
Right Hand
Right Leg Left Leg
Left Hand
+
Analog
Frond End
Electrodes
Multiplexer
and Isolator
Figure 10-6: ECG Analog Front End
Figure 9-6: ECG Analog Front End
Left
Electrode
Right
Electrode
100K
100K
Differential Amplifier
+
Left
Leg
I
+
Instrumentation
Amplifier
Feedback Network
–
Left
Arm
X
Band Pass
Filter
Filter Network
To MCU
ADC input
Output