Medical Design Briefs - March 2022 - 18

Electrocardiogram Monitor
Fig. 10 - GPIO settings.
This example uses slow tuning of the
instrumentation amplifier reference
point to cancel out op amp offset voltages.
Multiplied by a large gain factor,
these voltages will overload the amplifier
output without compensation. A slow
floating offset voltage presented by muscle
shaking, somatic tremor, or galvanic
contact of electrode metals to the skin
are all sources of interference that must
be canceled out. The entire circuit of the
comparator, reference voltage, potentiometers,
and a clock signal slowly adjust
an average DC level of the amplifier to a
reference voltage level, which is set to
VDD/2. The rate at which the voltage at
the potentiometer output is changing is
proportional to a clock frequency, fed
into a potentiometer block. Assuming
the potentiometer has 1023 steps, is connected
to a supply voltage of 5 V, and has
a clock of 170 Hz, the auto-trim control
loop will have a slew rate of 0.83 V/s. It
will also act as a high-pass filter with a
very low-frequency cutoff.
Fig. 11 - Seven second heart waveform (500 mV/div, 500 ms/div).
It is important to consider the error
presented by the constant auto-trim
process, but in most use cases, this is
negligible and can be tolerated. If it is
undesirable, it can be configured to be
switched off after a trimming period
using the internal SLG47004 logic. A reference
point of VDD/2 potential is fed
into the leg electrode, which also be -
comes present at both sides of capacitors
C3 and C4, preventing them from charging
(so you don't have to wait until the
output signal settles due to the input
capacitors' charging time).
Prototype Testing
Fig. 12 - Zoomed heart waveform (100 mV/div, 500 ms/div).
3
2.5
2
1.5
1
2×103
Fig. 13 - Raw digitized signal.
offset voltage. This will be amplified by a
factor of 910 and can produce up to 1 V
of output voltage offset and reduce
headroom for the signal, clipping it to
GND or VCC (keep in mind that VCC is
only 5 V).
18
Cov
To compensate for that error, an autotrim
circuit with a digital potentiometer is
used. This uses an analog comparator to
switch the direction of the digital po -
tentiometer change (up/down), which is
clocked by the oscillator.
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ToC
4×103
6×103
8×103
n
Data [n], V
Figures 11 and 12 show the waveform
at the analog output of the SLG47004.5
The raw analog output signal has a high
level (50 Hz) of noise. Such measurements
can't be analyzed, and this is a typical
problem for electrocardiography.
For this reason, additional filtering
should be applied to the signal.
The common approach is to apply
digital filters to remove power grid
noise as well as other noise components
2.2 Many articles describe and compare
different digital filtering approaches.2,3
Since the purpose of this discussion is
to show the performance of the analog
front-end, this article skips the digital
filter synthesis procedure and shows
the filtered signal. Figure 13 depicts the
raw digitized signal. At the first filtering
stage, 50 Hz noise was removed (see
Figure 14). At the second filtering
Medical Design Briefs, March 2022
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Medical Design Briefs - March 2022

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