Medical Design Briefs - March 2022 - 17

AC power interference is accepted by the
electrodes. Instrumen ta tion amplifiers
are good at rejecting common- mode signals,
and they also have high-impedance
inputs, which are needed for amplification
of the very small signal produced by
heart muscles in a noisy environment.
AC signals from arm electrodes go to
an RC high-pass filter (R3, R4, C3, and
C4 in Figure 4) with a low cutoff frequency,
which eliminates a DC offset
that might be present at the input due to
the electrochemical processes between
the electrode, conductive paste, and the
patient's skin.
Fig. 5 - Operational amplifier 0 and 1 settings.
Fig. 6 - Internal OpAmp settings.
The rheostat in this project is configured
as a digital potentiometer. Its configuration
is shown in Figure 8. The
configurations of oscillators 0 and 1 are
shown in Figure 9. The I2C macrocell
uses default configurations, and the
general-purpose input/output (GPIO)
pins 17, 18, 19, 10, and 11 have the
default configuration. The remaining
GPIO configurations are shown in
Figure 10.
Normal Operation Mode
Power-up pin 15 has a 100 kΩ pull-up
resistor, and if left floating, it will by
default power up the oscillator, all three
op amps, and the rheostat. When pin 15
is pulled down by external components,
all
internal blocks shut off, and the
SLG47004 goes into power-saving mode.
Electrodes on the skin and body have a
very high impedance, so common- mode
An even DC bias of VDD/2 is introduced
to both of the op amp inputs to
shift the operation point of the op amp,
preventing it from going beyond rails.
The voltage divider from R6, R7, and R5
(see Figure 4) forms an external negative
feedback loop for an instrumentation
amplifier from three op amps in the
SLG47004 and has a gain of 910.
The internal op amp has four resistors
with the same value, forming a differential
amplifier with a gain of 1. Op amps
have an input offset voltage. In this case,
the SLG47004 has a max 1000 µV input
Fig. 7 - HD buffer and OpAmp0 Vref settings.
Fig. 9 - Oscillator 0 and 1 settings.
Fig. 8 - Digital rheostat 0 and 1 settings.
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Medical Design Briefs - March 2022

Table of Contents for the Digital Edition of Medical Design Briefs - March 2022

Medical Design Briefs - March 2022 - Intro
Medical Design Briefs - March 2022 - Cov4
Medical Design Briefs - March 2022 - Cov1a
Medical Design Briefs - March 2022 - Cov1b
Medical Design Briefs - March 2022 - Cov1
Medical Design Briefs - March 2022 - Cov2
Medical Design Briefs - March 2022 - 1
Medical Design Briefs - March 2022 - 2
Medical Design Briefs - March 2022 - 3
Medical Design Briefs - March 2022 - 4
Medical Design Briefs - March 2022 - 5
Medical Design Briefs - March 2022 - 6
Medical Design Briefs - March 2022 - 7
Medical Design Briefs - March 2022 - 8
Medical Design Briefs - March 2022 - 9
Medical Design Briefs - March 2022 - 10
Medical Design Briefs - March 2022 - 11
Medical Design Briefs - March 2022 - 12
Medical Design Briefs - March 2022 - 13
Medical Design Briefs - March 2022 - 14
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Medical Design Briefs - March 2022 - Cov3
Medical Design Briefs - March 2022 - Cov4
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