Make With Mouser IoT Design Contest - Baby Vital Monitor_03 ECG

This blog entry is posted as a part of the "Make With Mouser IOT Design Contest".
This is my 3rd blog entry regarding the project "Baby Vital Monitor". I'll be covering some basics of single LEAD ECG and share my experience of acquiring ECG by constructing a simple Biopotential measurement circuit using Instrumentation amplifier and some basic analog filters. I conducted this experiment before receiving the ADPD4100 Evaluation Board. At the end of this blog, I'll share the configuration of ADPD 4100 to get single LEAD ECG. In another blog, I'll try to compare the output of two different approaches.

My previous blog entries for this project -
Make With Mouser IoT Design Contest - Baby Vital Monitor_01 Introduction
Make With Mouser IoT Design Contest - Baby Vital Monitor_02 SPO2 [PPG]

This blog has three parts -
  1. Background study on ECG leads
  2. constructing a Biopotential measuring circuit using instrumentation amplifier
  3. ADPD4100 configuration for ECG

ECG LEAD arrangement:

Einthoven’s Triangle-

LA = Left arm
RA = Right arm
LL = left leg


Conventional LEAD configuration-

Conventional configuration that is followed in wearable health tech is LEAD I, as it can be implemented in a chest belt. Where the RLD [right leg drive] is introduced as the alternative ground or reference path to cancel out the common noise to improve the SNR [signal to noise ratio] by improving the CMRR [common mode rejection ratio] of the bio amp.

One time / single use ECG electrodes from 'TOP TRACE' were used in experimental purpose. It has Ag-AgCl [silver-silver chloride] electrode with adhesive gel for skin-electrode coupling to provide good contact. Though Ag-AgCl has a little health issue for longer use, but it's only for experiment. I would like to go for carbon electrodes in my final design if it's possible.

For LEAD I configuration, the conventional LEAD arrangement produces an output of about 0.6 Vp-p. The resultant signal has 50 Hz [in my country] component contributed by the body [common mode signal] and the mains ground of the oscilloscope and hence the computer. In real life the 50 Hz component will be less as because the whole system will be driven by battery power supply and there will be less 50 Hz interference in an open playground. However, a digital filter can be implemented in the software end, but we'll lose information [like P wave will be flattened]. It's not a diagnosis purpose ECG and a very few amount of information can be found by only one LEAD. So, considering that the detailed wave shape is not much important here, we can implement digital filter if it's necessary later on.

Bioamp Circuit-
ecg bio amp basic 200.jpg

LEAD1 arrangement1.jpg

Figure: LEAD 1 captured from arm with commercial clip electrodes

LEAD1 arrangement2.jpg

Figure: LEAD 1 captured from chest region using Ag-AgCl one time electrode

For chest arrangement, the electrodes are placed too closed to RLD or the reference of Einthoven's triangle, which results an ECG output with slightly different shape for T wave and comparatively reduced QRS complex. However, the overall peak to peak voltage is increased from 0.6 Vp-p to about 2 Vp-p for chest arrangement.
the output seems to have high 50 Hz component. If we look into the spectrum view -
LEAD2 arrangement3 50hz a.jpg

AD8232 as Bioamp -
I have tried to compare the output signal of a single LEAD ECG captured by AD8232, a single chip ECG Bioamp designed by Analog Devices Inc.
20150315_231758_Tv Tower Rd.jpg

Output -
non isolated.jpg

Figure: noisy output as I have not used the opto-isolator

no FR.jpg

Figure: output using opto-isolator, which inverts the signal

ADPD4100 Configuration -
ECG configuration.png

source: Multiparameter Vital Signs Monitoring Is Easier Than Ever Before by Electronics Maker November 21, 2020

"The ADPD4100/ADPD4101 takes a novel approach by using a pas sive resistor-capacitor (RC) circuit to follow the differential voltage across a pair of electrodes. The passive RC circuit can be as simple as three components, two resistors RS and a capacitor CS, as shown in Figure 3a. It is a two-step process for each sample of the ECG data.

The two input pins (IN7 and IN8) float during the charging step. The charge on the capacitor CS is proportional to the differential voltage across the two electrodes if the charging time is >3τ where τ is the time constant defined by the RS and CS, τ=2RSCS. During the charge transferring step, the capacitor is connected to the TIA and the charge is transferred to the AFE for measurement. This charge measurement-based ECG solution offers several advantages, including eliminating the buffers and the third electrode for RLD, shrinking the system size through fewer external components, and saving power.

It is convenient to add lead-off detection to this ECG solution with the design flexibility of the ADPD4100/ADPD4101 using a bioimpedance-based approach. "

I'll make another blog entry to compare the output of ADPD4100 with the traditional approach.
My next blog entry will be on Impedance measurement. I'll share my experience of designing a tetra polar bioimpedance system to get the skin impedance to predict dehydration. That blog will cover the potential of using ADPD4100 for both dehydration and respiration measurement.

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