ECG Analog FE. Heading in the right direction? - Page 1
Forum section: electronics / projects

author: Dmeads
author_rank: Regular Contributor
timestamp: May 25, 2024, 07:29:39 am
content: Hello all!After some cool work in my DSP class on ECG signals, I would like to work with my own on an arduino (real-time).I am starting with the analog front end, and wondering if my circuit so far looks okay?Im concerned about the modeling of the actual right and left arm signals. I would like to add 60 Hz noise as well as the electrode offset (which I think is opposite polarity with abs of 300mV?). Is this simulated correctly?I am using the AD8232https://www.analog.com/media/en/technical-documentation/data-sheets/ad8232.pdfMy LTspice schematic is zipped with the ad8232.cir file, and pictures are below.The output waveform shape looks great, however the gain is not what I expected. I am using the reference circuit from the AD8232 datasheet for cardiac monitoring (pg 28). Gain should be 1100, but I am not getting that. input waveform is max 2mV, getting 10mV out. The gain coming out of "IAOUT" looks correct @ 100, its the lowpass filter on the "OPAMP+" and "OPAMP-" terminals that seems to be messing it up.Additionally, is it correct to connect the right leg drive feedback to the input right arm and left arm signals to close the loop? I have never simulated with feedback other than normal opamp feedback loops.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 25, 2024, 10:36:46 am
content: RLD is right leg for a reason, it’s as far away from the measuring electrodes as you can get.This helps balance out the source impedances for the common mode signal seen at the measuring electodes.Note that RLD provcies some DC feedback to reduce the effects of the half cell potentials.It can be tricky to get right and poor phase margin will cause intsability.Use the circuits in the data sheet unmodified for best performance. You can buy the ready assembled module for $3 on alaiexpress.I have found that the back of the right shoulder was a good enough CM drive point.Like all bioelectrode amplifiers it wont work properly if it has a path to mains ground. Any path, even high impedance via a power supply or a scope or next signal chain item will pretty much guarantee poor or no performance. Even with RLD the preamp CMRR is never enough to deal with the high 50/60Hz fields encountered in moden environments.The options here are an all battery set-up or full isolation of the AFE ciruits. Icouplers and iso DC-DC stuff for lab work is cheap enough.Not in the small print of this particular devcie is this fact that is intended for use in portable systems without a connection the mains earth.

author: daisizhou
author_rank: Super Contributor
timestamp: May 25, 2024, 11:06:31 am
content: Do you have plans to build a patient simulator?

author: Dmeads
author_rank: Regular Contributor
timestamp: May 25, 2024, 06:43:46 pm
content: probably not, just this simple AFE. I am more focused on DSP, but want to make sure I have basic grasp of a single lead ecg front end.

author: Dmeads
author_rank: Regular Contributor
timestamp: May 25, 2024, 06:53:18 pm
content: Quote from: Terry Bites on May 25, 2024, 10:36:46 amgotcha thanks. I was going to use the 5V on the Arduino board but will opt for a battery instead. Looks like max +Vs is 3.5 anyway.Thanks for the pdfs. I can see how they modeled the electrodes which is helpful.

author: daisizhou
author_rank: Super Contributor
timestamp: May 25, 2024, 07:22:46 pm
content: Quote from: Dmeads on May 25, 2024, 07:29:39 amAs a digression, are you familiar with this DSP chip?