Sensing EMG Signal - Page 1
Forum section: electronics / projects

author: Wob_Bidlar
author_rank: Newbie
timestamp: September 01, 2016, 08:17:18 pm
content: HiI wanted to build my own little PCB for sensing EMG-Signals...I actually wanted to do it without looking it up on the Internet, but now i ran into some problems.Attached you find the Schematic of my design.The Problem is, that I just occasionally get a good reading of the muscle-contraction. 90% of the time i either get nothing, or the amplitude of the signal falls and rises on its own without me doing anything. (Measuring right after the INA333...I haven't checked the rest of the circuit so far!!)Is there a flaw in my design (I think thats very much the case)??My initial thought was, that I use the wrong Electrodes (Link to my Electrodes is down below) OR that I need proper shielding (I don't have any)...but that won't explain the flawless functioning which sometimes happens.https://www.amazon.com/Premium-Electrode-Reusable-Carbon-Electrodes/dp/B00VTVWJOE/ref=sr_1_4_a_it?ie=UTF8&qid=1472759787&sr=8-4&keywords=electrodes

author: Andy Watson
author_rank: Super Contributor
timestamp: September 01, 2016, 08:26:38 pm
content: You need to provide a path(s) for the input bias current of the INA333 . See section 8.2.2.5 of the datasheet.

author: Wob_Bidlar
author_rank: Newbie
timestamp: September 01, 2016, 08:35:01 pm
content: OK...I totally overlooked that.If that solves the Problem I owe you a big one.Thank you very much!!!

author: Audioguru
author_rank: Super Contributor
timestamp: September 01, 2016, 11:47:36 pm
content: The input pins of your INA333 are floating without a DC reference voltage of 0V so the INA333 will not work properly. An ECG circuit is similar and usesDC coupling(no input coupling capacitors) from the patient, and uses shielded cables to the sensors, and the patient is biased at a DC reference of 0V with inverted common mode AC and DC signals to cancel any interference picked up. Here is a typical ECG circuit:

author: Wob_Bidlar
author_rank: Newbie
timestamp: September 25, 2016, 06:51:13 pm
content: Hi All!Sorry for the delay!Quote from: Andy Watson on September 01, 2016, 08:26:38 pmI've put a 280k resistor going to ground on each input...AND it worked great right away. Thank you very much Andy Watson!!!I attache a Screenshot of the Output-Signal on my Oscilloscope (Input amplified vs Output)!!!Second, I wanted to try the Method Audioguru was mentioning...BUT I really didn't see a reason for that. My circuit was working really good, even WITHOUT a Reference-Electrode. So basically I have Sense- and Sense+...do I miss something here?Do I really need the Reference-Electrode???Updates on the Schematic:* Bias resistors for the INA333...tested* Active High-Pass filter right after the INA333...design phase* Higher gain...design phase* Changed Diodes to Schottkys...design phase* Passive Low-Pass filter right after the rectifier...design phase* Clamping-Diodes at the rectifier and the last amplifier stage...design phase* BNC-Connectors for the Electrodes (Shielded Cables)...design phase* Changed OpAmps...design phase* Added Arduino and Connectors for LCD and Buttons...design phase

author: trophosphere
author_rank: Frequent Contributor
timestamp: September 25, 2016, 08:07:24 pm
content: You don't absolutely need a reference electrode since you are already feeding a reference voltage into your inputs via grounding your inputs through those 280k resistors. The reference electrode arrangement given by Audioguru is used to feed a scaled and inverted version of the common-mode voltage from the instrumentation amplifier. This is done in order to further suppress the common-mode signal between the inputs.

author: Audioguru
author_rank: Super Contributor
timestamp: September 26, 2016, 01:41:53 am
content: The common-mode interference is mains hum picked up by the ungrounded patient. There is also DC common mode interference from the patient's muscles.

author: Wob_Bidlar
author_rank: Newbie
timestamp: November 30, 2017, 08:06:38 pm
content: Hi allThanks for all your help!!!!!!!!!!!!!!!!!!!Finished the project almost a year ago...here is the result.Works like a treat, btw...