Instrumentation Amplifier Issue - Page 1
Forum section: electronics / beginners

author: CrazzzyEE
author_rank: Newbie
timestamp: January 14, 2021, 01:37:49 pm
content: Hello All! My name is Alex and I am student at Royal Holloway university of London.I am making an ECG circuit for fun, but recently it became less and less fun. I have designed a full circuit in EasyEDA --https://oshwlab.com/lobkoalex/ecgpcb_copy/. The circuit doesn't work. I have tested each part individualy, coming to the conclusion. The In-Amp is not working, it does not amplify the ecg signals.At this point I was a bit confused. The whole thing did work during the lab work and I have proves(I can not show). Ive asked my lecturer to help. After weeks of work, new design was made. EasyEDA --https://oshwlab.com/lobkoalex/ecg_circuit_v3/.This new design aimed to solve couple problems. Notch Filter and HP LP filters were changed for the Second Order Band Stop Multisim --https://www.multisim.com/content/gCdGvN9NHDfZiuPdmrebLh/2nd-order-band-pass-filter/open/. This circuit, in theory should have better noise filtering qualities. This circuit didn't work. The new Band Stop filter works perfect when tested with function generator separate from the In Amp.Now is the point when I am confused as a mammoth. I have no idea why the instrumentation amplifier IC won't work. I tested 3 different ones. I have built the In Amp circuit on the breadboard Multisim --https://www.multisim.com/content/ZbWMaH8bJVRbFXdxiWqkN6/inamp-circuit/open/. And it didn't amplify my ECG signals. I take signals from my arms: right arm, right arm ref, left arm. And I have tested other people signals as well. Now I am completely stuck in the middle of this fun project.The In Amp just might be to complex for me. Is it possible I just dont know how to connect it properly?I would be forever grateful for any help or advice.Links in text:Original Circuit Design --https://oshwlab.com/lobkoalex/ecgpcb_copy/New Version --https://oshwlab.com/lobkoalex/ecg_circuit_v3/BandStop Filter --https://www.multisim.com/content/gCdGvN9NHDfZiuPdmrebLh/2nd-order-band-pass-filter/open/In Amp using Op Amps --https://www.multisim.com/content/ZbWMaH8bJVRbFXdxiWqkN6/inamp-circuit/open/

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2021, 05:25:21 pm
content: A very important rule with op-amps is both inputs need to have a DC connection to a power supply rail: normally 0V. The inputs to the instrumentation amplifier are floating, which is causing the common mode range to be exceeded. Connect them to 0V, via a high value resistor (say 10M) in the simulation. In the real circuit, connect the person to 0V, either directly, or via a high value resistor.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 14, 2021, 06:00:26 pm
content: Which bit of stuff you've posted is the actual circuit diagram your are having trouble?with Getting clean ECG signal is not trivial. You can buy ready made modules e.g.https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2047675.m570.l1313&_nkw=ECG+module+AD8232+&_sacat=0see the AD8232 datasheetBut if you need to DIY don't try to build an instrumentation amp out of parts- Just buy one. It will massively outperform your home-brew and probably cost a lot less. An AD623 as shown, for instance can be had for less than a fiver.Points to note. The gain of the instrumentation amplifier needs to be low- say about 5x. This is because the electrodes can generate >100mV DC volts from your sweat which will swamp your mV of ECG. DC overload and recovery is a bugger in these systems. At a gain of 100 the output would saturate. This needs to be AC coupled to the next gain stage (100x) and your filter chain. Aim at an over all gain of 1000 or so.One of the main reasons DIY ECG fails (even when the circuit is correct) is connecting the output to a mains operated Scope or PC. Referencing to mains ground will stop the ECG amp from working properly. All battery systems work best. Stay safe.This schematic illustrates the usual topology.Still, at about 5 quid the AD8232 is better solution!

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2021, 08:46:20 am
content: If DC overload is an issue, then why not AC couple the inputs to the instrumentation amplifier?The circuit you've attached, has the same issue as the original poster's: the inputs are floating and not biased to 0V. Refer to page 27 of the AD623 data sheet: "Ground Returns for Input Bias Currents"https://www.analog.com/media/en/technical-documentation/data-sheets/ad623.pdf

author: CrazzzyEE
author_rank: Newbie
timestamp: January 15, 2021, 10:17:31 am
content: I did built the In Amp form the Op Amps just to wast time, not going to use it.The actual circuit I have right now ishttps://easyeda.com/lobkoalex/ecg_circuit_v3It has one problem. The In Amp doesn't amplify the signal. The Band Pass works fine with the function generator but as soon as its connected to the In Amp the circuit breakes. So Ive checked the In Amp individually from the filter. The IC just doesn't amplify.The actual electrode placement is demonstrated belowAlso my In Amp circuit on is connected as shown on the picture belowThe output goes to the filter, on the picture its HP LP with the Op Amp, but in my actual circuit it the Band Pass Filter. The gain is set to around 300 with 330 ohm resistor. Is it too high?

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2021, 11:02:44 am
content: Quote from: CrazzzyEE on January 15, 2021, 10:17:31 amThat's better. Connecting the 0V to another electrode, provides a path for the bias currents.What do you mean by the instrumentation amplifier doesn't amplify the signal? Does its output sit at 0V, or close to either supply rail? Have you tried connecting one input to 0V and the other to a signal generator?If the instrumentation amplifier's output is sitting near either supply rail, then it's most likely due to it being overloaded by the voltage generated from sweat, on the electrodes, as mentioned above. You could try AC coupling the input. Connect the inputs to 0V, via high value resistors, say 10M, and connect them to the electrodes via 470nF capacitors. This will block the DC signal, but it may take 30 seconds or so after connecting the electrodes for the voltages across the capacitors to stabilise.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 15, 2021, 06:14:05 pm
content: Is that really a 330 ohm resistor I can see? That will give a gain of about 300. Your electrodes form little batteries where the contact your skin. A 100mv would become 30V! 20k is nearer the mark.https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwih3pC0xJ7uAhW7VRUIHSlRCZEQFjAAegQIBBAC&url=https%3A%2F%2Fwww.analog.com%2Fmedia%2Fen%2Ftechnical-documentation%2Ftechnical-articles%2FECG-EEG-EMG_FINAL.pdf&usg=AOvVaw2yiIQ-3sCrdS7P1JvGeaOL

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 01:12:21 pm
content: How about this coupling that straightens out your DC from the get go without degrading your CMRR like a a simple RC coupling will. See attached. give it a simulation. Works great even with low tolerance parts. Without a DC component you can be more risqué with your front end gain.seehttps://www.researchgate.net/figure/Fully-differential-passive-ac-coupling-network-built-by-mirroring-two-single-ended_fig4_224320850full credit to these guys.BTW I'm retired having spent 40 years designing medical equipment.