Pls simulate this active electrode using Ltspice or other simulators - Page 1
Forum section: electronics / general

author: loop123
author_rank: Frequent Contributor
timestamp: March 03, 2024, 05:57:58 am
content: Hi can you pls simulate this active electrode circuit in Ltspice or other simulators? I dont know how and cant risk experiment with output that is unknown.https://www.olimex.com/Products/EEG/Electrodes/EEG-AE/open-source-hardwaresee clearer schematic aboveThe output plug is GndA, Chx_in and 5V. It is supposed to plug to their EEG-SMT but if the purpose is to get 5V to power it then get the Chx-in data to further amplify in the SMT amp. Then we can easily build adapter to power it with 5V and acquire the data to input any amplifier?I just want to confirm. It has gain of 1 and no further amplification? Its only purpose is impedance converter to create high impedance input and low impedance output? So if I input 10microVolt or 1 milliVolt. It will also output same 10uV and 1 mV?Thank you.

author: ataradov
author_rank: Super Contributor
timestamp: March 03, 2024, 06:06:43 am
content: This is a very basic opamp amplifier in a unity gain configuration. It is just a buffer, there is no need to simulate it.But if you are afraid that it will fry some sensitive equipment if gain is higher, then you absolutely need to do measurements on the real hardware before you plug it into anything. Just one poorly soldered pin and it will output full 5V.

author: loop123
author_rank: Frequent Contributor
timestamp: March 03, 2024, 06:32:13 am
content: Quote from: ataradov on March 03, 2024, 06:06:43 amWhat do you mean in second paragraph it can fry some sensitive equipment if gain is higher. I thought you said it is a buffer which means it has gain of 1. Also the output is always 5V? If gain is higher. It cant increase voltage since its powered by maximum 5V already so how can it fry other equipments?

author: Someone
author_rank: Super Contributor
timestamp: March 03, 2024, 06:36:11 am
content: Posters might want to check the OP's topic history, its all pretty much on the evolution of one (questionable) project.

author: ataradov
author_rank: Super Contributor
timestamp: March 03, 2024, 06:36:44 am
content: The schematic and practical implementation are two different things. The schematic is a unity gain buffer. But if in practice you forget to solder pin 2 or route something incorrectly, then it will output whatever voltage (up to 5V, of course). This is why you test stuff after assembly with test equipment before using in a real system.I don't know how sensitive your equipment is. A lot of stuff can be damaged by 5V, especially if it only expects 10 mV.

author: loop123
author_rank: Frequent Contributor
timestamp: March 03, 2024, 07:09:13 am
content: Thanks Id keep the 5V output in mind.I saw the specsheet of the TL062CD used in that.https://www.mouser.com/ProductDetail/Texas-Instruments/TL062CD?qs=LfG3tU9ud8Dn8hoZrJ7g3A%3D%3DIt has input offset voltage of 3mV when the input is supposed to be EEG range of just 10microVolt. How would it affect it?Also the noise is about 50nV/sqrt (Hz) so if the bandwidth is 100 Hz. The noise in rms is already 500nV or 0.5microVolt.So even in unity gain 1 mode. The noise of 50nV/sqrt(Hz) is valid so for active amplifiers. The amplifier chosen is still crucial even if the gain is only 1? So the olimex active electrode is just bad, isnt it.

author: ataradov
author_rank: Super Contributor
timestamp: March 03, 2024, 07:14:50 am
content: You are looking at some amateur garbage circuits. Why do you expect good performance from them?

author: loop123
author_rank: Frequent Contributor
timestamp: March 03, 2024, 07:40:22 am
content: There is a good active electrode at $159 im looking at but it needs $1600 driver box (g.Gammabox). Their electrode uses only 2 wire. They put the signal on the dc wire as described below. What is the scheme called? And how easy it is to build circuit to filter signal from dc output so I can build a custom box to run their active amplifiers?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2024, 09:36:05 pm
content: The TL062 is a rather noisy low cost, low power op-amp. There are many alternatives, though low noise often comes with a high supply current. A still not very expensive option could be an OPA377. If one cares more about the low frequency part also the OPA202 could be an option. If cost is less an issue, consider the OPA145.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 03, 2024, 10:22:40 pm
content: And beyond the performance of the opamp, I would draw attention to safety, if this is meant to be used on humans, but as someone else pointed out already, I think this discussion with the OP has been going on for a long time already, so pretty sure we mentioned it several times.Power supplies and everything connected to it must be fully isolated (and be aware obviously that isolation is very easy to break just by connecting say one output of your devices to a scope the ground of which will be connected to earth.Even if isolation is guaranted, since the electrode side is not AC-coupled and the circuit is on a single supply, you will also probably have a DC voltage between the electrode and its return path, which we don't see here. What's the return electrode connected to? If this is to your circuit's ground, that's not great.

author: loop123
author_rank: Frequent Contributor
timestamp: March 03, 2024, 10:38:04 pm
content: QuoteExpensive is no problem, I mean it's not we are talking about a $1000 Op-amp but perhaps $50 vs $5. So $50 is fine. So what is best highest performing Op-amp in the world with frequency from 0 to 20,000Hz with noise of 5nV/sqrt (Hz) or lower. If say in rare chance the pin is similiar to the Olimex active electrode shown in original message. Can I just replace the TL062 chip with it and every other components the same or does one have to create a new circuit from scratch? It is difficult to miniaturize it.My units are fully battery powered. And even have resistor and DC-converter protection to limit all DC to mere 60mA, it has full fledge isolator too but people just keep convincing me 9V can't electrocute you and the sailor story is a myth. So I may not use the isolator because it contributes to a lot of noises that makes microvolt resolution too noisy.QuoteWhat do you mean there is DC voltage between the electrode and its return path? The DC power will pass through the leads. And it will be connected to ground and 5V in the main amplifier. And what has it got to do with not being AC-coupled. Kindly explain more. Thanks.