Instrumentation Amplifier Modification or Replacement

[loop123]

Ok. I'll get an active electrode then. But I'm being quoted for $2000 just for one electrode with the 9V supply box.

Can you guys teach me how to create an active electrode.

Where do we begin? What chip to use? Can it be bigger like an INA114 so imagine your electrode look like  a matchbox.. or should it be miniaturized chip? What is commonly used?

[Andy Chee]

Simple active electrode here

https://www.olimex.com/Products/EEG/Electrodes/EEG-AE/open-source-hardware

I haven't analysed whether it's compatible with your existing bioamplifier.

[loop123]

unless by means of EMI? Do you have illustrations of waveforms of common causes of the noises? They are easy to recognize. Even just running a sine wave generator will make them show  up, Isn't it?

No.  Sometimes EMI has a known pattern.  But EMI can equally be random.

If you don't want to use a faraday cage, then you should analyse the surrounding EMI in your intended operating environment, so that either;

a) design the hardware to cope with the environment, or
b) program the EEG/EMG software with your EMI analysis, and the software will do the noise reduction for you.

https://eeglab.org/tutorials/ConceptsGuide/Setting_up_your_EEG_lab.html

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312138/pdf/june-17-10.pdf

Say. Is Mathcad compatible with direct output like you display in Audacity? or can it only accept proprietary files? can you use Mathcad on direct output to filter certain artifacts or interferences?

Besides Mathcad or maybe EEGlab. What is the most advanced post processing software that can remove for example ECG artifacts when you are doing EMG etc?

[Andy Chee]

Say. Is Mathcad compatible with direct output like you display in Audacity? or can it only accept proprietary files? can you use Mathcad on direct output to filter certain artifacts or interferences?

I don't use Mathcad, so I don't know for sure.  But if it doesn't have a compatibility layer for importing generic file formats, you could probably use Python to program something to translate the file format.

Besides Mathcad or maybe EEGlab. What is the most advanced post processing software that can remove for example ECG artifacts when you are doing EMG etc?

Hard to say.  Normally a researcher cannot afford to spend money on multiple software to compare features.  Normally a researcher learns to use whatever the university or hospital is using.

Given that you are asking about amplifier repair, I am guessing you don't want to spend money on software either.  In which case, I highly recommend open-source projects such as EEGlab.

[loop123]

unless by means of EMI? Do you have illustrations of waveforms of common causes of the noises? They are easy to recognize. Even just running a sine wave generator will make them show  up, Isn't it?

No.  Sometimes EMI has a known pattern.  But EMI can equally be random.

Just to clarity. I asked if just running a sine wave generator will make the interferences show up. And you said "No.  Sometimes EMI has a known pattern.  But EMI can equally be random". But if you run the sine wave for 1 hour and no interference, then chances are there is just no interference or not constant enough to disturb your output that you get for only 2 minutes, right?

Simple active electrode here

https://www.olimex.com/Products/EEG/Electrodes/EEG-AE/open-source-hardware

I haven't analysed whether it's compatible with your existing bioamplifier.

Are you saying that active electrodes can only be made by a company who has to miniaturize the chips and they can't be built DIY like us? Why can't you use for example an INA114 and put it in a small matchbox and place it on your skin directly with electrode cup glued to the matchbox and not using any cable before the preamp?

[Andy Chee]

unless by means of EMI? Do you have illustrations of waveforms of common causes of the noises? They are easy to recognize. Even just running a sine wave generator will make them show  up, Isn't it?

No.  Sometimes EMI has a known pattern.  But EMI can equally be random.

Just to clarity. I asked if just running a sine wave generator will make the interferences show up. And you said "No.  Sometimes EMI has a known pattern.  But EMI can equally be random". But if you run the sine wave for 1 hour and no interference, then chances are there is just no interference or not constant enough to disturb your output that you get for only 2 minutes, right?

No, that's incorrect.  A sinewave only contains one single frequency.  EMI can potentially contain many frequencies, all at the same time, and different amplitudes.

Simple active electrode here

https://www.olimex.com/Products/EEG/Electrodes/EEG-AE/open-source-hardware

I haven't analysed whether it's compatible with your existing bioamplifier.

Are you saying that active electrodes can only be made by a company who has to miniaturize the chips and they can't be built DIY like us? Why can't you use for example an INA114 and put it in a small matchbox and place it on your skin directly with electrode cup glued to the matchbox and not using any cable before the preamp?

No, I am not saying that at all.  Yes, you could put INA114 directly on the skin to make your own active electrode. 

Remember though, due to the heavy weight of large active probes, they tend to wobble and move around on the skin, so you may need to use a strap or something to hold it onto the head/face or whatever you're measuring.  A wobbling electrode will create a noisy signal.

[loop123]

unless by means of EMI? Do you have illustrations of waveforms of common causes of the noises? They are easy to recognize. Even just running a sine wave generator will make them show  up, Isn't it?

No.  Sometimes EMI has a known pattern.  But EMI can equally be random.

Just to clarity. I asked if just running a sine wave generator will make the interferences show up. And you said "No.  Sometimes EMI has a known pattern.  But EMI can equally be random". But if you run the sine wave for 1 hour and no interference, then chances are there is just no interference or not constant enough to disturb your output that you get for only 2 minutes, right?

No, that's incorrect.  A sinewave only contains one single frequency.  EMI can potentially contain many frequencies, all at the same time, and different amplitudes.

If that's the case. How come the output doesn't show this "potentially contain many frequencies, all at the same time, and different amplitudes."?  The output is all sine wave single frequency. The following is different bandwidth selected (main unit uses 2 pole Butteworth filters), and it's all uniform sine wave even for 20 minutes and all day.

10 microvolt input, 90Hz signal, 50000 gain, 100Hz bandwidth selected:



10 microvolt input, 90Hz signal, 50000 gain, 1000Hz bandwidth selected:



10 microvolt input, 90Hz signal, 50000 gain, 3000Hz bandwidth selected:

[Kleinstein]

The actual EMI background can vary from location to location.  The cableing and local configuration can effect which frequencies are picked up strong. Also the circuit can be more sensitive to some frequencies and less to others.  So while there is a mix of different frequencies, one may well end up with 1 frequency to dominate. However this can also change over time. It can also as well look like noise or one can get a radio program.

A uniform sine like background could indicate one specific source of interference. One may be able to identify and eliminate it.
In the old times classic such frequencies where the CRT horizontal frequencies. Today SMPS are common sources. Even if the testinstrument is battery powered it likely includes some SMPS.

A shielded passive probe may not be that different when it comes to sensitivity to EMI. In a difficult environment both version will struggle and in a quite environment both should work OK.  The extra weight at the electrodes could also be a real hassel and limited size may mean less EMI filtering than at a good central amplifier.

[Andy Chee]

If that's the case. How come the output doesn't show this "potentially contain many frequencies, all at the same time, and different amplitudes."?  The output is all sine wave single frequency. The following is different bandwidth selected (main unit uses 2 pole Butteworth filters), and it's all uniform sine wave even for 20 minutes and all day.

I can guarantee that if you took your equipment down to the train station (electric train) and measure the EMI again, you will definitely get something different!

[loop123]

If that's the case. How come the output doesn't show this "potentially contain many frequencies, all at the same time, and different amplitudes."?  The output is all sine wave single frequency. The following is different bandwidth selected (main unit uses 2 pole Butteworth filters), and it's all uniform sine wave even for 20 minutes and all day.

I can guarantee that if you took your equipment down to the train station (electric train) and measure the EMI again, you will definitely get something different!

The 60Hz AC noises are only present if the main amp is not connected to any source (like a sine wave generator). See below. But if it connected to any source, there is no interference. So how do you characterize (or compute) the strength of interference before the sine wave input can be affected? I guess the 10 microvolt sine wave generator has same strength as the skin 10 microvolt, isn't it.. since they are 10 microvolt.

The following is when no leads were connected. I didn't disconnect the extension wire in table (so there is 60Hz AC capacitive coupling or electric field), gain of 10000, 1000Hz bandwidth



The following is when gain is adjusted from 10000 to 5000. Why is there less noise?



The following is when Audacity is zoomed to show the 60Hz AC waveforms



When 11 feet cable with open end was connected, the following is the result:



The following with sine wave generator inputted 10 microvolt, 50Hz, 5000 gain, 60 Hz AC on (extension plugged in),  1000Hz Bandwidth



The following when extension cable unplugged (no 60Hz AC (extension wire unplugged))



When there is sine wave generator input, the output is not affected anymore whether the extension wire is plugged (with capacitive coupling) and not plugged (with no capacitive coupling). So what can you say about any built in filters that activates when there is sine wave input??

[Andy Chee]

So how do you characterize (or compute) the strength of interference before the sine wave input can be affected? I guess the 10 microvolt sine wave generator has same strength as the skin 10 microvolt, isn't it.. since they are 10 microvolt.

This is where the software analysis of the waveform comes in.

Have you heard of the mathematical technique, convolution/deconvolution?

[loop123]

I watched it but I need to implement the filter right at the input. How do I know if the existng one has it already?

I also ran the above waveforms again by adjust the bandwidth to 100Hz, 1000Hz, 3000hz, the noise is the same so it means it is a low frequency noise.

However, when I plugged in the 60Hz AC/DC adaptor to the laptop. I saw major interference to the 10 microvolt 50Hz signal input. The following is without AC/DC adaptor plugged in the laptop.
 


The following is the AC/DC adaptor plugged in the laptop. It totally distorted the 10 microvolt 50Hz Netech sine wave input. Do you think it's caused by interference from the current itself or from EMI? 



If the Netech is set to 60Hz sine wave. No interference but the amplitude get about 15% larger. 60Hz from line or EMI?

Anyway my worry is strong signal in the input clipping the microVolt signal. How do you compute how strong must be the RF interference enough to influence microvolt signal?

[Andy Chee]

I watched it but I need to implement the filter right at the input. How do I know if the existng one has it already?

You will need to trace the circuit board for the amplifier, just like you did with the ISO-Z.

But my guess is that there is no hardware filter, and they are relying on software filtering.

[Kleinstein]

Filtering the lower frequency background, like 60/120 Hz mains hum would likely be done in software. A hardware filter (if really used) could also be further down the path, e.g. ater the 1st amplifier stage.

The RF filtering should be hardware and right at the input. The filter would also be part of the ESD protection. A simple form to ckeck are clip on ferrites to on the electrode cables.
How sensitive a front end is to RF interference can vary quite a bit, not just by the chips used but also layout details. For testing one would want something like an AM modulated REF signal.
Just from looking at the low frequency signal one can not tell if the interference is directly a low frequency signal or a modulated RF signal that causes the problem or extra noise.

[Andy Chee]

Are there RFI filters? How do RFI filters look like?

I expect them to look like this, but I don't see any on your board:



Don't forget to check the ISO-Z board as well, they might be on there.

[loop123]

Are there many ways to implement RFI fillter? for example, one can use IC?  one can use a combination of resistors and capacitors?  One can use an inductor?  Can you give examples of RFI filter circuits?

Also won't there be a generic module that you can put in the input to filter RFIs?

[Kleinstein]

The typical RFI filters would be ferrite beads and capacitors. It can also work with resistors and capacitors. A simple form is a Pi type filter with a ferrite (and or resistor) in series and a capacitor to ground on both ends.  With the differential signals and relatively high source impedane the capacitoance to ground would however reduce the common mode rejection for the high frequencies. So one would want relatively small capacitors, maybe 100 pF or so. The cable capacitance may replace the capacitance on the input side.

[loop123]

The loss in signal amplitude should not be an issue: the amplifier input impedance should be well larger than 10 K and also the capacitive loading from a shielded cable is not that much, unless one is interested in the really high frequencies.

EMI can be a point - it is a bit tricky to design a high impedance filter to keep EMI away from the amplfier.  Once the very high frequency EMI part hits an amplifier it can get demodulated and effect the frequency band of interrest. After that filtering can no longer separate it from the signal.

It may be relevant how good the amplifier can tolerate RF band interference (e.g. WLAN, mobile phones, TV signals, radio). Not all amplifiers react the same way in this respect.
If really deparate one could have an RF receiver to detect the interferening signal and than subtract it. Still tricky when the cables move.

I see an only moderate advantage for active electrodes - maybe a reason to charge more money for them  .

Adding shielding at least adds inconvenience - like move to a special room instead of moving the intrument to the patients room. A shilded room would also need filtering for the supplies and light. RF tight doors are also a thing that is not easy.

When I read the above 10 days ago. It gave me impression active electrodes didn't have that much advantage because the loss of signal amplitude is not an issue since the input impedance of an amplifier is way higher than say the 10kOhm electrode impedance, and I lost interest in active electrodes. However, when I read the following today. It seems to indicate the entire microVolt signal can get drown when electrode to skin impedance is 10kOhm or even 5kOhm!  I read.

https://www.sciencedirect.com/science/article/abs/pii/S0165027014001666

"Active electrodes are often billed as enabling this mode of data collection, and this feature is taken as an additional justification of their relatively high cost. The follow example illustrates why active electrodes should have this property. In passive amplification systems, interference currents that come from the main power and “wirelessly” couple to the participant and to the electrode wires (capacitive coupling), multiply by the interelectrode impedance that gives as a result the interference voltage that corrupts the EEG signal before it gets to the amplifier (Metting Van Rijn et al., 1990). A typical interference current is of the order of 20 nA, which, given an interelectrode impedance of even 10 kΩ, yields an interference voltage of 200 μV, a magnitude capable of drowning the EEG signal being measured, which is normally between 10 and 100 μV (Aurlien et al., 2004)."

200uV interference voltage can totally down any 10uV signal. When you mentioned shielded wire. Can it totally eliminate any interference current of 20nA or so? How much interference current is retained using shielded wire? Do you have any data?  I got the Metting Van Rijn reference and saw this figure:

[Kleinstein]

Shielding the wires should eliminate essentially all the capacitive coupling via Cca and Ccb. For the RF part (e.g. > 10 MHz) a little of the singal may still reach the amplifier, as the shield impedance is not zero.
The current would than instead flow towards the supply. So Cca and Ccb would add to Csup.
Chances are that an active electrode would use some kind of shielded cable too - so it would also see the added current to the supply.

I see a very limited advantage for active electrodes over a shielded cable.


The specs for the Gtec amplifier don't look that impressive. <400 nV_RMS for the 1-30 Hz noise looks rather high  - it should be quite a bit smaller (e.g. 100 nV range). The high input impedance suggests that they use a FET basd amplifier. The 200 pF suggests that there is RF fitlering included, as the amplifiers itself have usually only some 10 pF or less of input capacitance.
Maybe they want an extra preamplifier in front.

[loop123]

Shielding the wires should eliminate essentially all the capacitive coupling via Cca and Ccb. For the RF part (e.g. > 10 MHz) a little of the singal may still reach the amplifier, as the shield impedance is not zero.
The current would than instead flow towards the supply. So Cca and Ccb would add to Csup.
Chances are that an active electrode would use some kind of shielded cable too - so it would also see the added current to the supply.

I see a very limited advantage for active electrodes over a shielded cable.


The specs for the Gtec amplifier don't look that impressive. <400 nV_RMS for the 1-30 Hz noise looks rather high  - it should be quite a bit smaller (e.g. 100 nV range). The high input impedance suggests that they use a FET basd amplifier. The 200 pF suggests that there is RF fitlering included, as the amplifiers itself have usually only some 10 pF or less of input capacitance.
Maybe they want an extra preamplifier in front.

I can only buy cables like this. They don't make shielded ones.



I will work mostly in microvolt like 10uV. How do you convert the 400nV RMS noise at 1-30Hz for the Gtec amplifier to nV/sqrt (Hz)?  For the AMP01 amp in BMA-200. 5nV/sqrt (Hz) x sqrt (30 Hz bandwidth) = 5nV * 5.47722  = 27.386 nV.  Is this right? If so, then the BMA-200 even have better noise figure than the $16750 Gtec?

I tried to order this Borescope to peek inside. I don't know if it is too big. What is a smaller one that is very clear?

[Andy Chee]

Shielded electrodes have two plugs per electrode (also a ferrite suppression core):



https://m-cdn.adinstruments.com/product-data-cards/MLA4105-DCW-16A.pdf

[Kleinstein]

How do you convert the 400nV RMS noise at 1-30Hz for the Gtec amplifier to nV/sqrt (Hz)?  For the AMP01 amp in BMA-200. 5nV/sqrt (Hz) x sqrt (30 Hz bandwidth) = 5nV * 5.47722  = 27.386 nV.  Is this right? If so, then the BMA-200 even have better noise figure than the $16750 Gtec?

In principle the calculation is right. Using 30Hz -1 Hz for the bandwidth does not make a big difference. There is however likely a little extra 1/f noise from the amplifier. So the 5 nV/sqrt(Hz) is not valid all the way to 1 Hz, though close and not that much difference here too.

The specs for the Gtec part indeed don't look good and suggest a rather poor noise figure. For the comparison one still has to include the protection part needed and take into account that the INA noise on the front page is for a high gain. With less gain the noise tends to be higher too.

[loop123]

How do you convert the 400nV RMS noise at 1-30Hz for the Gtec amplifier to nV/sqrt (Hz)?  For the AMP01 amp in BMA-200. 5nV/sqrt (Hz) x sqrt (30 Hz bandwidth) = 5nV * 5.47722  = 27.386 nV.  Is this right? If so, then the BMA-200 even have better noise figure than the $16750 Gtec?

In principle the calculation is right. Using 30Hz -1 Hz for the bandwidth does not make a big difference. There is however likely a little extra 1/f noise from the amplifier. So the 5 nV/sqrt(Hz) is not valid all the way to 1 Hz, though close and not that much difference here too.

The specs for the Gtec part indeed don't look good and suggest a rather poor noise figure. For the comparison one still has to include the protection part needed and take into account that the INA noise on the front page is for a high gain. With less gain the noise tends to be higher too.

I wonder if we made a mistake somewhere because I read in

https://arxiv.org/pdf/1606.02438.pdf

"Comparison of an open-hardware electroencephalography amplifier with medical grade device in brain-computer interface applications"

"No matter the financial aspects, the qualities of the g.USBamp amplifier make it the perfect baseline to gauge new challengers. This is also true for the electrodes developed by its manufacturer".

It is supposed to be the perfect baseline. It is what I owned too. Most institutions used it for cutting edge projects and experiments. For example:

https://cordis.europa.eu/docs/projects/cnect/5/257695/080/deliverables/001-D32FirstPortotypeBBCIFinal.pdf

I'm thinking whether to buy g.Gammabox (with one set of active electrodes) for $2000.



It says active electrodes allows for larger impedance. They are already using shielded cables. So why are they still concerned about the impedance? Maybe even without full shielded cables. There is still interference current leakage of some sort? Are you 100% sure it is eliminated with shielded cable? Then why do they still seek lowering impedance?

[loop123]

Shielded electrodes have two plugs per electrode (also a ferrite suppression core):

(Attachment Link)

https://m-cdn.adinstruments.com/product-data-cards/MLA4105-DCW-16A.pdf

That's also and the only shield one I saw. But why are there 2 leads? What is the 2nd lead for? It says it is for use for the a main cable. See https://m-cdn.adinstruments.com/product-data-cards/MLA4105-DCW-16A.pdf

Both my BMA and Gtec amplifiers only accept 1.5mm touchproof plug. So it's not compatible even if I'd just insert one lead (the 2nd lead is ground)? There seems to be no 1.5mm touchproof shielded leads available.

[Andy Chee]

Shielded electrodes have two plugs per electrode (also a ferrite suppression core):

https://m-cdn.adinstruments.com/product-data-cards/MLA4105-DCW-16A.pdf

That's also and the only shield one I saw. But why are there 2 leads? What is the 2nd lead for?

The first plug is the electrode, just like a regular unshielded version.

The second plug is for the shield, it needs to connect to ground.  Your bioamp may not be compatible if it doesn't have multiple ground connections for multiple shields.

Think of the shield as a miniature faraday cage surrounding the wire.  At the very least it stops the wire picking up EMI noise.  It doesn't stop the human body picking up EMI noise though, which is why EEG experiments are usually (but not always) performed in a faraday shielded laboratory (making shielded electrode wires unnecessary).