Instrumentation Amplifier Modification or Replacement

[Kleinstein]

A shilded cable should elimiate capacitive coupling. It can however not help against effects from the cable movement. Here the shielded cable can actually add it's own interference signal. So one would want a low noise isolation in the cable, like in some microphone cables.
The shielded cables need an amplifier or adapter to provide the extra ground terminals.  For some reasons it seems that shielding is not that common and not all setups may support it.


It looks like the "amplifier" / ADC box still needs the preamplifier box. The noise specs may be for the large +-250 mV range and thus with low gain. For the ADC performance this is not that bad.

Even with active electrodes one would still want low resistance electrodes. The electrode resistance alone add thermal noise, even without extra interference. With a high impedance, like >20 K this noise can be higher than the amplifier noise.

[loop123]

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).

I found out Biopac where I bought my Electrode Checker has shielded cable for 1.5mm touchproof that can work with snap on electrodes. It has 2 leads too. the 2nd for ground as your said. If I'd connect it to my BMA-200 which I can custom adjust. Can I just connect the ground lead to the ground of BMA? With the USBamp it is more difficult since I can't open the unit. But with my 2 units of BMA, I can alter it at will. 

[loop123]

I ordered the Biopac shielded leads. If it needs extra circuit in the BMA. Then please help me build one. Remember I bought a 2nd BMA to replace the AMP01 amplifier but you didn't have any suggestion for better amp with significant improvement. So I'll add the ground circuit (does it need any extra component or just connecting it to the terminal)?  If it won't work, then last resort is the $2000 g.GAMMAbox to connect active electrodes to the USBamp. Thanks.

[Kleinstein]

The shield part would only need the link to circuit ground and the connectors. So no active components. Additional ferrites may help in some cases, but thus could be clip on ones.

[SiliconWizard]

What are these bodge wires? I would assume that this "bioamplifier" is not meant for human use.

[loop123]

Its made for mice. But then humans and mice share 97.5% of DNA so the measuring device for rats need not be less sensitive. The company CWE deals with products related to animals. By the way. Why do they need 50,000Hz for animal use? And i always forget to ask. Can I transmit music into the wired differential inputs so the build in speakers can produce the same music? How do I encode music into differential signals? Great to do while waiting for my shielded leads. Oh the ISO-Z is designed for human use and convert main amp to human use.

[Andy Chee]

Why do they need 50,000Hz for animal use?

Decades of animal experimentation have found that some animal brain EEG waves (especially small animals like mice) can have extremely high frequencies, much higher than humans.

The Nazis in WW2 did awful experiments on humans, which is why modern researchers now experiment on animals first before humans. 

[loop123]

I read that dealing with a bio signal of 10uV is no easy thing. That this signal is much smaller than a typical IA offset voltage and, even a small electrode resistance under the influence of the IA bias and offset currents will typically produce an even bigger offset. So you will need to deal with the fact that a good proportion of the signal you will amplify will have an offset bigger than the signal and will need to be removed at some point before your circuits run out of headroom. And next comes the interferences. These can dwarf the offsets and everything else if your signal chain and signal processing is not handled very carefully. One of the biggest complicating factors here is that you cannot use the protective earth to shunt any of this noise away. You will need to either use concepts such as the RLD reimagined for your application or use an amplifier circuit embedded in the electrode itself or one of the thousands of other ways to deal with 10uV bio signals.

There are hundreds of EEG products. So how do they deal with the above problems?

And what is the problem if the signal is smaller than typical IA offset voltage?  For example. Many IA has input offset voltage of 50uV. What would happen to the 10uV input signal in face of such huge input offset voltage?  For illustration:

[loop123]

A shilded cable should elimiate capacitive coupling. It can however not help against effects from the cable movement. Here the shielded cable can actually add it's own interference signal. So one would want a low noise isolation in the cable, like in some microphone cables.
The shielded cables need an amplifier or adapter to provide the extra ground terminals.  For some reasons it seems that shielding is not that common and not all setups may support it.


It looks like the "amplifier" / ADC box still needs the preamplifier box. The noise specs may be for the large +-250 mV range and thus with low gain. For the ADC performance this is not that bad.

Even with active electrodes one would still want low resistance electrodes. The electrode resistance alone add thermal noise, even without extra interference. With a high impedance, like >20 K this noise can be higher than the amplifier noise.

I was looking for the specs of the gtec active electrodes to see what gain it makes. But couldn't find the information. Then I read this

https://www.biosemi.nl/forum/viewtopic.php?t=486

"(2) BIOSEMI ACTIVE ELECTRODES - A "pre-amplifier" is inserted just after the electrode and before the long wire that goes to the amplifier. This amplifier does not give the signal any gain. "

Active electrodes don't produce any gain?  And the only purpose is impedance transformation to get rid of the interference signal (explained many times in the link above)?

If the gtec active electrodes don't have any gain. Then the main amp noise specs is as is (meaning it's not for low gain).

[Kleinstein]

The offset voltage should be no longer a big problem, as the modern system tend to use rather high resolution ADCs and can live with some offset. Besides offset there is often also some hum as background. The background from hum and offsets limits on how much gain can be used, but with a good ADC there is enough dynamic range and not that much gain needed. If a 16 bit ADC works with LSB steps of some 100 nV for the input before the amplifier, it can still read signals up to some +-3.2 mV.  So the DC offset and hum only need to be smaller than some 2 mV_peak for this example.
In the old days the likely used more analog filtering (e.g. 50 Hz notch), offset trim and maybe AC coupling with a low cross over, to get away with less ADC resolution or direct plot to paper.

I have the feeling that the signals are in many cases not actually limited by the amplifier or ADC noise, but some background from EMI, hum and persons moving / breathing. So there may not be a need to have the absolute lowest noise amplifiers. In many cases the designs also care about power to allow battery operation and this way get good isolation.
It may be a bit more critical with small animals in a well shielded lab environment, compared to a more normal hospital environment.

Edit: with more critical, I mean less background and thus a possible use for a lower noise amplifier.

[loop123]

So is it true, all active electrodes have no gain and its only purpose is impedance converter?

If so, then it is very easy to build. Just use the smallest unity gain amplifier.  To start, what chip can you suggest? I'll try building one for both my BMA and USBamp and save over $2000.  What smallest unity gain amplifier has differential output too? Because if it's only single ended. How will I connect it to the differential inputs of both the BMA and USBamp?

[loop123]

The offset voltage should be no longer a big problem, as the modern system tend to use rather high resolution ADCs and can live with some offset. Besides offset there is often also some hum as background. The background from hum and offsets limits on how much gain can be used, but with a good ADC there is enough dynamic range and not that much gain needed. If a 16 bit ADC works with LSB steps of some 100 nV for the input before the amplifier, it can still read signals up to some +-3.2 mV.  So the DC offset and hum only need to be smaller than some 2 mV_peak for this example.
In the old days the likely used more analog filtering (e.g. 50 Hz notch), offset trim and maybe AC coupling with a low cross over, to get away with less ADC resolution or direct plot to paper.

I have the feeling that the signals are in many cases not actually limited by the amplifier or ADC noise, but some background from EMI, hum and persons moving / breathing. So there may not be a need to have the absolute lowest noise amplifiers. In many cases the designs also care about power to allow battery operation and this way get good isolation.
It may be a bit more critical with small animals in a well shielded lab environment, compared to a more normal hospital environment.

Edit: with more critical, I mean less background and thus a possible use for a lower noise amplifier.

Capacitive coupling has frequency. So the AC capacitive coupling frequency is 60Hz. If you filter all signal below 100Hz. Then no need for shielded cable or active amplifier? What you think?  Of course one can argue it will filter everything below 100Hz. But supposed your application is higher frequency and you want to get rid of 60Hz capacitive coupling. What happens if you filter out everything below 100Hz?

[Kleinstein]

AFAIK the relevant frequenices are from some 1 Hz (maybe a little lower) to maybe some 40 Hz or even a bit higher. WIth small animals even higher frequencies may be relevant.

So filtering out 50/60 Hz interference is tricky and would need a rather steep flilter, like a high Q notch. It is still not ideal and it really helps to keep the hum out. New ADC have a high resolution and thus large dynamic range, but there can still be too much hum and background. A changing level of mains hum would include also frequencies away from the 60 Hz nominal frequency that may interfere with some signal and not get filtered out.  Also other low frequency background could couple capacitively, e.g. from movement and electrostitic charges. So it is not only 60 Hz and harmonics.

[Andy Chee]

What happens if you filter out everything below 100Hz?

You will filter out low frequency biosignals as well.  The proper procedure is a notch filter at 60Hz. 

Hardware notch filter has performance limitations, which is why such filtering is almost always performed in software.

A software notch filter can be extremely sharp, for example, filter everything between 59.99999Hz and 60.00001Hz 

https://www.learningeeg.com/artifacts  (scroll down to the section about Electrical Artifacts)

[loop123]

AFAIK the relevant frequenices are from some 1 Hz (maybe a little lower) to maybe some 40 Hz or even a bit higher. WIth small animals even higher frequencies may be relevant.

So filtering out 50/60 Hz interference is tricky and would need a rather steep flilter, like a high Q notch. It is still not ideal and it really helps to keep the hum out. New ADC have a high resolution and thus large dynamic range, but there can still be too much hum and background. A changing level of mains hum would include also frequencies away from the 60 Hz nominal frequency that may interfere with some signal and not get filtered out.  Also other low frequency background could couple capacitively, e.g. from movement and electrostitic charges. So it is not only 60 Hz and harmonics.

Have you handled real active electrodes before and thats why you can say they are not really very good?

Anyway before investing in $2000 active electrode system. I want to build and taste one first with gain of 1. What kind of amplifier where the input and output is both differential? Because most Instrumentation amplifiers I saw has singular Vout and not differential Vout.

[Kleinstein]

I have not work with bio-electricity measurements in any way or seen an active probe in real life. It very much depends on the amplifier if it really needs a true differential signal or is just ground an signal.
Yes most INA have a single ended output, but I don't think one would need an INA for an active probe. From what I have read, the active probles seem to be simple buffer amplifiers with 1 input and 1 output.  An INA would be for 2 electrodes and would only make sense with gain. If really needed one could add a complementary output to  an INA if really needed.

[loop123]

I have not work with bio-electricity measurements in any way or seen an active probe in real life. It very much depends on the amplifier if it really needs a true differential signal or is just ground an signal.
Yes most INA have a single ended output, but I don't think one would need an INA for an active probe. From what I have read, the active probles seem to be simple buffer amplifiers with 1 input and 1 output.  An INA would be for 2 electrodes and would only make sense with gain. If really needed one could add a complementary output to  an INA if really needed.

Yes I realized it yesterday, that was why I made a new thread asking one to model in Ltspice the Olimex active electrode shared by Andy to see if I could use it in my amplifiers. I found out I could but the noise spec is terrible at 30nV/sqrt (Hz). Do you know any buffer amp with spec of 5nV/sqrt (Hz)?

https://www.eevblog.com/forum/chat/pls-simulate-this-active-electrode-using-ltspice-or-other-simulators/

See the thread also and give suggestion how to build the $1600 g.GAMMAbox. Thanks.

[loop123]

A shilded cable should elimiate capacitive coupling. It can however not help against effects from the cable movement. Here the shielded cable can actually add it's own interference signal. So one would want a low noise isolation in the cable, like in some microphone cables.
The shielded cables need an amplifier or adapter to provide the extra ground terminals.  For some reasons it seems that shielding is not that common and not all setups may support it.


It looks like the "amplifier" / ADC box still needs the preamplifier box. The noise specs may be for the large +-250 mV range and thus with low gain. For the ADC performance this is not that bad.

Even with active electrodes one would still want low resistance electrodes. The electrode resistance alone add thermal noise, even without extra interference. With a high impedance, like >20 K this noise can be higher than the amplifier noise.

Someone told me he thought the g.USBamp didn't even have any amplifier. This is because you need amplifier to map into the voltage of the ADC like 1V. But with the USAamp. It is already +-250mV range and has sensitivity of 85.7nV. So it is possible there is really no amplifier in the analog front end, agree?

[loop123]

I don't want to start a new thread just to ask the following, so please tell me how to do it.  I read that

"If you connect a pair of electrodes using a R-ohm resistor, at room temperature, with f-Hz bandwidth, you will get 0.13*sqrt(R*f) nV (rms) noise.  It is very easy to verify, since by using different resistors, you can observe in the digital data what kind of resistor is needed to change the noise floor."

I have a box full of resistors of different values. I'll connect the resistor to the 2 leads V+ and V- of the BMA or the g.USBamp? What will I get with the 5 nV/sqrt (Hz) figure? Is this to test for thermal noise? what would different resistors values produce in the output? please give some idea before I even connect the resistors. Thanks!

[Kleinstein]

Using the resistors as a noise reference is not that easy. The problem is that there can also be current noise from the amplfier, especially with BJT based amplifiers. The external resistor would convert the current noise to voltage noise proportional to the resistor.  It can work the FET based amplifiers that have very low current noise.

The more usual way is to measure the votlage gain / scale factor with sine or similar signal and than in a 2nd step measure the noise with a shorted input or a resisor of about the values expected for the source (electrodes).

It is not clear if the USBamp has gain in front. It would make sense to have some gain in front, but I just don't know if they have it.

[loop123]

Using the resistors as a noise reference is not that easy. The problem is that there can also be current noise from the amplfier, especially with BJT based amplifiers. The external resistor would convert the current noise to voltage noise proportional to the resistor.  It can work the FET based amplifiers that have very low current noise.

The more usual way is to measure the votlage gain / scale factor with sine or similar signal and than in a 2nd step measure the noise with a shorted input or a resisor of about the values expected for the source (electrodes).

It is not clear if the USBamp has gain in front. It would make sense to have some gain in front, but I just don't know if they have it.

How about testing just the BMA which we know it uses the AMP01. I want to see the effects of thermal noise using resistors? Does it mean if I put 500kohm resistor vs 100kohm resistor across the leads, the noise would increase? just want to have idea.

I'll just test using the BMA since I have 2 pcs and if one gets damaged, I have another one (costs $150 used) versus the $16750 g.USBamp which I can't find another used working for $1000. Also I don't  have software to run it. BCI2000 and OpenVibe are just so difficult to use. You have to know programming just to run the recording part?

[Kleinstein]

One can use resistors to get a crude idea of the noise. With the amp1 one will see a mix of the voltage noise and current noise. 100 K and even more 500 K would be in the range that is dominated by current noise. So 5 x more noise with 500 K.

[loop123]

Using the resistors as a noise reference is not that easy. The problem is that there can also be current noise from the amplfier, especially with BJT based amplifiers. The external resistor would convert the current noise to voltage noise proportional to the resistor.  It can work the FET based amplifiers that have very low current noise.

The more usual way is to measure the voltage gain / scale factor with sine or similar signal and than in a 2nd step measure the noise with a shorted input or a resisor of about the values expected for the source (electrodes).

It is not clear if the USBamp has gain in front. It would make sense to have some gain in front, but I just don't know if they have it.

Please give example  how to do this usual way to measure "the voltage gain / scale factor with sine or similar signal and than in a 2nd step measure the noise with a shorted input or a resisor of about the values expected for the source (electrodes).".

I have a Netech ECG simulator and a Netech EEG simulator. What setting must I use and what scale factor  you mean and what resistor values. Please give values of the example so I can actually try it out. Thanks.

[loop123]

A shilded cable should elimiate capacitive coupling. It can however not help against effects from the cable movement. Here the shielded cable can actually add it's own interference signal. So one would want a low noise isolation in the cable, like in some microphone cables.
The shielded cables need an amplifier or adapter to provide the extra ground terminals.  For some reasons it seems that shielding is not that common and not all setups may support it.


It looks like the "amplifier" / ADC box still needs the preamplifier box. The noise specs may be for the large +-250 mV range and thus with low gain. For the ADC performance this is not that bad.

Even with active electrodes one would still want low resistance electrodes. The electrode resistance alone add thermal noise, even without extra interference. With a high impedance, like >20 K this noise can be higher than the amplifier noise.

Let's use your example above. Let's say the interference current is taken care of. But there is leftover 20 Kohm impedance with bandwidth of 1000Hz. So to use the formula = 0.13*sqrt(R*f) nV (rms) noise = 0.13 sqrt (20,000 x 1000 Hz) nv = 0.13 x 4472 = 581 nV/ sqrt (Hz) noise addition versus the 5 nV/sqrt (Hz) of the AMP01. Is this correct? Do I have to add the AMP01  noise 5nV/sqrt (Hz) x sqrt (1000Hz) = 158nV noise to the 581nV resistor noise? so total noise is 739nV or 0.739uV?

[Kleinstein]

The noise from the resistor is 581 nV rms, not nv/sqrt(Hz). The noise from the resistor and amplifier add geometrical. So as sqrt(581²+158²), not as a direct sum.

Another part to add is the current noise multiplied with the source resistance. So some 0.15 pA/sqrt(Hz) * 20 K = 3 nV/sqrt(Hz). It is not that bad with 20 K , but it would with 500 K.

The EEG simulator should give a defined voltage. The ratio of the amplitude after the amplifier to the give votlage for the source is the gain factor for the amplifier. One can than divider the signal seen as noise by the gain to get the amplitude to the noise as voltage and not just output units.  It is well possible that the amplifier already has a defined gain and does the conversion with it's nominal gain. Usually that would be good enough for the noise testing.