Can changing amplifier in Sullen-Key filter affect the frequency response much?

[loop123]

I was mistaken. 332 capacitor means 3300pF. so it's really Sullen and key of 6800pF and 3300pF with the LF411CP. This means low pass of less than 10kHz.

With 6.8n/3.3n the corner frequency is at ~4.7kHz (see attached screenshot).

Compare this to the original datasheet Sullen and Key 1 000pF and 220pF with cutoff of 50,000kHz. What does this do to the ripples? Does it mean the ripples at all frequencies will be less when the 50kHz cutoff is used versus the say 7kHz cutoff? Or does it mean only ripples below 7kHz would be cutoff in the latter?

I don't know what frequency the ripples have. The screenshot you provided was unclear because you cut off the units respectively the values of the time and amplitude scale.

If you use the 4.7kHz filter variant, the amplitude of the ripples that are far above the corner frequency will be less compared to the 50kHz filter. Ripples inside the filter passband can obviously not be attenuated at all.

Also more poles means more abrupt cutoff. What has this got to do with suppressing the ripples? Kindly explain because I want to understand the mechanism.

As already mentioned, you can only suppress ripples that are outside of the filter passband. A steeper filter (more poles) ensures ripple voltage(s) with lesser amplitude.

It's just an amplifier. I don't need 50kHz but 7kHz may be too low for say audio use.

So 20kHz bandwidth will suffice?

Did you use R1=4.75k, R2=9.76k, C1=3300pF, C2=6800pF?  Why did online calculator shows cutoff of only 4.934kHz?? Instead yours show more than 50kHz??

20kHz will suffice but I want to change the C1=1000pF, C2=220pF, and use OPA602 just like in the original ISO122P datasheet. This is in order to hopefully remove all the ripples in the following.

https://www.youtube.com/watch?v=DO58qtAMYBg&feature=youtu.be

It started with the high pass filter at 100Hz (clean signal). This is irregardless if the input frequency is 10 Hz or 40Hz or 90Hz or higher). Then I switched it to 1000Hz, then 3000 Hz, 10000Hz, 30000Hz, 50000Hz (Yes my amplifier has dials for all those frequencies). The noises keep getting worse. You can hear the click at the video as I adjusted them. Halfway I returned from 50kHz back to 100Hz.

Going to the 6800pF, 3300pF, LF411CP which the actual circuit is using. If it filters anything above 5kHz. Why is the output showing up to 50kHz?? This greatly puzzles me now.

[loop123]

Can't you tell the frequency of the ripple even with the datasheet? It says the 20mV peak to peak came from carrier modulation of 500kHz. What is the ripple frequency?

https://www.ti.com/lit/ds/symlink/iso122.pdf?ts=1707196390069&ref_url=https%253A%252F%252Fwww.google.com%252F



Remember it mentioned in page 10 the Sullen-Key low pass filter up to 50,000Hz can remove the ripples. You were saying out the 50kHz and not within? But what is the frequency of the ripples in the first place?

[RFDx]

Did you use R1=4.75k, R2=9.76k, C1=3300pF, C2=6800pF?

Yes.

Why did online calculator shows cutoff of only 4.934kHz?? Instead yours show more than 50kHz??

My (last) screenshot shows a corner frequency of ~4.656kHz @ -3dB and that is including the 1st order lowpass inside the ISO122 amp.

20kHz will suffice but I want to change the C1=1000pF, C2=220pF, and use OPA602 just like in the original ISO122P datasheet. This is in order to hopefully remove all the ripples in the following.

More bandwidth means more noise. Increasing the bandwidth unnecessarily from 20kHz to 50kHz doesn't make much sense if you want less noise/ripples at the output.

https://www.youtube.com/watch?v=DO58qtAMYBg&feature=youtu.be

It started with the high pass filter at 100Hz (clean signal). This is irregardless if the input frequency is 10 Hz or 40Hz or 90Hz or higher). Then I switched it to 1000Hz, then 3000 Hz, 10000Hz, 30000Hz, 50000Hz (Yes my amplifier has dials for all those frequencies). The noises keep getting worse. You can hear the click at the video as I adjusted them. Halfway I returned from 50kHz back to 100Hz.

The video shows that the frequency of the input signal fed to the amplifier is always the same. The "frequency dial" your amplifier has, seems to be an option to change the bandwidth of the amplifier and not the frequency of an input signal. I don't think your amplifier has a built in signal generator. If you increase the bandwith of the amplifier more noise comes through (which btw is perfectly normal).

Going to the 6800pF, 3300pF, LF411CP which the actual circuit is using. If it filters anything above 5kHz. Why is the output showing up to 50kHz?? This greatly puzzles me now.

See above response. You never applied 50kHz to the amplifier so you can't really tell if 50kHz is showing up or not. With 50kHz at the input of the ISO122 amp and with the additional Sallen-Key filter (6.8n/3.3n), the output must show an attenuation > 40dB (that's less than 1/100 in amplitude) of the input signal.

[RFDx]

Can't you tell the frequency of the ripple even with the datasheet? It says the 20mV peak to peak came from carrier modulation of 500kHz. What is the ripple frequency?

The frequency would be 500kHz, that is the frequency the modulator is clocked with. This unwanted product shows up, according to the datasheet, with typ. 20mVpp at the output (that is after the 50kHz 1st order lowpass filter inside the ISO122).
With the additional 2nd order 50kHz (1nF/220pF, OPA602 or LF411) Sallen-Key lowpass filter, the amplitude of the 500kHz clock signal will be further attenuated by a factor of 100 (40dB) and the remaining amplitude should be ~0.2mVpp.
With the corner frequency of the filter lowered to 4.656kHz (6.8nF/3.3nF, LF411), the amplitude of the 500kHz clock signal will be attenuated by ~75dB and should be ~3.5uVpp.
Do you have test equipment (AC voltmeter, spectrum analyzer etc.) that goes up to 500kHz and can measure such low voltages?

[loop123]

By the way. The input is 1mV about 80 Hz using signal generator (I tried 2 brands) passing through the ISO122p unit. When main unit passband is set to 100Hz. No ripple. When it is set to 1000Hz. There are already ripples as you can see in the video.

I thought this occurred because the manufacturer didn't follow the datasheet of 1000pF, 220pF, OPA602 and instead using 6800pF, 3300pF, LF411CP which I thought meant the filtered noise is only 0 to 4.75kHz. I thought using the datasheet original 50kHz passband means everything inside 50kHz will be ripple free. But it's the opposite as you explained.

But how come at mere 1000Hz pass band switch, ripples at 500kHz already showed up with the input of 1mV, 80Hz via the ISO122?

Note that without the ISO122. You can input even 30kHz into main unit without any ripples. Does this mean the ISO122 is only good up to 1mV 100Hz input only?

[RFDx]

By the way. The input is 1mV about 80 Hz using signal generator (I tried 2 brands) passing through the ISO122p unit. When main unit passband is set to 100Hz. No ripple. When it is set to 1000Hz. There are already ripples as you can see in the video.

A 1mV input signal is way to small. The ISO-amp produces itself almost 1mV of noise with a bandwidth of 50kHz, which means the signal to noise ratio (SNR) will be very bad. This noise is inside the passband and can not be removed.

To increase the SNR you can either increase the input signal or decrease the bandwidth of the ISO-amp. With a power supply of +/-15V the ISO amp accepts an input signal of max. +/-12.5V.

[loop123]

By the way. The input is 1mV about 80 Hz using signal generator (I tried 2 brands) passing through the ISO122p unit. When main unit passband is set to 100Hz. No ripple. When it is set to 1000Hz. There are already ripples as you can see in the video.

A 1mV input signal is way to small. The ISO-amp produces itself almost 1mV of noise with a bandwidth of 50kHz, which means the signal to noise ratio (SNR) will be very bad. This noise is inside the passband and can not be removed.

To increase the SNR you can either increase the input signal or decrease the bandwidth of the ISO-amp. With a power supply of +/-15V the ISO amp accepts an input signal of max. +/-12.5V.

But why is the ISO122 signal clean if the filter is set to 100Hz? Also even without using the Sullen-key and tapping directly the ISO122, the 90Hz, 2mV signal is clean with 100Hz cutoff selected in the main unit.

How about an opto-isolator? Can it accept 50kHz without any ripple? What is the maximum bandwidth of opticoupler?

Btw.. i couldn't find in the ISO122P data sheet the noise at 1mV. Where did you read it?

[loop123]

I don't know how to use Ltspice except to delete components, change values and run the simulator.

This is your fundamental problem.
Learning how to add voltage sources, draw connections and maybe use net labels would take not two days but less than two hours.

Tip: F2, F3, F4, respectively.

I have spent just that 2 hours putting the power supply. I did. However the frequency response is still the same with or without a power supply (15V vs 0V/not connected).



The above is with 15V inputted. Below is with 0V (same as cutting the power supply)



This is DC check for both V+ and V- for 15V.





Without power supply. The frequency response came directly from the connected resistors and capacitors, even in real circuit, right? With this. Even if the real amp such as the LF411CP is defective. It can still show frequency response? I was wondering if my real LF411CP is defective and the frequency response came from the R1,R2, C1,C2 in the 2 pole Sullen-Key

In LTspice, how come the frequency response is same with both power supply 15V inputted and no power supply?

here is the file, pls check it out, remove the battery and see the frequency response is the same

[RFDx]

But why is the ISO122 signal clean if the filter is set to 100Hz? Also even without using the Sullen-key and tapping directly the ISO122, the 90Hz, 2mV signal is clean with 100Hz cutoff selected in the main unit.

Less bandwidth equals less noise voltage at the output riding on top of the actual signal. The overall bandwidth is determined by the 100Hz bandwidth of the main amplifier. Compared to the full 50kHz, the noise voltage will be smaller by a factor of SQR(50000/100)= 22.36 when the bandwidth is switched to 100Hz.

How about an opto-isolator? Can it accept 50kHz without any ripple? What is the maximum bandwidth of opticoupler?

You would need an analog optocoupler. The datasheet will probably give a hint to how much noise is to be expected. Depending on the mode of operation, the max bandwidth is maybe 50...100kHz.

How about putting a low noise preamp with a gain of 100 (or even 1000) in front of the ISO-amp?

Btw.. i couldn't find in the ISO122P data sheet the noise at 1mV. Where did you read it?

The noise spectral density of the ISO122 is 4uV/SQR(Hz) ("Noise" on p.5 of the datasheet). To get the noise voltage (RMS) multiply with SQR(bandwidth-in-Hz). With the bandwidth switched to the max. of 50kHz, the noise voltage at the output would be 0.894mV RMS. That is alot for an input signal of only 1mV (RMS?). With a bandwidth of only 100Hz, the noise voltage is only 40uV RMS. The main amplifier also contributes some noise to the output but it is probably very little in comparison to the ISO-amp and can be ignored.

Fig. 12 in the datasheet shows the unfiltered 20mVpp ripple from the 500kHz clock feedthrough plus the broadband noise from the ISO-amp. Fig. 13 shows only the broadband noise at the output after being filtered by the additional 50kHz Sallen-Key lowpass filter. The 500kHz ripple is gone (buried in noise). The broadband noise has at least 6...7mVpp. Divide this by 6.6 to get the RMS value.

In LTspice, how come the frequency response is same with both power supply 15V inputted and no power supply?
here is the file, pls check it out, remove the battery and see the frequency response is the same

I don't have LTspice or else I would have already done the necessary changes for you. In the simulation software I am using, there are virtual opamps that have default/built in supply voltages and hence don't need external power supplies. I guess LTspice has something similar?

[magic]

I have spent just that 2 hours putting the power supply. I did. However the frequency response is still the same with or without a power supply (15V vs 0V/not connected).

Sorry, it hasn't occurred to me that you are using an opamp model which ignores power supplies.
I downloaded this file and it appears to be working correctly now, with or without the 15V supplies.
I don't know why it was showing -160dB initially, you must have changed something that fixed it.

[loop123]

I have spent just that 2 hours putting the power supply. I did. However the frequency response is still the same with or without a power supply (15V vs 0V/not connected).

Sorry, it hasn't occurred to me that you are using an opamp model which ignores power supplies.
I downloaded this file and it appears to be working correctly now, with or without the 15V supplies.
I don't know why it was showing -160dB initially, you must have changed something that fixed it.

The circuit was originally a 3rd order Butterworth using the MCP607. I edited the drawing to delete some lines and change the resistors and capacitors. When a 2nd order has short from V- to output. I tried to use draw lines but incorrect added lines so the line is not connected. After I change the line again from -V to output, it connects and the values are correct.

In 2 Pole Sullen-Key. You can put any op-amp and the frequency response is mostly identical? Because the amp in all files is still modeled after MCP607. I didn't change anything and just assume the LF411CP has same values or functions.

[Smokey]

Just a heads up since people are still typing "Sullen"... it's "Sallen-Key" with an "a".  It's two dudes' names.
https://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology

The dudes were R.P. Sallen and E.L. Key of MIT's Lincoln Labs in 1955.

[magic]

In 2 Pole Sullen-Key. You can put any op-amp and the frequency response is mostly identical? Because the amp in all files is still modeled after MCP607. I didn't change anything and just assume the LF411CP has same values or functions.

Closed loop response is determined by the feedback network, as long as gain bandwidth product is high enough. For only a few kHz BW and unity gain anything works, with exception of some exotic ultra low power types.

[RFDx]

1000Hz is enough for me because EMG is just 1000Hz and below. So how do I remove the ripples at 1000Hz? Maybe by changing the capacitor values to higher ones? what values for C1 and C2 do you recommend for the ripples to vanish, something where corner frequency is just about 1kHz

The ripples are not at 1000Hz. The "ripple" seen in Audacity is actually broadband noise that occupies the entire effective bandwidth. You can't get rid of the noise but you can get around it.
The first step, as you also suggested, would be to change the bandwidth of the S&K LP-filter to 1kHz and thereby lowering the noise voltage contributed by the ISO-amp by a factor of ~7.07 [SQR(50000/1000)]. Two suggestions (2nd & 3rd order S&K LPF) with component values are attached.

Your input signal is very small and the noise voltage is still substantial, even after the 1kHz filter, which means the signal to noise ratio (SNR) after the ISO-amp/filter will be rather modest. To raise the SNR, run the input signal through a low noise preamp with a gain of 100 (or even 1000). The amplified signal then passes through the noisy ISO-amp, the S&K LP-filter and last but not least, if neccessary, through a 100:1 (1000:1) attenuator to recover the original amplitude of the input signal (2mV). Despite the attenuation stage, the SNR will be (mostly) retained and the signal will be free of visible noise and look clean. The attached screenshot shows the difference between the two noisy output voltages with no amplification at all and amplification by a factor of 100, including the recovery of the original 2mV signal with the help of a simple attenuation stage.

Now that you know the ripple is 500kHz.. So there are no ripples inside the filter passband (which obviously can't be attenuated at all as your described)? So the noises I'm seeing in the waveforms below (see the message) are not from the ripples but from an inherent "noise spectral density of the ISO122" being 4uV/SQR(Hz) that is not related to the ripples at all?  These are two independent thing? I can remove the ripples using bigger capacitor values to make the bandwidth smaller but I can't remove the noise spectrual density because it is within the passband. is all these what you mean?

Exactly. The sufficiently suppressed 500kHz ripple form the modulator is not the actual problem, the broadband noise, added by the ISO-amp to the signal, is.

Please confirm so I know how junk the ISO122p is. However, if I put 100 or 1000 gain before the ISO. Would it produce clean signal at all? That means the amplifier before it would be almost like the main amplifier, i don't know if the DC-DC converter can supply current to it. And the output will be 1V and the main amplifier won't be useful at all because it would just accept it as 1X?

The ISO-amp is definitely not junk, it just doesn't do well with small input signals. Before adding a low noise preamp, check out yourself what happens when you increase the input signal. Inject 202mV instead of 2mV into the amp, insert a 100:1 attenuator (1kOhm + 10Ohm resistors) after the S&K LP-filter  and take a look at the output signal in Audacity. Does it look cleaner?
A low noise amp needs only a few mA. I don't see the DC-DC converter having problems supplying the additional current.

[loop123]

But why is the ISO122 signal clean if the filter is set to 100Hz? Also even without using the Sullen-key and tapping directly the ISO122, the 90Hz, 2mV signal is clean with 100Hz cutoff selected in the main unit.

Less bandwidth equals less noise voltage at the output riding on top of the actual signal. The overall bandwidth is determined by the 100Hz bandwidth of the main amplifier. Compared to the full 50kHz, the noise voltage will be smaller by a factor of SQR(50000/100)= 22.36 when the bandwidth is switched to 100Hz.

How about an opto-isolator? Can it accept 50kHz without any ripple? What is the maximum bandwidth of opticoupler?

You would need an analog optocoupler. The datasheet will probably give a hint to how much noise is to be expected. Depending on the mode of operation, the max bandwidth is maybe 50...100kHz.

How about putting a low noise preamp with a gain of 100 (or even 1000) in front of the ISO-amp?

Btw.. i couldn't find in the ISO122P data sheet the noise at 1mV. Where did you read it?

The noise spectral density of the ISO122 is 4uV/SQR(Hz) ("Noise" on p.5 of the datasheet). To get the noise voltage (RMS) multiply with SQR(bandwidth-in-Hz). With the bandwidth switched to the max. of 50kHz, the noise voltage at the output would be 0.894mV RMS. That is alot for an input signal of only 1mV (RMS?). With a bandwidth of only 100Hz, the noise voltage is only 40uV RMS. The main amplifier also contributes some noise to the output but it is probably very little in comparison to the ISO-amp and can be ignored.

In your comments above. The application of the formulas for suppressing broadband noise is via selecting 100Hz in the switch in the main amplifier. You were not describing changing the 2 pole LF411 capacitor values. But in the following statement. You were describing same formulas but changing the 2 pole values. Im a bit confused. If suppression of the broadband noises can occur (right?) from either changing 2 pole capacitor values or selecting the 1000Hz cut off filter switch in main amplifier. why cant I just select the switch to 1000Hz and avoid changing the 2 pole capacitor values to 1000Hz cutoff? What would happen when both 2 pole values is changed and selecting the main switch to 1000Hz filter? Isnt it either will work. what is advantage by doing both? Thanks. you wrote:

"The first step, as you also suggested, would be to change the bandwidth of the S&K LP-filter to 1kHz and thereby lowering the noise voltage contributed by the ISO-amp by a factor of ~7.07 [SQR(50000/1000)]. Two suggestions (2nd & 3rd order S&K LPF) with component values are attached."

[RFDx]

Im a bit confused. If suppression of the broadband noises can occur (right?) from either changing 2 pole capacitor values or selecting the 1000Hz cut off filter switch in main amplifier. why cant I just select the switch to 1000Hz and avoid changing the 2 pole capacitor values to 1000Hz cutoff?

I had the impression you wanted to lower the bandwith to 1kHz, my bad. It is perfectly fine to keep the original components (6.8n/3.3n/LF411) in the S&K LPF and switch the biomed amp to 1kHz.

What would happen when both 2 pole values is changed and selecting the main switch to 1000Hz filter? Isnt it either will work. what is advantage by doing both? Thanks.

Not much. The overall filtering would be tighter with somewhat less noise but not enough to make a real difference in the end.

[Andy Chee]

I'll just put a resistor in series to the batteries to make sure it won't reach greater than 40mA for example, and avoid the ISO122 and DCP010512DBP  altogether.

Thanks a lot!

No.  The correct location for the resistor is in series with your skin probe wire connection.

[RFDx]

Thanks. The ISO122 is powered by an isolated DC-DC converter called  DCP010512DBP see datasheet at  https://rocelec.widen.net/view/pdf/6mnzrokr1h/sbvs012f.pdf?t.download=true&u=5oefqw

It has input of 5 Volts and output of 12 Volts 83mA. However my ISO-Z isolation head stage and the main unit is only powered by batteries. 12 pcs of 1.2v rechargeable NiMH batteries for total of 1.2V x 12 = 14.4V, but since the main unit uses V+ and V- then it got it from the following configuration where each V+ is 7.2V and V- is -7.5V. I never use any AC-DC adaptor so no problem about isolating ground of AC.

You don't need an ISO-amp or a DC-DC converter at all. The biomed amp is powered from batteries, is not earth referenced and already floating.

Most uses of Isolated DC-DC Converter is to isolate the main power ground. But my use is to protect from 90mA from batteries. A sailor who used multimeter died from 9V battery after it reach 90mA (maybe because of defective multimeter and he punctured his skin), see https://darwinawards.com/darwin/darwin1999-50.html It's in the official Navy guidelines, true?

Dying from a 9V block battery? That's BS. A fairytale probably told to gullible new navy cadets for some reason. 

Do you think the all the components can be supported by 83mA?

It has the following chips:

1 pc AMP01
1 pc 555 tiimer
1 pc G6H-2 relay
1pc LF412CP
3 pcs TLE2061AIP
1 pc LM386.
and other components like capacitors, resistors, diodes, etc.

Can they be powered by 83mA?

I have no clue. How about measuring the current?

[loop123]

Thanks. The ISO122 is powered by an isolated DC-DC converter called  DCP010512DBP see datasheet at  https://rocelec.widen.net/view/pdf/6mnzrokr1h/sbvs012f.pdf?t.download=true&u=5oefqw

It has input of 5 Volts and output of 12 Volts 83mA. However my ISO-Z isolation head stage and the main unit is only powered by batteries. 12 pcs of 1.2v rechargeable NiMH batteries for total of 1.2V x 12 = 14.4V, but since the main unit uses V+ and V- then it got it from the following configuration where each V+ is 7.2V and V- is -7.5V. I never use any AC-DC adaptor so no problem about isolating ground of AC.

You don't need an ISO-amp or a DC-DC converter at all. The biomed amp is powered from batteries, is not earth referenced and already floating.

Most uses of Isolated DC-DC Converter is to isolate the main power ground. But my use is to protect from 90mA from batteries. A sailor who used multimeter died from 9V battery after it reach 90mA (maybe because of defective multimeter and he punctured his skin), see https://darwinawards.com/darwin/darwin1999-50.html It's in the official Navy guidelines, true?

Dying from a 9V block battery? That's BS. A fairytale probably told to gullible new navy cadets for some reason. 

Do you think the all the components can be supported by 83mA?

It has the following chips:

1 pc AMP01
1 pc 555 tiimer
1 pc G6H-2 relay
1pc LF412CP
3 pcs TLE2061AIP
1 pc LM386.
and other components like capacitors, resistors, diodes, etc.

Can they be powered by 83mA?

I have no clue. How about measuring the current?

But remember when measuring using ECG or EEG. the skin is abraded to lower the resistance (from average surface resistance of 50kOhm or more). What if there is wound not seen, then the resistance can go to 100 Ohm. and using the 9V battery example...  I =V/R = 9/100Ohm = 90mA. If you use 15V battery. Even high amps. What if it gets into contact with the electrode leads for example, an insect crawl inside the unit and Vs got shorted to electrode.  Why is this not caused for concern?

Btw.  When I used the main amp directly injecting it with 90Hz 5mV signal generator using Netech ECG simulator. It is ok with switch of  100Hz or even 1000Hz in main amp. But when I switched it to 3000Hz filter. I noticed the sine wave is more jagged line. I can't decide if it is the sine wave generator that can do that. Does sine wave generator also produce noise in the component with uniform high frequency noise even if the sine wave is only 90Hz?  Or is it the main amp noise? But at only 3000Hz. Why does noise come out already? It used the low noise AMP01 Instrumentation amplifier.. See    https://www.analog.com/media/en/technical-documentation/data-sheets/amp01.pdf                 the noise seems to be at nV only.   What is the real noise of AMP01 at 3000Hz?

[Andy Chee]

Most uses of Isolated DC-DC Converter is to isolate the main power ground. But my use is to protect from 90mA from batteries. A sailor who used multimeter died from 9V battery after it reach 90mA (maybe because of defective multimeter and he punctured his skin), see https://darwinawards.com/darwin/darwin1999-50.html It's in the official Navy guidelines, true?

Dying from a 9V block battery? That's BS. A fairytale probably told to gullible new navy cadets for some reason. 

But remember when measuring using ECG or EEG. the skin is abraded to lower the resistance (from average surface resistance of 50kOhm or more). What if there is wound not seen, then the resistance can go to 100 Ohm. and using the 9V battery example...  I =V/R = 9/100Ohm = 90mA. If you use 15V battery. Even high amps. What if it gets into contact with the electrode leads for example, an insect crawl inside the unit and Vs got shorted to electrode.  Why is this not caused for concern?

You need to think about battery internal impedance.  You can test this yourself!  Put a 100ohm resistor between +ve and -ve and measure the battery voltage, and watch it go down.  Lower voltage means lower current.

[loop123]

Most uses of Isolated DC-DC Converter is to isolate the main power ground. But my use is to protect from 90mA from batteries. A sailor who used multimeter died from 9V battery after it reach 90mA (maybe because of defective multimeter and he punctured his skin), see https://darwinawards.com/darwin/darwin1999-50.html It's in the official Navy guidelines, true?

Dying from a 9V block battery? That's BS. A fairytale probably told to gullible new navy cadets for some reason. 

But remember when measuring using ECG or EEG. the skin is abraded to lower the resistance (from average surface resistance of 50kOhm or more). What if there is wound not seen, then the resistance can go to 100 Ohm. and using the 9V battery example...  I =V/R = 9/100Ohm = 90mA. If you use 15V battery. Even high amps. What if it gets into contact with the electrode leads for example, an insect crawl inside the unit and Vs got shorted to electrode.  Why is this not caused for concern?

You need to think about battery internal impedance.  You can test this yourself!  Put a 100ohm resistor between +ve and -ve and measure the battery voltage, and watch it go down.  Lower voltage means lower current.

I read this thread where the professor himself (IanB)  wrote:

https://www.eevblog.com/forum/beginners/eneloop-aa-battery-short-circuit-current-and-other-questions/



I used the same 1.2V Eneloop rechargeable batteries NiMH 2500mAH. He wrote it could even reach 24A! 90mA and above could already be lethal. So I cant dare short the battery. I guess it is one of those "Do no try this at home".  Please share what current can go to your finger if  you have micro wounds and your resistance can get 100 Ohm. If the voltage is say 15V D.C.. It can reach I=V/R = 15V/100ohm = 150mA enough to stop your heart.  Please share the counterarguments because it seems I  heard little of electronic enthusiasts who got electrocuted by their circuit powered by batteries only.

Also if I'd put the resistor in series with the resistor. Let's say it is to avoid amperage greater than 5mA. Then the resistor has to be R=V/I = 15/0.005 = 3kOhm? Should it be put in all 3 input wires (+, -, ground/common) of the bioamp?

How do you know if the existing device has resistor in series in the input already? how to measure the presence of any resistor and values? Thanks.

[RFDx]

But remember when measuring using ECG or EEG. the skin is abraded to lower the resistance (from average surface resistance of 50kOhm or more). What if there is wound not seen, then the resistance can go to 100 Ohm. and using the 9V battery example...  I =V/R = 9/100Ohm = 90mA. If you use 15V battery. Even high amps. What if it gets into contact with the electrode leads for example, an insect crawl inside the unit and Vs got shorted to electrode.  Why is this not caused for concern?

You're confusing the impedance between electrode and skin, which is imo irrelevant in this context and at best a few kOhms considering wet electrodes, and the high to very high impedance between two arbitrary points on the human body. Never ever would a 9V potential difference be able to drive 90mA through the body nor can a few Volts more (15V) drive "high amps". Who told you this nonsense?

Btw.  When I used the main amp directly injecting it with 90Hz 5mA signal generator using Netech ECG simulator. It is ok with switch of  100Hz or even 1000Hz in main amp. But when I switched it to 3000Hz filter. I noticed the sine wave is more jagged line. I can't decide if it is the sine wave generator that can do that.

What is there to decide? It's very easy to find out by measuring the signal generator on it's own.
By the looks of it, the sinwave is generated digitally and then converted to analog by a low resolution DAC. A proper reconstruction filter also seems to be missing. You can even count the discrete steps on the sinewave.

[Andy Chee]

How do you know if the existing device has resistor in series in the input already? how to measure the presence of any resistor and values? Thanks.

You have already identified the resistors here:

[Andy Chee]

How do you know if the existing device has resistor in series in the input already? how to measure the presence of any resistor and values? Thanks.

You have already identified the resistors here:

No. The above was the schematic of the ISO-Z where the ISO122P is located. It's not the main amp which I haven't trace yet.

If you want electric shock protection, then use the same resistor connection as the schematic.  That's how the unit is designed for safety.

[Andy Chee]

How do you know if the existing device has resistor in series in the input already? how to measure the presence of any resistor and values? Thanks.

You have already identified the resistors here:

No. The above was the schematic of the ISO-Z where the ISO122P is located. It's not the main amp which I haven't trace yet.

If you want electric shock protection, then use the same resistor connection as the schematic.  That's how the unit is designed for safety.

My bad. The pcb in my last message was still the ISO-Z. The following is the main amp pcb I haven't traced yet. Maybe it already has the resistor?

As the two units are designed to connect together and work together, I am going to guess no.  In other words, the ISO-Z does all the safety protection.