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

[loop123]

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.

In the ISO-Z.  The resistors have color code yellow-white-white-brown (which you can clearly see in the pcb, the blue resistors in bottom left). which is code for 4990 Ohm. Now if the input is 15V. the current is 15V/4990 = 0.003.. or 3mA. So you are right. it may be designed just to limit it to 3mA. But why didn't they just use 5000 Ohm instead of 4990 Ohm.

The circuit of the main amp is complicated. If there is also a resistor inside, then it would become series to existing and become 15/10k = 1.5mA. But wait, at 1mV input differential voltage, what is the current?

Btw.. the main amp is for use in rats. Yes. It's a biopotential for use in rats. They designed the ISO-Z for use in humans. But since I'd use batteries only. I can use the main amp without the ISO-Z. That's why I have to put safety in place because it made for rats!

[loop123]

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?

So the impedance requirements of 5kOhm (from 50kOhm using abrasive gel) is between electrodes and skin. But if there is a wound between the 2 electrodes. What is the lowest resistance the internal body can have, can't it be 100 Ohm? In the Darwinian article, it says I=V/R=9Volt/100 = 90mA.
 

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.

Oh. I didn't post the 1000 Hz noise and 3000 Hz noise when the Netech simulator is connected directly to the main amp (without using the ISO-Z).

In the following, the 1000Hz has less noise. The 3000Hz is noisy. You are saying both are made of discrete steps using low resolution DAC. I haven't thought of that. Thanks. But why is it that at 3000Hz. the discrete steps show more?

1000Hz:



3000Hz:



This is important because as I wrote in the message to Andy. I mentioned there is an INA114AP just before the ISO122 and I was able to use resistor to produce gain of 100 just like you said. And it has no visible broadband noise at 1000Hz. But at 3000Hz. There is a noise which seemed to be from the discrete steps. When I used the Netech directly into the main amp. There is the above discrete step noise at 3000Hz showing more than the 1000Hz. Again why? Note the LF411CP, 6800pf, 3300pf producing cutoff of 4.65kZ still has the 3kHz inside it so it shouldn't be producing broadband noise at 3000Hz.

With just batteries powering the main amp and ISO-Z. The ISO-Z may not be useful anymore except for the resistors which I can just add in the main amp. But just to be sure the ISO-Z can just add noise paths to the signal. What noises are in the isolated DC-DC converter. I heard it can produce noises and some avoid it like the plagues, but I haven't seen the noise. It is what can overwhelming make me not use the ISO-Z anymore. Many thanks!

[Andy Chee]

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?

So the impedance requirements of 5kOhm (from 50kOhm using abrasive gel) is between electrodes and skin. But if there is a wound between the 2 electrodes. What is the lowest resistance the internal body can have, can't it be 100 Ohm? In the Darwinian article, it says I=V/R=9Volt/100 = 90mA.
 

The lowest resistance is dependent on electrode separation and applied voltage. 

But for reference, the IEC states that at 25 volts dry skin, the hand-to-hand resistance for 95% of subjects it is <6100 ohms, for 50% of human test subjects is <3250 ohms, and for 5% of subjects it is <1750 ohms.

[loop123]

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?

So the impedance requirements of 5kOhm (from 50kOhm using abrasive gel) is between electrodes and skin. But if there is a wound between the 2 electrodes. What is the lowest resistance the internal body can have, can't it be 100 Ohm? In the Darwinian article, it says I=V/R=9Volt/100 = 90mA.
 

The lowest resistance is dependent on electrode separation and applied voltage. 

But for reference, the IEC states that at 25 volts dry skin, the hand-to-hand resistance for 95% of subjects it is <6100 ohms, for 50% of human test subjects is <3250 ohms, and for 5% of subjects it is <1750 ohms.

I read the resistance between ears is 100 Ohms. What would happen when one wires a 9V or 12V battery to the ears, it should produce 12/100Ohms = 120mA current to his brain? Would you feel a zap? 



https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html#:~:text=It's%20The%20Current%20That%20Kills&text=Individuals%20have%20been%20electrocuted%20by,as%2042%20volts%20direct%20current

[MrAl]

Yes, it ran. But do you see the vertical scale?

At -160dB you are simulating parasitic coupling through the feedback network or things like that...

There is a V1, AC1 in the left side. What is it? I thought it was the power supply. Where and how do you add the power supply? I don't know how to use LTspice Pls download the file in original thread and add it and upload it back so I can run the frequency response plot. Thanks.

V1 is the input signal.

LT Spice, in their wisdom, always use a DC generator symbol with + & - polarity markings on it, unlike the ac generator symbol which I am familiar with from decades of seeing it used.
That is why LTSpice annoys me so much.

If the generator is so marked, it is easy for someone to think it is a DC power supply source.

Just to note, AC generators also must have a polarity.  That's because they have a specific phase reference.
If you flip an AC generator, you reverse the phase so it would be 180 degrees out of phase with an AC generator that is not flipped.
If there were no polarity markings, you would not be able to tell which phase it was.

[MrAl]

Consider this Sullen-Key low pass filter.

(Attachment Link)

If you change the amplifier to LF411CP (with the same pins). How much would the frequency response change?

When I increase the value of the capacitor from 1000pF to 6800pF, the frequency get lowered. Why is that?

I found a Ltspice file using the third order filter (see attached). I changed it to the 2nd order Sullen-key by deleting some parts and changing the values. Please check if the entry is correct. Also the voltage source is reverse. This is ok for AC, isn't it? Second. What amplifier is in the Ltspice attached Ltspice file?. How do you change it to the LF411CP? I want to see if the frequency response would change. If it's hard to change. Please change it to LF411CP and attached the edited Ltspice.

(Attachment Link)

(Attachment Link)

What does ".ac oct 20 10 10000" mean? If I removed it, I can't run the simulator anymore.

Thank you.

When you use an op amp you also have to think about the slew rate.  The minimum slew rate for an op amp is:
sr=A*w
where
sr is the slew rate in volts per second,
A is the maximum peak amplitude of the sine wave,
w is the angular frequency 2*pi*f, with f in Hertz.

[Andy Chee]

I read the resistance between ears is 100 Ohms. What would happen when one wires a 9V or 12V battery to the ears, it should produce 12/100Ohms = 120mA current to his brain? Would you feel a zap? 

https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html#:~:text=It's%20The%20Current%20That%20Kills&text=Individuals%20have%20been%20electrocuted%20by,as%2042%20volts%20direct%20current

Please examine the voltages on your picture of the human body.  It is 110 volts, not 9 volts. 

The resistance of the human body does not behave like a standard component resistor.  At 9 volts, the human body resistance will be higher than your diagram.

[loop123]

I read the resistance between ears is 100 Ohms. What would happen when one wires a 9V or 12V battery to the ears, it should produce 12/100Ohms = 120mA current to his brain? Would you feel a zap? 

https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html#:~:text=It's%20The%20Current%20That%20Kills&text=Individuals%20have%20been%20electrocuted%20by,as%2042%20volts%20direct%20current

Please examine the voltages on your picture of the human body.  It is 110 volts, not 9 volts. 

The resistance of the human body does not behave like a standard component resistor.  At 9 volts, the human body resistance will be higher than your diagram.

Are you saying that for any ECG equipment this is battery operated There is 0% risk? No scenerio at all it can be dangerous? For example if there are wounds in the chest and the electrodes got short to Vs. There will still be high resistance at all time in the chest inches from heart even with micro wounds at skin surface? But if the dc voltages keep getting higher. what dc voltages would make it hazardous?
 

[Andy Chee]

I read the resistance between ears is 100 Ohms. What would happen when one wires a 9V or 12V battery to the ears, it should produce 12/100Ohms = 120mA current to his brain? Would you feel a zap? 

https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html#:~:text=It's%20The%20Current%20That%20Kills&text=Individuals%20have%20been%20electrocuted%20by,as%2042%20volts%20direct%20current

Please examine the voltages on your picture of the human body.  It is 110 volts, not 9 volts. 

The resistance of the human body does not behave like a standard component resistor.  At 9 volts, the human body resistance will be higher than your diagram.

Are you saying that for any ECG equipment this is battery operated There is 0% risk? No scenerio at all it can be dangerous? For example if there are wounds in the chest and the electrodes got short to Vs. There will still be high resistance at all time in the chest inches from heart even with micro wounds at skin surface? But if the dc voltages keep getting higher. what dc voltages would make it hazardous?

Please study the document standard IEC 60601 which relates to medical equipment.  This standard contains many of the answers to your questions regarding medical equipment electrical safety.

https://webstore.iec.ch/preview/info_iec60601-2-26%7Bed3.0%7Db.pdf
https://numlor.fr/tp/docs/IEC60601-1.pdf

https://en.wikipedia.org/wiki/IEC_60601

IEC 60601-2-25 Medical electrical equipment - Part 2-25: Particular requirements for the basic safety and essential performance of electrocardiographs
IEC 60601-2-26 Medical electrical equipment - Part 2-26: Particular requirements for the basic safety and essential performance of electroencephalographs
IEC 60601-2-40 Medical electrical equipment - Part 2-40: Particular requirements for the basic safety and essential performance of electromyographs and evoked response equipment

[loop123]

RFdx. To illustrate what I meant below. Here are the waveforms using the ISO-Z with the 100gain before the ISO122. There is no more bandwidth noise at 1000Hz. But at 3000Hz the discrete steps can be seen just like using the sine wave generator directly on the main amp. Maybe because of increased resolution that makes the steps more visible?





Setting is 90Hz, 1mV in Netech simulator. Gain in main amp is set to 50X because 100X can make it clip.
For the 1mV input, and gain of 100X before ISO122. The voltage in the output of the ISO-Z is 0.1V. With 50X gain in main unit. It's like the maximum voltage it can display without clipping is about 0.1x50 = 5Volt. I wonder if this is the limitation of Audacity or the main unit. I can't use 1000 gain before the ISO122 because with ISO-Z 1Volt output and 10X mininum gain in the main unit, 10V will clip, or maybe there is setting in Audacity to make it display 10V?

But where are the infamous noises of the isolated DC-DC converter in all the waveforms? If it doesn't appear, maybe the single 0.1uF capacitor at Vs of the isolated DC-DC converter removed all noises? If it's that simple, why so many complains about the noises of isolated DC-DC converters in general?

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?

So the impedance requirements of 5kOhm (from 50kOhm using abrasive gel) is between electrodes and skin. But if there is a wound between the 2 electrodes. What is the lowest resistance the internal body can have, can't it be 100 Ohm? In the Darwinian article, it says I=V/R=9Volt/100 = 90mA.
 

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.

Oh. I didn't post the 1000 Hz noise and 3000 Hz noise when the Netech simulator is connected directly to the main amp (without using the ISO-Z).

In the following, the 1000Hz has less noise. The 3000Hz is noisy. You are saying both are made of discrete steps using low resolution DAC. I haven't thought of that. Thanks. But why is it that at 3000Hz. the discrete steps show more?

1000Hz:



3000Hz:



This is important because as I wrote in the message to Andy. I mentioned there is an INA114AP just before the ISO122 and I was able to use resistor to produce gain of 100 just like you said. And it has no visible broadband noise at 1000Hz. But at 3000Hz. There is a noise which seemed to be from the discrete steps. When I used the Netech directly into the main amp. There is the above discrete step noise at 3000Hz showing more than the 1000Hz. Again why? Note the LF411CP, 6800pf, 3300pf producing cutoff of 4.65kZ still has the 3kHz inside it so it shouldn't be producing broadband noise at 3000Hz.

With just batteries powering the main amp and ISO-Z. The ISO-Z may not be useful anymore except for the resistors which I can just add in the main amp. But just to be sure the ISO-Z can just add noise paths to the signal. What noises are in the isolated DC-DC converter. I heard it can produce noises and some avoid it like the plagues, but I haven't seen the noise. It is what can overwhelming make me not use the ISO-Z anymore. Many thanks!

[RFDx]

RFdx. To illustrate what I meant below. Here are the waveforms using the ISO-Z with the 100gain before the ISO122. There is no more bandwidth noise at 1000Hz. But at 3000Hz the discrete steps can be seen just like using the sine wave generator directly on the main amp. Maybe because of increased resolution that makes the steps more visible?

The switchable filter in the main unit acts as reconstruction filter for the digitally generated, low samplerate sinewave from the Netech simulator. The sinewave becomes smoother as you lower the bandwidth.

Setting is 90Hz, 1mV in Netech simulator. Gain in main amp is set to 50X because 100X can make it clip.
For the 1mV input, and gain of 100X before ISO122. The voltage in the output of the ISO-Z is 0.1V. With 50X gain in main unit. It's like the maximum voltage it can display without clipping is about 0.1x50 = 5Volt. I wonder if this is the limitation of Audacity or the main unit. I can't use 1000 gain before the ISO122 because with ISO-Z 1Volt output and 10X mininum gain in the main unit, 10V will clip, or maybe there is setting in Audacity to make it display 10V?

Audacity is just a software. The limitation lies in the soundcard used to make the measurements. The max. input voltage the soundcard can handle is unknown.

But where are the infamous noises of the isolated DC-DC converter in all the waveforms? If it doesn't appear, maybe the single 0.1uF capacitor at Vs of the isolated DC-DC converter removed all noises? If it's that simple, why so many complains about the noises of isolated DC-DC converters in general?

The noise of the DC-DC converter is already quite low, even with small value decoupling capacitors. As the switching frequency of the converter is very high (400kHz) and there are multiple active filters with low cuttoff frequencies fitted to the ISO-Z/main amp, you won't see any noise at the output.

[loop123]

RFdx. To illustrate what I meant below. Here are the waveforms using the ISO-Z with the 100gain before the ISO122. There is no more bandwidth noise at 1000Hz. But at 3000Hz the discrete steps can be seen just like using the sine wave generator directly on the main amp. Maybe because of increased resolution that makes the steps more visible?

The switchable filter in the main unit acts as reconstruction filter for the digitally generated, low samplerate sinewave from the Netech simulator. The sinewave becomes smoother as you lower the bandwidth.

Setting is 90Hz, 1mV in Netech simulator. Gain in main amp is set to 50X because 100X can make it clip.
For the 1mV input, and gain of 100X before ISO122. The voltage in the output of the ISO-Z is 0.1V. With 50X gain in main unit. It's like the maximum voltage it can display without clipping is about 0.1x50 = 5Volt. I wonder if this is the limitation of Audacity or the main unit. I can't use 1000 gain before the ISO122 because with ISO-Z 1Volt output and 10X mininum gain in the main unit, 10V will clip, or maybe there is setting in Audacity to make it display 10V?

Audacity is just a software. The limitation lies in the soundcard used to make the measurements. The max. input voltage the soundcard can handle is unknown.

But where are the infamous noises of the isolated DC-DC converter in all the waveforms? If it doesn't appear, maybe the single 0.1uF capacitor at Vs of the isolated DC-DC converter removed all noises? If it's that simple, why so many complains about the noises of isolated DC-DC converters in general?

The noise of the DC-DC converter is already quite low, even with small value decoupling capacitors. As the switching frequency of the converter is very high (400kHz) and there are multiple active filters with low cuttoff frequencies fitted to the ISO-Z/main amp, you won't see any noise at the output.

We were talking about milliVolt input mostly prior. But now I'm focusing on microVolt input signal so I can zoom in the milliVolt to microVolt at will when changing between EMG and EEG in controlling the augmented cytoskeleton. You said above the noise of DC-DC converter is quite low. But this is with respect to milliVolt. In terms of microVolt, what is the noise of the DC-DC converter? And what kind of noise waveform to look for?

I plan to just use the DC-DC converter directly on the battery pack of the main amp. So it's bypassing those "multiple active filters with low cuttoff frequencies fitted to the ISO-Z/main amp".

Btw.. I received the DIP version of the OPA602 for the 2 pole Sallen-key but don't seem to need it now. But I'll try it when my 2nd sets of units arrived. I bought the duplicate to experiment on upgrading the main amp. But it seems the conclusions of the amp expert is there may be nothing significant to replace it with. Maybe you know something better than the AMP01. Thanks.

[Andy Chee]

But now I'm focusing on microVolt input signal so I can zoom in the milliVolt to microVolt at will when changing between EMG and EEG in controlling the augmented cytoskeleton.

If you are not doing so already, I suggest you begin familiarising yourself with existing BCI methodology.  You don't have to reinvent the wheel, you can learn from other people's experiments.

Use the academic search engine  https://scholar.google.com  to look for relevant scientific papers, like those attached below.

[MrAl]

Consider this Sullen-Key low pass filter.

(Attachment Link)

If you change the amplifier to LF411CP (with the same pins). How much would the frequency response change?

When I increase the value of the capacitor from 1000pF to 6800pF, the frequency get lowered. Why is that?

I found a Ltspice file using the third order filter (see attached). I changed it to the 2nd order Sullen-key by deleting some parts and changing the values. Please check if the entry is correct. Also the voltage source is reverse. This is ok for AC, isn't it? Second. What amplifier is in the Ltspice attached Ltspice file?. How do you change it to the LF411CP? I want to see if the frequency response would change. If it's hard to change. Please change it to LF411CP and attached the edited Ltspice.

(Attachment Link)

(Attachment Link)

What does ".ac oct 20 10 10000" mean? If I removed it, I can't run the simulator anymore.

Thank you.

Hi,

The most obvious answer is that this depends on the operating frequency vs the power bandwidth of the op amp.
The power bandwidth is a lower frequency number than the gain bandwidth product and that is because the slew rate comes into effect also.
If you do not want to calculate this then a very rough guide is to divide the bandwidth by 30 to 50.
For example, an LM358 has a gain bandwidth of 1MHz, but its power bandwidth is only around 30kHz with a 5v peak AC output.
It does depend on the output voltage amplitude too though, so if the output amplitude was lower the power bandwidth would go up.

If you choose a higher bandwidth op amp then it is not likely to alter the frequency response, but then you have to think about oscillations.

In short, op amps have their own frequency response and to know the effects of changing one out for another you have to do a few little calculations.  The power bandwidth calculation is the most important or else you could get a lot of distortion on the output.

The maximum frequency for any or the usual op amps is:
Fmax=2*pi*A
where
A is the peak amplitude of the output voltage of the op amp in volts.

It's interesting that the peak output of the op amp is more important to know than the gain bandwidth product.  That's because the slew rate is usually the limiting factor not the gain bandwidth product, and the slew rate works with the output amplitude in determining the max allowable operating frequency, so you also have to determine what your max output voltage peak will be.