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using audio ADC for instrumentation use and probing noise floor of amplifiers

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loop123:

--- Quote from: WatchfulEye on March 27, 2024, 04:05:12 pm ---
--- Quote from: loop123 on March 27, 2024, 03:23:42 pm ---
By shorting the inputs (NOT using the Netech) and all settings identical, just replacing the OPA2132P and LF412 to compare. The OPA2132P has lower noise! 26mV (541.6uV/Sqrt (Hz) vs 40mV (855.2uV/Sqrt (Hz) to the LF412. See below. But with the Netech connected, the noise is the same. Can the waveforms seen at Audacity be significant for 26mV vs 40mV noise?

--- End quote ---

As expected, the measurements of the amplifier itself show significantly lower noise with the newer op amps.

However, when making a real measurement: there is also the signal source resistance. Any resistor will generate noise, and that includes the resistance of an electrode or biological specimen.

If your waveform generator is simulating a physiological signal with a high resistance, then there will be a noise contribution from the source resistance. Depending on the resistance, this could  be the largest source of noise in your experiment.

Also remember that because noise adds as a root-sum-of-squares (you square the noise amplitude of all sources, add them, then take the square root), it is changes to the largest noise source which make the biggest difference, with changes to minor sources having disproportionaltely small effects.

If you are trying to measure noise density with your simulator connected, there is a problem. Your simulator produces a low fidelity sine wave with huge quantisation noise - the quantisation noise is broadband and not readily distinguishable from other sources of broadband noise. This will prevent any meaningful noise density measurement, even though the quantisation noise is visibly distinct on a waveform.

If you want to get a feel for the amplitude of the noise waveform, you could just record the noise with audacity and inspect the waveform. You could then repeat the recording with a 10k or 20k resistor, to see how much noise the resistance adds. The same experiment could be done with REW to get an rms measurement.

--- End quote ---

What frequency in the Netech output should you use to compare with the resistor noise?  The one at 0.5Hz, 10uV setting at Netech simulator was close to the 10k Ohm resistance shorted value. Note it's difficult to compare noises by looking at the amplitudes at Audacity. They have somewhat same amplitudes, but it is only when the REW RTA was used that the noises could be distinguished.

The following was when 1k Ohm resistor was shorted in the input of the BMA. It's closed to the value when the input is totally shorted. But looking at Audacity, how can you compare the display at 1k ohm vs 10 k ohm? All results were from the OPA2132P chip.





The following is when 10k ohm shorted at input





The following is when the Netech simulator was used with input frequency of 0.1Hz and 10uV. I chose 0.1 Hz because it was the only way for the sine wave to be almost straight at the horizontal, to compare with the resistors shorted displays. It is at this 0.1Hz, 10uV Netech output setting where the RTA noise is similar to the 10k ohm resistor shorted value above.





The following is when the Frequency was increased to 2 Hz. There are 0.1Hz, 2Hz, 5Hz, 50Hz, 60Hz frequency options. I displayed them to ask you what frequency must I use to compare it to the 1k and 10k resistors shorted. As the frequency increases, the noises increased. The display at Audacity just show more sine wave as the amplitude is increased.



The following is when the frequency was increased to 5Hz.



The following is when the frequency was increased to 50Hz.




Since the noises at RTA matched at Netech 0.1Hz 10uV output and the 10k resistor shorted. Does it mean the noise of the Netech is like 10k resistor? If it is, then it's bad because how can you decrease the resistance of the skin to mere 100ohms. Even then, the shorted input noise is already high at 26.4mV as shared in last message.

loop123:

--- Quote from: WatchfulEye on March 27, 2024, 04:05:12 pm ---
--- Quote from: loop123 on March 27, 2024, 03:23:42 pm ---
By shorting the inputs (NOT using the Netech) and all settings identical, just replacing the OPA2132P and LF412 to compare. The OPA2132P has lower noise! 26mV (541.6uV/Sqrt (Hz) vs 40mV (855.2uV/Sqrt (Hz) to the LF412. See below. But with the Netech connected, the noise is the same. Can the waveforms seen at Audacity be significant for 26mV vs 40mV noise?

--- End quote ---

As expected, the measurements of the amplifier itself show significantly lower noise with the newer op amps.

However, when making a real measurement: there is also the signal source resistance. Any resistor will generate noise, and that includes the resistance of an electrode or biological specimen.

If your waveform generator is simulating a physiological signal with a high resistance, then there will be a noise contribution from the source resistance. Depending on the resistance, this could  be the largest source of noise in your experiment.

Also remember that because noise adds as a root-sum-of-squares (you square the noise amplitude of all sources, add them, then take the square root), it is changes to the largest noise source which make the biggest difference, with changes to minor sources having disproportionaltely small effects.

If you are trying to measure noise density with your simulator connected, there is a problem. Your simulator produces a low fidelity sine wave with huge quantisation noise - the quantisation noise is broadband and not readily distinguishable from other sources of broadband noise. This will prevent any meaningful noise density measurement, even though the quantisation noise is visibly distinct on a waveform.

If you want to get a feel for the amplitude of the noise waveform, you could just record the noise with audacity and inspect the waveform. You could then repeat the recording with a 10k or 20k resistor, to see how much noise the resistance adds. The same experiment could be done with REW to get an rms measurement.

--- End quote ---

The above tests used the latest OPA2132P. When I used the old LF412. I can see that even the output of the Netech Simulator has more noise (something I couldn't easily detect just looking at Audacity). I'd just share the results below of the  Netech output of 0.5Hz, 10uV vs 10k Ohm resistor connected to the BMA at 50k gain and 1000Hz.

This is Netech 0.1Hz, 10uV RTA using the LF412. This has more noise compared to the 36.73mV result using the OPA2132P chip.



This is 10k resistor shorted  at the BMA  RTA using LF412 too. This has more noise compared to the 33.97mV  result using the OPA2132P chip.



So the Netech at 10uV is like simulating 10k Ohm. But you said something about "If you are trying to measure noise density with your simulator connected, there is a problem. Your simulator produces a low fidelity sine wave with huge quantisation noise - the quantisation noise is broadband and not readily distinguishable from other sources of broadband noise. This will prevent any meaningful noise density measurement, even though the quantisation noise is visibly distinct on a waveform."

You mean the Netech at 10uV can't be likened to 10k ohm noise? But why is its quantization noise like 10k ohm?  but in last paragraph you just said to try comparing them.

Whatever. Since changing the buffer amp can lower the noise in both Netech and resistor tests. Then putting the 1nV/Sqrt (Hz) INA849 can produce even lesser noise at the Netech and 10k resistor tests??

What are the formulas again to compute what the noise in nV contributed by resistors? I want to see the total noise when the INA849 will be put (is this compatible by putting a second socket on top of the existing socket and rewiring the top socket of the AMP01??)


WatchfulEye:
I think you need to take a step back and consider what you are trying to do.

This thread was about suitability of an audio ADC for instrumentation use. The main problems mentioned are calibrating the gain and the frequency response. At present, the gain has been guessed but not checked, and frequency response remains unknown. It is also not clear what software you plan to use for analysis of your recordings and what it needs for calibration of gain.

There has been a lot of discussion about noise, but you don't seem to know what signal-to-noise ratio you need for your analysis. You have been trying to track down noise which appears in measurements of a simulator, but the amount and type of noise generated by the simulator is unknown, meaning that it may or may not be representative of real data. The noise hunt has been made more difficult because the noise measurements may be inaccurate as the gain of your measurement system is not calibrated.

I would suggest taking some time to calculate where your major noise sources are, and how they impact your signal-to-noise ratio, and in turn how they affect your analysis.

The formula for thermal noise in a resistor is easily found with google, and a writeup is found at https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise#Noise_voltage_and_power.

If you want to reanalyse your amplifier to see what changing various components makes then, I think main noise sources in the signal path are:
Source resistance (e.g. 10 kOhm)
2x 5k protection resistors
2x op amps (20 nV/sqrt Hz for the original LF412)
Instrumentation amp (5 nV/sqrt Hz for the AMP01).

You are interested in a bandwidth of 1 kHz:
So the calculation becomes:
10k Source resistance: 0.13 * Sqrt (10000) * Sqrt (BW) = 411 nV rms
2x 5k Protection resistors: Sqrt(2) * 0.13 * Sqrt (5000) * Sqrt (BW) = 411 nV rms
2x OP amps: Sqrt(2) * 20 * Sqrt (BW) = 894 nV rms
I amp: 5 * Sqrt (BW) = 158 nV rms

The noise powers sum:
Total = Sqrt (411 ^2 + 411 ^2 + 894 ^2 + 158 ^2) = 1078 nV rms










Andy Chee:

--- Quote from: loop123 on March 28, 2024, 10:32:47 pm ---
--- Quote from: WatchfulEye on March 28, 2024, 07:48:51 pm ---I think you need to take a step back and consider what you are trying to do.

--- End quote ---
I'm doing a critical CERN-like experiment so need the most sensitive instrument like they do at the Large Hadron Collider. Remember the movie Interstellar where they need to collect data to finish the equations of quantum gravity and save the future. My experiment is like it.

--- End quote ---
This is a red herring!  The poster is using the instrumentation to detect bioelectrical signals like EEG, EMG and ECG.  They are definitely NOT performing a particle physics experiment!

That's not to say their experiment is any more or less in value to the scientific world.  Just that from an engineering technical perspective, the poster is dealing with human biosignals.

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