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

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loop123:
above is 2.5mV, 50Hz set in Netech connected directly to the E1DA without the BMA.

Below is 10uV, 50Hz set in Netech connected directly to the E1DA without the BMA.

Noise is 1.245uV. Is it accurate? does it mean with 10uV Netech output, there is noise of 1.245uV. And if the BMA is connected, it adds to the BMA 0.746uV noise to produce roughly 2uV noise in the 10uV output?  Also let me know if the output in both 2.5mV and 10uV is in rms or peak-to-peak. Many thanks guys.

WatchfulEye:
I think what the last few measurements from the signal simulator show is the biggest problem of using an audio ADC and generic software for measurement. The software and the hardware are not integrated together and the settings are wrong in some way.

There appears to be a significant gain error, as the signal amplitude measured in the software does not correspond to the signal amplitude expected. We had guessed that the simulator generated a 10 uV p-p signal (3.4 uV rms), which after 50k gain, should have been measured as 170 mV - but instead the measurement is 127 mV.  Earlier on we had assumed that the ADC was calibrated for gain, and that the 1.7 V setting corresponded to a 1.7 V rms sine wave (5V p-p). However, this may not be accurate.

Other possibilities are that the signal generator is not calibrated for amplitude, or that there is an error introduced because your amplifier is unable to accurately drive the low input impedance of your ADC. (The gain error is much larger when connecting the signal generator direct to the ADC, likely because as a biophysical simulator it simulates a high source resistance).

I think before proceeding further, it is necessary to verify the amplitude calibration of your measurements, as it is clear that something is wrong, and it is likely that all your measurements so far have been lower than the real value.

In the first instance, you could use the calibration signal in your amplifier, which should be 1 mV p-p, and using an appropriate setting (e.g. gain 1000) use the oscilloscope mode in your REW software to verify the amplitude of the resulting signal. There is theoretically another problem here which is that your amplifier's test signal is 10 Hz, and an audio ADC may not measure this faithfully. If you have a normal laboratory oscilloscope, you should also directly measure the output of your amplifier as a cross check. Similarly, a general purpose laboratory signal generator may also be useful as an additional signal source.

If you are going to be using this setup for scientific purposes, getting something as fundamental as amplitude correct is of great importance. Spending time getting this right strikes me as the most important step now.

loop123:

--- Quote from: WatchfulEye on March 23, 2024, 01:28:06 am ---I think what the last few measurements from the signal simulator show is the biggest problem of using an audio ADC and generic software for measurement. The software and the hardware are not integrated together and the settings are wrong in some way.

There appears to be a significant gain error, as the signal amplitude measured in the software does not correspond to the signal amplitude expected. We had guessed that the simulator generated a 10 uV p-p signal (3.4 uV rms), which after 50k gain, should have been measured as 170 mV - but instead the measurement is 127 mV.  Earlier on we had assumed that the ADC was calibrated for gain, and that the 1.7 V setting corresponded to a 1.7 V rms sine wave (5V p-p). However, this may not be accurate.

Other possibilities are that the signal generator is not calibrated for amplitude, or that there is an error introduced because your amplifier is unable to accurately drive the low input impedance of your ADC. (The gain error is much larger when connecting the signal generator direct to the ADC, likely because as a biophysical simulator it simulates a high source resistance).

I think before proceeding further, it is necessary to verify the amplitude calibration of your measurements, as it is clear that something is wrong, and it is likely that all your measurements so far have been lower than the real value.

In the first instance, you could use the calibration signal in your amplifier, which should be 1 mV p-p, and using an appropriate setting (e.g. gain 1000) use the oscilloscope mode in your REW software to verify the amplitude of the resulting signal. There is theoretically another problem here which is that your amplifier's test signal is 10 Hz, and an audio ADC may not measure this faithfully. If you have a normal laboratory oscilloscope, you should also directly measure the output of your amplifier as a cross check. Similarly, a general purpose laboratory signal generator may also be useful as an additional signal source.

If you are going to be using this setup for scientific purposes, getting something as fundamental as amplitude correct is of great importance. Spending time getting this right strikes me as the most important step now.

--- End quote ---

You are right. All the amplitudes dont correspond so let me calibrate first. In the following. I used the ADC to measure the Netech EEG Simulator set at 2.5mV, 50Hz. I selected the 1.7vrms switch at ADC and set also the FS sine Vrms in REW RTA to 1.7. But why do I get only 0.544mV instead of 2.5mV in the RTA voltage display attached? Is 544.5uV the noise or the signal amplitude? how do you make it display the 2.5mV signal input at RTA?  is it even possible to measure real voltage in RTA?

WatchfulEye:
The E1DA Cosmos ADC likely cannot accurately measure the netech simulator directly. The ADC has an input impedance of 640 Ohms, and is only suitable for measuring low impedance sources (the source impedance must be much lower than 640 Ohms whereas I would expect the netech simulator to simulate a higher source impedance).

The amplifier has a calibration signal source so you may as well use it. However, it is a square wave and the filters will distort it, so you will need to measure it with some oscilloscope software, rather than try to use rms measurements.

loop123:

--- Quote from: WatchfulEye on March 23, 2024, 01:59:39 am ---The E1DA Cosmos ADC likely cannot accurately measure the netech simulator directly. The ADC has an input impedance of 640 Ohms, and is only suitable for measuring low impedance sources (the source impedance must be much lower than 640 Ohms whereas I would expect the netech simulator to simulate a higher source impedance).

The amplifier has a calibration signal source so you may as well use it. However, it is a square wave and the filters will distort it, so you will need to measure it with some oscilloscope software, rather than try to use rms measurements.

--- End quote ---

What oscilloscope software do you recommend (that you have personally tested)?  You mean REW RTA cant measure peak to peak?

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