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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 20, 2024, 07:18:58 pm ---
--- Quote from: loop123 on March 20, 2024, 02:12:06 pm ---Are you supposed to look at the peak or black lines? What do they mean? In the 1000Hz bandwidth case, the peak is 3.91uV/Sqrt (Hz), divided it by 10X gain is 0.391uV/Sqrt (Hz) or 391nV/Sqrt (Hz). Is this noise only for 1000Hz?  But then is it not you multiply 391nV/Sqrt (Hz) x Sqrt (Bandwidth) to get noise Vrms. So 391nV/Sqrt (Hz) x Sqrt (1000) = 391nv x 31.62 = 12364nV ??

--- End quote ---

Ignore the peak line. Only the black "mean" line is relevant. The black line is the output-referred noise density, it tells you the amount of noise contained with a portion of the frequency spectrum.

If you have a frequency range of interest, then to compute the rms noise, requires integration. Where the noise density is constant (white noise) over the frequency band of interest, this can be simplified to N * Sqrt (BW) where BW is the bandwidth, and N is the representative noise density within the band. If the noise density is not constant, then numerical integration is required.


--- Quote ---In the case of the 100Hz bandwidth, the peak is 469.7nV/Sqrt (Hz) / 10 gain = 46.97nV/Sqrt (Hz)  Should you multiply this by Sqrt (100Hz bandwidth) to get nV rms?

--- End quote ---

The peak measure is not relevant. Also, the 469.7 nV/Sqrt(Hz) measure appears to have been taken at 1000 Hz, which is outside of the band you are interested in and not representative of the band you are interested in, so this measure is irrelevant.

If all you are interested in is noise rms, then you don't need the noise density. You just need to filter your signal, and measure the amplitude rms directly. You already have an amplifier with built in filters. Conveniently, the REW software will measure and display rms voltage at the top right of the display.

So, by way of example, looking at your 1000 Hz filtered recording analysis. The overall noise amplitude at amplifier output is 74.5 uV, giving an input referred noise of 7.45 uV rms.

You mentioned in another thread, that a test signal you want to measure has an amplitude of 3.4 uV rms (10 uV p-p sine wave). As you can, see the expected signal to noise ratio is less than 0.5, meaning that your signal is unlikely to be recovered in a useful manner.

Your amplifier and recording setup will likely have much better performance with the amplifier set to higher gain. You should try repeating the measurements with the amplifier configured more appropriately for measuring signals of such a low amplitude (try a gain of 10k).

--- End quote ---

the following is taken with 10k gain (in both 1000Hz bandwidth selected)



so the Vrms of 36.28mV /10000 gain = 0.000003628 or 3.628uV rms noise?

and the noise density is 459.3uV/sqrt (Hz) /10000 gain = 0.0000000459 or 46nV/Sqrt (Hz)

for the following taken with maximum 50k gain



noise Vrms = 2.123V / 50k = 0.00004246  or 42.46uV rms... does it make sense? remember for my previous 10uV 50Hz signal you analyzed; the sine wave could still be resolved. Here the noise is much larger than it.

for noise density:  280.5uV/sqrt (Hz) / 50k gain = 5.61nV/Sqrt (Hz). 

Should the noise density be about 45nV/Sqrt (Hz)?   (5nV/Sqrt (Hz) for AMP01,  25nV/Sqrt (Hz) for LF412, 15nV/Sqrt (Hz) for other components like resistors in the circuit) Why 5.61nV/Sqrt(Hz)?

Also what are those peaks at 2kHz and so on not there at 10k gain?

WatchfulEye:

--- Quote from: loop123 on March 20, 2024, 10:51:00 pm ---noise Vrms = 2.123V / 50k = 0.00004246  or 42.46uV rms... does it make sense? remember for my previous 10uV 50Hz signal you analyzed; the sine wave could still be resolved. Here the noise is much larger than it.

--- End quote ---

Something is wrong at 50k gain. The ADC is clipping, leading to severe signal distortion (You can see the "max sample 0 db" which indicates that the ADC has clipped). There is probably a DC offset somewhere. When doing these noise measurements, you are shorting the amplifier inputs to each other and to common, aren't you? If the inputs are properly shorted, you will need to adjust the DC offset on the amplifier to bring it back into range.

At these high gains, you may need to adjust the DC offset from time to time.


--- Quote ---Should the noise density be about 45nV/Sqrt (Hz)?   (5nV/Sqrt (Hz) for AMP01,  25nV/Sqrt (Hz) for LF412, 15nV/Sqrt (Hz) for other components like resistors in the circuit) Why 5.61nV/Sqrt(Hz)?

--- End quote ---

That sounds about right - there are 2x LF412, 2x 5k resistors, and the rest of the amp. So, from datasheet figures that adds up to somewhere in the region of 40 nV/Sqrt Hz at 1 kHz.   

loop123:

--- Quote from: WatchfulEye on March 21, 2024, 12:40:14 am ---
--- Quote from: loop123 on March 20, 2024, 10:51:00 pm ---noise Vrms = 2.123V / 50k = 0.00004246  or 42.46uV rms... does it make sense? remember for my previous 10uV 50Hz signal you analyzed; the sine wave could still be resolved. Here the noise is much larger than it.

--- End quote ---

Something is wrong at 50k gain. The ADC is clipping, leading to severe signal distortion (You can see the "max sample 0 db" which indicates that the ADC has clipped). There is probably a DC offset somewhere. When doing these noise measurements, you are shorting the amplifier inputs to each other and to common, aren't you? If the inputs are properly shorted, you will need to adjust the DC offset on the amplifier to bring it back into range.

At these high gains, you may need to adjust the DC offset from time to time.


--- Quote ---Should the noise density be about 45nV/Sqrt (Hz)?   (5nV/Sqrt (Hz) for AMP01,  25nV/Sqrt (Hz) for LF412, 15nV/Sqrt (Hz) for other components like resistors in the circuit) Why 5.61nV/Sqrt(Hz)?

--- End quote ---

That sounds about right - there are 2x LF412, 2x 5k resistors, and the rest of the amp. So, from datasheet figures that adds up to somewhere in the region of 40 nV/Sqrt Hz at 1 kHz.

--- End quote ---

The inputs were not shorted but all floating. you mean I should short the +in, -in and ground together?

For the 10k gain. is the 46nV/Sqrt(Hz) (from 459.3uV/sqrt (Hz) /10000 gain) only for the 1kHz bandwidth selected or is it the formula for all bandwidth? I mean you multiply it by Sqrt (Hz bandwidth) to get the Vrms. but if the noise density is different for each bandwidth. how can you multiply each by Sqrt ( Hz bandwidth)? do you get what im saying here? I asked this several times pls elaborate on this. Many tnx.

loop123:

--- Quote from: loop123 on March 21, 2024, 01:06:03 am ---
--- Quote from: WatchfulEye on March 21, 2024, 12:40:14 am ---
--- Quote from: loop123 on March 20, 2024, 10:51:00 pm ---noise Vrms = 2.123V / 50k = 0.00004246  or 42.46uV rms... does it make sense? remember for my previous 10uV 50Hz signal you analyzed; the sine wave could still be resolved. Here the noise is much larger than it.

--- End quote ---

Something is wrong at 50k gain. The ADC is clipping, leading to severe signal distortion (You can see the "max sample 0 db" which indicates that the ADC has clipped). There is probably a DC offset somewhere. When doing these noise measurements, you are shorting the amplifier inputs to each other and to common, aren't you? If the inputs are properly shorted, you will need to adjust the DC offset on the amplifier to bring it back into range.

At these high gains, you may need to adjust the DC offset from time to time.


--- Quote ---Should the noise density be about 45nV/Sqrt (Hz)?   (5nV/Sqrt (Hz) for AMP01,  25nV/Sqrt (Hz) for LF412, 15nV/Sqrt (Hz) for other components like resistors in the circuit) Why 5.61nV/Sqrt(Hz)?

--- End quote ---

That sounds about right - there are 2x LF412, 2x 5k resistors, and the rest of the amp. So, from datasheet figures that adds up to somewhere in the region of 40 nV/Sqrt Hz at 1 kHz.

--- End quote ---

The inputs were not shorted but all floating. you mean I should short the +in, -in and ground together?

For the 10k gain. is the 46nV/Sqrt(Hz) (from 459.3uV/sqrt (Hz) /10000 gain) only for the 1kHz bandwidth selected or is it the formula for all bandwidth? I mean you multiply it by Sqrt (Hz bandwidth) to get the Vrms. but if the noise density is different for each bandwidth. how can you multiply each by Sqrt ( Hz bandwidth)? do you get what im saying here? I asked this several times pls elaborate on this. Many tnx.

--- End quote ---



Here I tied up the +IN, -IN and ground as you suggested. set to 50000gain, 1000Hz bandwidth, and not clipping.

integrated noise is 40.82mV / 50000 gain = 816nV rms or 0.816uV rms   It tallies to the 10uV 50Hz noise  you saw?

But for the noise density of 877uV/Sqrt (Hz) (in the RTA above)  divided by 50000 gain = 17.54nV/Sqrt(Hz)

Why is it smaller than the 45nV/Sqrt(Hz) that the components actually produced?             

Kleinstein:
The data with the shorted inputs looks already much better.
The point at 1000 Hz is aready at a point where the response / gain goes down. A more reasonable frequency to look at is 200 Hz or so, so well within the flat region.
The noise there is higher by maybe a factor 2 or a little less. This may still be a bit less than the estimated 40 or 45 nV/sqrt(Hz) for the front end, but no longer much. The stimates for the noise are a bit crude, as the actual noise performance of the LF412 may scatter - some can be better and some can be worse.

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