General > General Technical Chat
using audio ADC for instrumentation use and probing noise floor of amplifiers
loop123:
Watchfuleye, what software is the RTA in your last message?
If it's the REW RTA.. how did you run it without the hardware? Did you use the 1000 Hz noise file?
the above was the RTA setting I used. Why can't you use FFT length of 4M instead of your 64k? or Averages of Exponential 0.97 instead of Forever?
You mean it's possible the E1DA has read them wrong reporting the 100nV/Sqrt (Hz) at 100Hz and 1uV/Sqrt (Hz) at 1000Hz? that was why you couldn't figure how they were derived? My original message was also about whether the E1DA can read noise floor accurate. It can't?
Edit: Or did you mean I must change the 1V rms to 1.7V rms in the RTA and the output will be accurate? Or must it be 5V peak to peak (changed to rms)?
WatchfulEye:
--- Quote from: loop123 on March 20, 2024, 03:50:58 am ---Watchfuleye, what software is the RTA in your last message?
If it's the REW RTA.. how did you run it without the hardware? Did you use the 1000 Hz noise file?
(Attachment Link)
--- End quote ---
It's the same software - REW. I just used my audio interface hardware.
--- Quote ---the above was the RTA setting I used. Why can't you use FFT length of 4M instead of your 64k? or Averages of Exponential 0.97 instead of Forever?
--- End quote ---
Longer FFT gives more frequency resolution but takes longer - You don't need the frequency resolution or low frequencies.
Because noise is noisy, you need good amplitude resolution to see the patterns. This needs a lot of averages. The default exponential moving average in the REW software is not enough.
--- Quote ---You mean it's possible the E1DA has read them wrong reporting the 100nV/Sqrt (Hz) at 100Hz and 1uV/Sqrt (Hz) at 1000Hz? that was why you couldn't figure how they were derived? My original message was also about whether the E1DA can read noise floor accurate. It can't?
Edit: Or did you mean I must change the 1V rms to 1.7V rms in the RTA and the output will be accurate? Or must it be 5V peak to peak (changed to rms)?
--- End quote ---
If you set the software scale to 1.7 V rms, I am confident that you will be getting accurate readings of output referred noise. You will need to divide by amplifier gain to get the input referred noise.
It will just be more useful if you have a clean plot - and this is obtained most quickly with short FFTs and lots of averages.
dobsonr741:
Given you only inspect the noise floor, and not a signal vs. the noise it’s a problem in the software domain, not an ADC part selection. It allows you to mismatch the dynamic range of the ADC and the DUT.
loop123:
--- Quote from: WatchfulEye on March 20, 2024, 10:51:10 am ---
--- Quote from: loop123 on March 20, 2024, 03:50:58 am ---Watchfuleye, what software is the RTA in your last message?
If it's the REW RTA.. how did you run it without the hardware? Did you use the 1000 Hz noise file?
(Attachment Link)
--- End quote ---
It's the same software - REW. I just used my audio interface hardware.
--- Quote ---the above was the RTA setting I used. Why can't you use FFT length of 4M instead of your 64k? or Averages of Exponential 0.97 instead of Forever?
--- End quote ---
Longer FFT gives more frequency resolution but takes longer - You don't need the frequency resolution or low frequencies.
Because noise is noisy, you need good amplitude resolution to see the patterns. This needs a lot of averages. The default exponential moving average in the REW software is not enough.
--- Quote ---You mean it's possible the E1DA has read them wrong reporting the 100nV/Sqrt (Hz) at 100Hz and 1uV/Sqrt (Hz) at 1000Hz? that was why you couldn't figure how they were derived? My original message was also about whether the E1DA can read noise floor accurate. It can't?
Edit: Or did you mean I must change the 1V rms to 1.7V rms in the RTA and the output will be accurate? Or must it be 5V peak to peak (changed to rms)?
--- End quote ---
If you set the software scale to 1.7 V rms, I am confident that you will be getting accurate readings of output referred noise. You will need to divide by amplifier gain to get the input referred noise.
It will just be more useful if you have a clean plot - and this is obtained most quickly with short FFTs and lots of averages.
--- End quote ---
These are taken with your suggested 1.7Vrms, 64k and forever.
100Hz bandwidth, 10X gain
1000Hz bandwidth, 10X gain
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 ??
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?
But then, how do you derive the base formula for nV/Sqrt (Hz) where you can just multiply it by Sqrt (100Hz) or Sqrt (1000Hz) to get Vrms?
WatchfulEye:
--- 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).
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