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

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WatchfulEye:
Bf412 is a typo. The circuit analysis is also wrong at that time. Lf412 buffer stage has gain of 1.09 (just for trimming purposes).

45 nV/Sqrt Hz was miscalculation based on wrong resistor values in schematic and also included additional allowance for 1/f noise at 50Hz and 10kohm source impedance. Based on 2x Lf412 using datasheet spec + 2x 5k input protection resistors, a better estimate would be 37 nV/Sqrt Hz. Still the datasheet is old and it is not impossible that manufacturing processes have improved.

The Lf412 is a JFET input amplofier and as such has very high impedance, very low bias current and very low current noise. This allows it to have consistent performance even with a very high source impedance. This is why it is used as the input stage.

The AMP01 has more bias current and more current noise. These will limit performance and introduce errors and noise with a high impedance signal source. With the problems that come with modifying an existing circuit which has been factory trimmed for precision, I don't think a major modification like bypassing the input stage is going to be a good Idea.

gf:

--- Quote from: loop123 on March 21, 2024, 03:12:26 pm ---Also using the 200Hz flat response portion, the noise density is 1.323mV/Sqrt (Hz) / 50000 = 26.4nV/Sqrt (Hz). Still too low considering AMP01 has 5nV/rtHz and LF412 has 25nv/rtHz noise.

--- End quote ---

Why? With gain=1 for the LF412 input stage, the combined noise power of 25nV/sqrt(Hz) and 5nV/sqrt(Hz) is still only ~25.5nV/sqrt(Hz). Please remember what I worte:


--- Quote ---If you add two noise sources, then their powers (or squared voltages) sum up, not their voltages

--- End quote ---

Isn't 26.4 close to this value anyway? IMO it is in the ballpark.

loop123:
Here is the part I need clarification above.

"The dynamic range can limit how much amplification one can have before the ADC. With the modern high resolution ADCs this is usually no longer an issue. E.g. the noise would still be way small than the 250 mV range of the ADC."

Can you provide actual values or example to illustrate what this means "The dynamic range can limit how much amplification one can have before the ADC"?

as for the next statement:

"E.g. the noise would still be way small than the 250 mV range of the ADC."

If your noise is  20uV and your signal is 10uV.  What if the 20uV noise is too small than the 250mV range of the g.USBamp, you cant resolve the 10uV. Can you?

Note the BMA uses AMP01 to amplify signal. While the g.USBamp doesnt have amplifier bec you map the signal direct to the ADC.

So my question is. Using purely ADC based amplifier like the USBamp. Can you somehow magically retrieve the signal at 10uV at 1000Hz even if the noises are akin to the BMA? And in connection to this thread. How can I test using the REW RTA on the g.USBamp? maybe there is another RTA software to piggy back on its own software??

I want to justify spending $3000 on the software if the g.USBamp has some magic in it the BMA doesnt. One thing it has magic on is it has DSP that can oversample and remove noises using moving averages and almost brick wall frequency response. I want to test it using REW RTA or similar software.

loop123:

--- Quote from: gf on March 21, 2024, 04:32:47 pm ---
--- Quote from: loop123 on March 21, 2024, 03:12:26 pm ---Also using the 200Hz flat response portion, the noise density is 1.323mV/Sqrt (Hz) / 50000 = 26.4nV/Sqrt (Hz). Still too low considering AMP01 has 5nV/rtHz and LF412 has 25nv/rtHz noise.

--- End quote ---

Why? With gain=1 for the LF412 input stage, the combined noise power of 25nV/sqrt(Hz) and 5nV/sqrt(Hz) is still only ~25.5nV/sqrt(Hz). Please remember what I worte:


--- Quote ---If you add two noise sources, then their powers (or squared voltages) sum up, not their voltages

--- End quote ---

Isn't 26.4 close to this value anyway? IMO it is in the ballpark.

--- End quote ---

Going back to the BMA. I inputted a 10uV, 50Hz signal to the BMA at 1000Hz, 50000 gain and tested it with REW with the same E1DA and all same settings. The noise is higher, why?

In our last RTA. The noise os 0.746uV rms and 26.4nV/Sqrt(Hz).

Now with input signal, the noise becomes 2.548uV rms and the noise density is 36.8nV/Sqrt (Hz). Why does noise increase when there is actual input signal? The noise happens to be the one that is supported by Audacity showing the 10uV waveform below with 2.548uV noise.







loop123:
I measured the Netech EEG simulator directly set at 2.5mV, 50Hz  connected to the E1DA (without the BMA connected).  This is the RTA. I was wondering if the Netech introduced additional noise of more than 1.8uV (difference between the 2.548uV (with input) minus 0.746uV (without input)). Are the noises floor supposed to be the same with input and without input?

What does the RTA in the following say? Also can you tell if it outputs in rms or peak to peak? what voltage does it output?

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