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

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jonpaul:
read LT and ADI and BB notes and apps on noise in opamps.

See Jim Williams and Bob Pease.

j

gf:
Noise density is a function of frequency. If you want to specify it as a single number for a particular frequency range, then this number can only be the average density for that range. Only for white noise, this number is the same for any frequency range. I don't know the ENBW of the involved filter. If I assume 1kHz, then I'd roughly estimate an average density of 40.82mv/(sqrt(1000Hz)) = 1.29mV/(sqrt(Hz) for the region below 1kHz. Referred to the input, with 50000x gain, that would be ~26nV/sqrt(Hz) then. However, for an exact calculation of the average density in a particular frequency range, it would really be necessary to integrate power density over this range. [ Btw, note that you have to integrate power density over frequency, i.e. you integrate V²/Hz, not V/(sqrt(Hz)). If you add two noise sources, then their powers (or squared voltages) sum up, not their voltages. ]

loop123:



In the above I used 20Hz to 1000Hz in the distortion setting but the noise density didn't change, so it still integrates from 20Hz to 20kHz. How do you make it integrate over 20Hz to 1000Hz only? 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. It's not possible for the LF412 to become half noise only. That will be beyond milspec for them.

Can you manually try to integrate power density over frequency in the range above? using the V^2/Hz instead of V/Sqrt (Hz)? Would the result come close to 45nV/Sqrt (Hz)? Please compute.

Also I wonder if the gain of 50000 is really accurate. How do you test it? For example. For the 1.7Vrms in the E1DA. What voltage must appear in the 1 to -1 of Audacity. Is it 1.7Vrms? Maybe I can check whether 50000 gain is accurate by amplifying  say 10uV x 50000 times to come up with 0.5V and see if it would tally with the 1.7Vrms setting? I can use peak to peak or rms in the 10uV no problem since I'm not sure if the 10uV in the Netech signal generator is peak-to-peak or rms.

loop123:

--- Quote from: loop123 on March 21, 2024, 03:12:26 pm ---
(Attachment Link)

In the above I used 20Hz to 1000Hz in the distortion setting but the noise density didn't change, so it still integrates from 20Hz to 20kHz. How do you make it integrate over 20Hz to 1000Hz only? 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. It's not possible for the LF412 to become half noise only. That will be beyond milspec for them.

Can you manually try to integrate power density over frequency in the range above? using the V^2/Hz instead of V/Sqrt (Hz)? Would the result come close to 45nV/Sqrt (Hz)? Please compute.

Also I wonder if the gain of 50000 is really accurate. How do you test it? For example. For the 1.7Vrms in the E1DA. What voltage must appear in the 1 to -1 of Audacity. Is it 1.7Vrms? Maybe I can check whether 50000 gain is accurate by amplifying  say 10uV x 50000 times to come up with 0.5V and see if it would tally with the 1.7Vrms setting? I can use peak to peak or rms in the 10uV no problem since I'm not sure if the 10uV in the Netech signal generator is peak-to-peak or rms.

--- End quote ---



Here is another information. The above was taken with 10k gain (with same 1000Hz bandwidth chosen). You can easily calculate that the 271.8uV/sqrt (Hz) / 10000 gain = 27nV/Sqrt (Hz).  Still fall short of the expected 45nV/Sqrt (Hz) noise. Could it be the E1DA is not accurate at all? Or maybe the LF412 is not really used at all to condition signal to enter the AMP01? So the noises are due to the resistors and not really the LF412? WatchfulEye mentioned in another thread:

"The BF412 in the BMA-200 acts as a pre-amplifier with gain of 2 and very high input impedance. It also provides trimming for CMRR and DC offset.  Due to its near infinite input impedance and very low bias current, it has near zero current noise, so is ideal for buffering very high impedance signal sources.

The AMP01 is bipolar input, rather than FET input like the BF412. As such it has higher bias currents and higher current noise, which means that if the impedance of your signal is more than about 200 kOhms, it will likely give more noise than the BF412."

The input impedance is 10k-20k. My concern is. Could the AMP01 be used without the BF412 at all (why did he refer to the LF412 as BF412, are they identical?), like perhaps the AMP01 engineers have designed it such that you don't need another amplifier to require before it. So the LF412 (BF412) has other use?

Kleinstein:
The 200 Hz noise may still be plausible with the LF412. If would not be surprized to find lower noise with newer version and 20 nV/SQRT would still be plausible. So an overall noise of 26 nV/SQRT(Hz) would not be such a surprize (the noise adds as squares and thus not much added from the smaller parts). There is however still some oddity: the noise seems to be flat at all the way to some 20 Hz and thus only relatively little 1/f noise. So there may be something wrong with the front end / amplfiier circuit or a rather good example of the LF412. Less 1/f noise is possibly as an exception from better purity - not very likely, but possible.  I have seen TL031 with surprisingly low 1/f noise.

I don't see why the expected noise should be at 45 nV/sqrt(Hz) - more like 30 nV/sqrt(Hz with a bit more 1/f noise than observed.

A BF412 would normally be a BJT transistor and not an OP-amp.
As part of the amplifier there is also the possibility that the actually used part is not a LF412, but a different lower noise part.

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