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using audio ADC for instrumentation use and probing noise floor of amplifiers
loop123:
I have 2 questions.
1. What is the negative effects of using an audio ADC specifically the E1DA Cosmos ADC in sampling voltages for instrumentation use? I know it blocks low frequency like 10Hz but how is its frequency response? Can it somehow distort the voltage sampling of microvolt? The E1DA Cosmos ADC is not for general audio use like listening to music. It's built only to measure noises of audio amplifiers so it doesn't have the normal front end audio shaping like done in other full fledge audio devices. But for the chip used. How is its frequency response (see below)?
https://e1dashz.wixsite.com/index/cosmos-adc
If you will say audio ADC is not good for sampling voltages, then what is the best ADC for instrumentation use with noise floor in the -128dB (A). see below.
2. The E1DA Cosmos ADC is very sought after in the audio world because it has a -128 dB (A) noise floor (or 6nV-7nV/Sqrt (Hz)). Since most amplifiers installed in complete equipment have total noise of 10nV/Sqrt (Hz) or higher. Can it literally be used to measure the amplitude of noise floor of other amplifiers (with no input)? I have tested them and don't know how accurate it is:
The following is the noise floor of the E1DA Cosmos ADC. I tested the Cosmos using latest REW RTA without any input connected.
It has such low noise of -128.2db (A) (in A-weighted value) (between 6 and 7nV/Sqrt (Hz) voltage noise). Can you find other ADC with such good noise? Again it's marketed for people to measure noises of audio amplifiers and not for music use. The reason it's not for music use is because they minimize all components to avoid noise.
I used the E1DA to measure an amplifier with a LF412 conditioning chip connected to the AMP01 amplifier (with no input) with gain set for 10X gain and bandwidth of 100Hz. This is the noise floor as measured by the E1DA.
The following is the noise with 10X gain and 1000Hz bandwidth chosen in the AMP01 amplifier. I chose 10X gain because it is the minimum, there is no 0 gain in the switch.
How accurate are the tests?
Can you use the E1DA to test all existing amplifier equipments? Will it accurately measure the noise floor of them like the above? In audio tests. They have standard bandwidth of 20,000 Hz. But if you will vary it to different bandwidths. Can the E1DA measure each bandwidth accurately or is it biased to measure only 20,000Hz audio bandwidth, and what is the technical reasoning if this is true? (Is it?)
Thank you.
MasterT:
FYI:
SAR is preferable IMHO, since it allows to split conversion and data transfer phases.
DS (SD) ADC may pickup less noise from data bus only if oversampling is internaly implemented, thats seems not a case with audio adc.
Kleinstein:
The main limitations with audio ADCs are the low frequency cut off, possible extra LF noise (seems to be not the problem here) and often a not that stable gain (e.g. gain TC could be > 10 ppm/K). For the noise tests on amplifiers the ADC is not that critical at all. In the usual test the Amplfier to test provides quite some gain. If needed an additional amplifier is between the DUT and ADC. This way one can measure amplifier noise (referrred to the input) much smaller than the ADC noise. The gain of the amplifier(s) effectively attenuates the ADC noise.
It depends on the exact ADC / audio card how bad the LF cut of is. The noise curve alone may not show it. One could compensate some of the drop off, but only a limited amount.
A SD ADC also has it's pros: it has much of the AA filtering included and is thus easier to use. The higher speed of an SAR type ADC would not be really useful as much of the extra BW would be lost to the AA filter transition range. So the sampling rates are not directly comparable.
The software the the audio ADC should also be able to measure noise with different bandwidth or calculate spectral noise density values, as shown in the last 3 graphs. The accuracy should be good enough - with noise one rarely cares so much about high resolution or high accuracy. So the gain drift is not an issue here.
WatchfulEye:
Subject to the limitations given by Kleinstein above, there is no reason why an ADC shouldn't be a particular problem as long as you are only interested in noise in the audio band (50-20 kHz).
There is a potential problem with using analysis software with an ADC which is not properly calibrated (or one or both are incorrectly configured), in that there may be systematic errors. For example, if your ADC has a +/- 5V full scale range, but your analysis software assumes +/-1 V FS, then there will be a systematic error with measured voltages being off by a factor of 5. For example, in the screenshots provided, the software is configured for a 1 V rms sine wave being full-scale, but this may not be a range supported by the E1DA unit. It is said to have a 1.7 V measurement range (among several other selectable ranges), but I don't know whether that is +/- 1.7V or 1.7V rms. This is important if you want numbers out. This particular ADC seems to have an active community on audio forums, and the designer of it seems to be happy to answer questions directly, so you should be able to find out how to set up your software and ADC from people experienced in its use.
Of course, if you are just looking for qualitative information (e.g. amplifier A is better or worse than B, or setting C is better than setting D), then this is less important.
Measuring amplifier noise floor is often not particularly demanding - because you are normally interested in "input referred" noise - and the amplifier will amplify its own noise. This means you can measure the output noise, and then divide the measurements by the amplifier gain. (Don't forget that amplifier noise floor can be sensitive to amplifier settings - especially gain, noise floor may be much higher at low gain).
loop123:
Something puzzles me. The noise density in the nV/Sqrt (Hz) should not change with the bandwidth because the AMP01 has constant 5nV/Sqrt (Hz) from 12 Hz to 10kHz as shown below. But why does my REW RTA showed about 100nV/Sqrt (Hz) at 100Hz and about 1uV/Sqrt (H) at 1000Hz? Shouldn't nV/Sqrt (Hz) be constant since it is the noise density where you multiply by Sqrt (Bandwidth) to get the nV rms?
The input range I chose is 1.7Vrms. But isn't it 1 rms = 6.6 p-p so it's 1.7Vrms x 6.6 = 11.22 Volts P-P.
My Audacity has 1 to -1 for exactly 5V p - p. Why is 1 to -1 not 11.22 Volts p-p? Btw.. when you use multimeter or dc output, is it in rms or peak-peak usually?
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