General > General Technical Chat
using audio ADC for instrumentation use and probing noise floor of amplifiers
WatchfulEye:
--- Quote from: loop123 on March 23, 2024, 02:05:53 am ---What oscilloscope software do you recommend (that you have personally tested)? You mean REW RTA cant measure peak to peak?
--- End quote ---
REW scope is fine. It can measure p-p (use the voltage cursor tool) and it uses the calibration setting from the RTA page.
David Hess:
Getting back to your original question, ADCs designed for audio often have poor DC specifications.
The bump in noise at low frequencies might be from aliasing of high frequency noise.
WatchfulEye:
Have you measured the noise using an identical setup? For example, using the RTA app. In other words directly compare the different op amps with signal inputs shorted to ground and all other settings the same.
loop123:
--- Quote from: WatchfulEye on March 27, 2024, 12:19:44 pm ---Have you measured the noise using an identical setup? For example, using the RTA app. In other words directly compare the different op amps with signal inputs shorted to ground and all other settings the same.
--- End quote ---
By shorting the inputs (NOT using the Netech) and all settings identical, just replacing the OPA2132P and LF412 to compare. The OPA2132P has lower noise! 26mV (541.6uV/Sqrt (Hz) vs 40mV (855.2uV/Sqrt (Hz) to the LF412. See below. But with the Netech connected, the noise is the same. Can the waveforms seen at Audacity be significant for 26mV vs 40mV noise?
This is the for OPA2132P
This is for the LF412
WatchfulEye:
--- Quote from: loop123 on March 27, 2024, 03:23:42 pm ---
By shorting the inputs (NOT using the Netech) and all settings identical, just replacing the OPA2132P and LF412 to compare. The OPA2132P has lower noise! 26mV (541.6uV/Sqrt (Hz) vs 40mV (855.2uV/Sqrt (Hz) to the LF412. See below. But with the Netech connected, the noise is the same. Can the waveforms seen at Audacity be significant for 26mV vs 40mV noise?
--- End quote ---
As expected, the measurements of the amplifier itself show significantly lower noise with the newer op amps.
However, when making a real measurement: there is also the signal source resistance. Any resistor will generate noise, and that includes the resistance of an electrode or biological specimen.
If your waveform generator is simulating a physiological signal with a high resistance, then there will be a noise contribution from the source resistance. Depending on the resistance, this could be the largest source of noise in your experiment.
Also remember that because noise adds as a root-sum-of-squares (you square the noise amplitude of all sources, add them, then take the square root), it is changes to the largest noise source which make the biggest difference, with changes to minor sources having disproportionaltely small effects.
If you are trying to measure noise density with your simulator connected, there is a problem. Your simulator produces a low fidelity sine wave with huge quantisation noise - the quantisation noise is broadband and not readily distinguishable from other sources of broadband noise. This will prevent any meaningful noise density measurement, even though the quantisation noise is visibly distinct on a waveform.
If you want to get a feel for the amplitude of the noise waveform, you could just record the noise with audacity and inspect the waveform. You could then repeat the recording with a 10k or 20k resistor, to see how much noise the resistance adds. The same experiment could be done with REW to get an rms measurement.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version