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.
If you add two noise sources, then their powers (or squared voltages) sum up, not their voltages
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.
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:QuoteIf you add two noise sources, then their powers (or squared voltages) sum up, not their voltages
Isn't 26.4 close to this value anyway? IMO it is in the ballpark.
I think what the last few measurements from the signal simulator show is the biggest problem of using an audio ADC and generic software for measurement. The software and the hardware are not integrated together and the settings are wrong in some way.
There appears to be a significant gain error, as the signal amplitude measured in the software does not correspond to the signal amplitude expected. We had guessed that the simulator generated a 10 uV p-p signal (3.4 uV rms), which after 50k gain, should have been measured as 170 mV - but instead the measurement is 127 mV. Earlier on we had assumed that the ADC was calibrated for gain, and that the 1.7 V setting corresponded to a 1.7 V rms sine wave (5V p-p). However, this may not be accurate.
Other possibilities are that the signal generator is not calibrated for amplitude, or that there is an error introduced because your amplifier is unable to accurately drive the low input impedance of your ADC. (The gain error is much larger when connecting the signal generator direct to the ADC, likely because as a biophysical simulator it simulates a high source resistance).
I think before proceeding further, it is necessary to verify the amplitude calibration of your measurements, as it is clear that something is wrong, and it is likely that all your measurements so far have been lower than the real value.
In the first instance, you could use the calibration signal in your amplifier, which should be 1 mV p-p, and using an appropriate setting (e.g. gain 1000) use the oscilloscope mode in your REW software to verify the amplitude of the resulting signal. There is theoretically another problem here which is that your amplifier's test signal is 10 Hz, and an audio ADC may not measure this faithfully. If you have a normal laboratory oscilloscope, you should also directly measure the output of your amplifier as a cross check. Similarly, a general purpose laboratory signal generator may also be useful as an additional signal source.
If you are going to be using this setup for scientific purposes, getting something as fundamental as amplitude correct is of great importance. Spending time getting this right strikes me as the most important step now.
The E1DA Cosmos ADC likely cannot accurately measure the netech simulator directly. The ADC has an input impedance of 640 Ohms, and is only suitable for measuring low impedance sources (the source impedance must be much lower than 640 Ohms whereas I would expect the netech simulator to simulate a higher source impedance).
The amplifier has a calibration signal source so you may as well use it. However, it is a square wave and the filters will distort it, so you will need to measure it with some oscilloscope software, rather than try to use rms measurements.
What oscilloscope software do you recommend (that you have personally tested)? You mean REW RTA cant measure peak to peak?
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.
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?
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?
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.
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?
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.
I think you need to take a step back and consider what you are trying to do.I'm doing a critical CERN-like experiment so need the most sensitive instrument like they do at the Large Hadron Collider. Remember the movie Interstellar where they need to collect data to finish the equations of quantum gravity and save the future. My experiment is like it.