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Message ID: 13419     Entry time: Wed Nov 8 16:39:24 2017
 Author: Kira Type: Update Category: PEM Subject: ADC noise measurement

Gautam and I measured the noise of the ADC for channels 17, 18, and 19. We plan to use those channels for measuring the noise of the temperature sensors, and we need to figure out whether or not we will need whitening and if so, how much. The figure below shows the actual measurements (red, green and blue lines), and a rough fit. I used Gautam's elog here and used the same function, $\sqrt{a^{2}(\frac{b}{f^{2}})+c^{2}}$ (with units of nV/sqrt(Hz)) to fit our results. I used a = 1, b = 1e6, c = 2000. Since we are interested in measuring at lower frequencies, we must whiten the signal from the temperature sensors enough to have the ADC noise be negligible.

We want to be able to measure to $\inline 1mK/\sqrt{Hz}$ accuracy at 1Hz, which translates to about $\inline 1nA/\sqrt{Hz}$ current from the AD590 (because it gives $\inline 1\mu A/K$). Since we have a 10K resistor and V=IR, the voltage accuracy we want to measure will be $\inline 10^{-5}V/\sqrt{Hz}$. We would need whitening for lower frequencies to see such fluctuations.

To do the measurements, we put a $\inline 50\Omega$ BNC end cap on the channels we wanted to measure, then took measurements from 0-900Hz with a bandwidth of 0.001Hz. This setup is shown in the last two attachments. We used the ADC in 1X7.