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

To do the measurements, we put a 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.

Attachment 1: ADC-fit.png  45 kB  | Hide | Hide all | Show all
Attachment 2: IMG_20171108_162532.jpg  3.906 MB  | Show | Hide all | Show all
Attachment 3: IMG_20171108_162556.jpg  3.533 MB  | Show | Hide all | Show all
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