Noise Visualization

A project log for 22-bit Capacitance to Digital Converter

A new capacitance displacement sensor interface for my seismometer with improved performance and low cost.

bud-bennettBud Bennett 07/03/2019 at 16:010 Comments

The capacitance to digital converter is now working steadily at 2 sps. I have the data plotting to a web page to visualize the data. Here's raw data from the C2D:

You can see why the standard deviation of the noise varies from each group of samples. The signal has a non-time-invariant component. At first I thought that this was due to interference or temperature drift so I covered the board with electrical tape and then copper foil connected to GND. It did not change much. Here's a plot of 1 hour of data from the shielded unit:

There is a slight improvement, but not much. I have the unit located in an isolated room to reduce any external interference from me sitting next to it ('s that sensitive.) 

Eventually, started to think that the 1/f noise component of the OPA376 was causing a lot of what I was seeing. The plot above has a frequency range, or bandwidth, of roughly 0.0003Hz - 1Hz. If the noise density was flat there would be no increased noise voltage with an increased observation time period. But 1/f noise should show more noise as the observation period is increased. 

I resorted to Mathematica to help me understand what I thought I was seeing. Here's the analysis of the expected peak-peak counts observed over a 10 second period with a 2 sps ADC rate:

I just got lucky here -- it predicts about 10 counts of variation over a 10 second period. Thats about what I see on the above plots. I changed to observed period to 1 hour (3600 seconds) to see what to expect from the 1/f noise.

Lo and behold!!! The peak to peak variation rises to over 200 counts. So I'm betting that the low frequency noise terms will decrease dramatically when the MCP6V81 is substituted for the OPA376. The new opamps are in the mail. 

[Edit 2019-07-04: New information.]

I implemented a ringserver to collect data and allow high-pass and low-pass filtering of the data.  The C2D converter ran overnight and I collected the data: it's not noise...and it appears to be temperature related. The increasing drift as the house cools down at night is pretty obvious now. Along the way the various noise elements add and subtract from the trend at much lower numbers. The next step is to insulate the board and see if that reduces the temperature effect.

And here's the same data processed by a 0.01Hz to 1Hz 2-pole high-pass and 2-pole low-pass filters:

Just for grins and giggles I connected the AD7745 C2D sensor and plotted its filtered noise pattern without a sensor connected:

Both of these are ±4pF full scale output. It appears that we're moving in the right direction. The noise has improved by a factor ~4X over the AD7745, and I expect more improvement with better opamps.