Last days I tried to reduce noise levels in the ohmmeter and I can say that I got much better results than expected. Now the noise is between 2 and 3 uV peak to peak for period of 8.5 hours, instead 19 uV before the changes:
The drift which is visible on the chart is probably due the temperature changes last night. However, most of the noise was about 2uV and there were some spikes to up 3uV. Which is very close to the dominant peak to peak noise from datasheet of the LTC2052 op-amp buffer (1.5 uV). If I calculate the worst case peak to peak noise, it will be 2.69 uV : ADC 0.565 uV+ LTC2052 1.5 uV + LTC6655-2.5 0.625 uV.
The changes which I made to get these low noise levels are:
- Using separate supply for the uController. Before I used one shared USB power supply for both ADC and the uController. This change gave the biggest drop in the noise.
- Using a low noise LDO to supply the LTC2052 buffer of the ADC. This change gives additional 1-2 uV noise drop.
- Adding bypass capacitors for each IC on the breadboard.
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