I work on a 8 Channel 24-Bit A/D-Converter. I have an issue with the noise level of this converter. There are some "quiet" periods where the noise is low and acceptable and there are other times where the noise increases by a factor of 5. 
Y-Axis is in mV, X-Axis in minutes, sample rate is 2Hz, the pcb board electronic is in a aluminum case to reduce the general noise. The A/D conversion part of the electronics looks like this:
The AD converter is the AD7190 from analog devices. Prior to the A/D conversion part we use the following electronic to scale the incoming signals to the A/D converter range:
I tested the electronics connected to a function generator, with open inputs, in a case and without a case several times at different locations and i always get a behaviour like that…
My Question is what is the reason for this behavior and how can i "fix" it? many thanks in advance!
Best Answer
There could be many sources of noise, I'll cover a few:
1) Temperature. Temperature changes in the electronics can create noise in electronics, especially those of low level measurements. With ADC's you can find the temperature coefficents in the datasheet listed as ppm or in a graph like this:
Usually it's a good idea to temperature control low level analog electronics
The other problem is air on chips, take a cover and put it over the board if you haven't done so already.
2) Power and conducted emissions One of the biggest problems is common mode noise (noise getting in through the ground), if you have other loads that are switching these can cause problems (especially if the return currents for those loads cross the analog subsection of the PCB) DC to DC converters can wreck havoc on low level measurements, make sure your supplies are noise free, and you use good regulators for the power.
3) Radiated noise. Remember that 5/2^24 is 63nV, it's not that hard to get 63nV of noise from radio sources on your PCB (and that means even less before the amplifier, because the amplifier gains up noise). Make sure your inputs to the amplifiers and electronics are properly shielded.
Source: Art of Electronics 3rd ed