Besides the missing reliability of the Raspberry Pi, which could be solved by switching the power input to the GPIO pins, there was one point, which disturbed me a lot the last months: The high level of noise at the input of the ADS1262. To keep things simple I measured the input noise by short cutting the pin AIN0 and AONCOM. The measeared value should be zero but instead I got a noise with a size of around 16 bit. The noise level depends on many things like the protoboard design and the quality of the power source, but still, a noise level of around 16 bits was much to high in my opinion. That would mean, we could only use the left over 16 bit from the total of 32 bits, and then we could use a 16 bit ADC instead. So I tried all kind of improvements, like using a battery as a power supply, different other power supplies, different ground levels for AINCOM and some more things. Nothing really helped. Normally, using a battery as a power supply should give some better results, but since the ADS1262 was still coupled with the Raspberry Pi, the peaks from the power supply of the Raspberry couldn’t get removed. One reason of the peaks is probably the switch mode power supply unit on board of the Raspberry, which produces the 3.3 V from the 5 V input. In order to remove the coupling, optocouplers would be needed for all signal lines between the Raspberry Pi and the ADS1262, which would increase the complexity a lot.
Finally I tried one more thing: I used larger capacitors (47 µF and 100 nF) between the refout pin and ground and used the refout voltage as the potential for AINCOM. Wow, what a difference! The noise dropped by 4 bits downto 12 bits, which is a factor of 16! Now we have 20 noise free bits left at the ADC, which let me sleep happy again 🙂 .
There was a 1 µF capacitor between these pins already on the Protocentral breakout board, but maybe the capacitor was damaged since of my soldering from all the tests or the capacity was to small.
There are two capacitors (47 µF and 100 nF) between the refout pin and ground. The refout pin is pin 8 of the ADS1262, which doesn’t have an own pin at the Protocentral breakout board. Instead either pin 8 of the ADS1262 or the pin of the little capacitor on the board, which is connected to pin 8, must be used.
Also the other inputs AIN1 up to AIN9 are set to AINCOM level in order to prevent oscillation.