Ripple on ADC measurement - Page 1
Forum section: electronics / beginners

author: happy_hippo0
author_rank: Newbie
timestamp: March 14, 2021, 12:35:22 pm
content: Hello. This is my first post here and I'm quite new to practical electronics. I wanted to learn more by implementing an ADC board (ebay, ads1256 based) with an arduino, but I'm facing an issue that I cannot understand after a week of trying different things. I was wondering if someone could give me some advice or suggestions please.The board is a cheap ebay ads1256 implementationwith the schematic attached. I connected it to an arduino via SPI and programmed some code to read the data and convert to voltage (I think this part is fine). I then tried measuring some cheap power supplies 1-5V (I don't have a good one) and there was huge ripple - I though it was the quality of the supplies, so I switched to just a pack of AA batteries (1.5V). It sort of worked as expected, I did some offset and gain calibrations and was quite happy - I could measure at different data rates (averages) and reduce the noise reaching tens of uV level. Then I wanted to measure lower voltages, let's say in milli-volts and I connected the same battery (1.4299V) via a voltage divider: R1 = R2 = 5.388k Ohm and connected the ADC across one resistor, hoping to measure half of the value, which was ok, but with ~100-200 uV ripple.Looking in more detail at the frequency of the ripple - it is exactly 50 Hz, which I guess can come from the power supply. So, I disconnected the ads1256 board from the arduino 5V power output and connected a battery-powered 5V output, but it did not change the ripple (note the SPI lines were still connected). My only guess is that this is some sort of electromagnetic interference in my room of 50Hz that is picked up or could be bad board design where 50Hz ripple is somehow passed from the Arduino via the SPI lines(?). I don't quite understand why this was not picked up when measuring just the battery. I tried putting everything into an alumina foil enclosure (grounded), but it did not change much - maybe Al is not enough as shielding.If someone could clarify the reason why 50Hz ripple is picked up that would be very helpful. Thanks in advance.

author: happy_hippo0
author_rank: Newbie
timestamp: March 14, 2021, 04:17:25 pm
content: Sorry to post again, but it looks like I resolved my own issue - it was Electromagnetic interference after all. I put everything, the ADC, the voltage divider circuit, the battery enclosure and all the wires inside a box with aluminum foil and the ripple significantly decreased. In my previous test I left the battery enclosure and some wires outside, but it looks like even 5cm of wire was able to act like an antenna and bring tens to hundreds of uV or ripple. I guess just connecting to an AA battery works because it has low internal resistance. Thanks

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2021, 02:56:11 am
content: Besides shielding, the common solution for this, which multimeters and other instruments which make low level measurements use, is to make an integrated measurement over a whole number of power line cycles which notches out the power line frequency and its harmonics.Configuring the ADS1256 for 50 samples per second will accomplish this.

author: radiolistener
author_rank: Super Contributor
timestamp: March 15, 2021, 08:47:43 pm
content: Quote from: happy_hippo0 on March 14, 2021, 12:35:22 pmMost likely this is because your voltage divider circuit has longer total length of wires. It means it overlaps more large area in the space and as result it receive more power from the the same power flux density.