ADS1220 Delta-sigma 24bit ADC and Heltek Wifi32 Kit messurment error - Page 1
Forum section: electronics / general

author: MADxHAWK
author_rank: Contributor
timestamp: May 23, 2021, 09:20:08 am
content: Hello community,i recently bought the ADS1220 24Bit delta-sigma ADC Module like this one:https://protocentral.com/product/ads1220-24-bit-4-channel-low-noise-adc-breakout-board/and tried to use it wit a Heltec Wifi32 kit by using the Arduino Libraryhttps://github.com/Protocentral/Protocentral_ADS1220By playing around with the Examplecode i noticed the ADC seems to have a quite linear messurment Error.Compared to my ET3240 (4 1/2 Digit 20000 Count +/-0.03% +3) i got these results:Code:Multimeter: 50.34mV - ADC: 49.06mV ~ -1.5mv differenceMultimeter: 100.36mV - ADC; 97.90mV ~ -2.5mv differenceMultimeter; 200.15mV - ADC: 195.03mV ~ -5-6mv diffenreceMultimeter: 501.52mv - ADC: 489.11mV ~ -11-12mv differenceMultimeter: 1000.67mV - ADC: 977.05mV ~ -23-24mv diffenreceSince the TI ADS1220 should be a precise delta-sigma ADC i wonder about this quite huge messurment error on the ADC and even the ADS1115 16Bit ADC Module seems to be more precise.Anyone have some experience with the ADS1220 Breakout module and maybe have some informations?Im not even sure if i didnt got a faked chip on the module due to this messurment error.So any informations would be appreciated, thank youADS1220 Datasheet:https://www.ti.com/lit/ds/symlink/ads1220.pdf

author: exe
author_rank: Unknown
timestamp: May 23, 2021, 09:41:33 am
content: If it's a linear error, then it's easy to calibrate it out with a full-scale calibration. I'd say for an ADC stability is more important than offset and gain errors are those can be compensated.I'd check the voltage of internal reference to see if it's within specification. Afaik, I've seen in datasheet a way to expose internal voltage on IC pins.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 23, 2021, 01:23:37 pm
content: According to the "Internal Voltage Reference" section on page 7 of the datasheet, it has an initial accuracy between 2.045V (min), 2.048V (typ.), 2.051V (Max), so it should be within 3mV of nominal for a full scale measurement.So apparently there is something wrong here.I would start with checking power supply ripple and decoupling.Another important check is the current consumption of the ADS1220. If this is too high, then you possibly have ESD damage, but in that case a linear error would have been unlikely. (if the ESD damage would have been on the input terminals).Apparently the majority (around 80% ?) of ESD damage does not cause complete loss of function, but just degradation of parameters.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2021, 03:32:07 pm
content: The values look like they are some 2% off - this could be somerthing like a wrong reference values in the software (e.g. 2 V instead of 2.048 V).Anyway one should be able to correct this in software with a adjusted scale factor.

author: MADxHAWK
author_rank: Contributor
timestamp: May 23, 2021, 06:18:39 pm
content: Thanks for your replys guysIll guess i need to calibrate the ADS1120 according to its datasheet but have to figure out how to send this through SPI Bus (page 30 and 40 of the datasheet).Meanwhile i have adjusted the Vref in the Example Code from 2.048 to 2.100 and now the messurments fit quite nicely compared to the Multimeter.Sadly there is no documentation of the Library, at least i havent found one and it seems like there is no calibration function implemented.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 24, 2021, 05:04:42 am
content: How much software is there in such a lib?Maybe 2kB, takes half an hour to read through all the functions?And I won't be too quick in just calibrating the error away. The error is 5 times bigger then it should be according to the datasheet, and if you "just calibrate it away" then you may be hiding some underlying problem.

author: MADxHAWK
author_rank: Contributor
timestamp: May 24, 2021, 08:36:53 am
content: You might be right, but i have no idea what else i could do, at the moment the messurment error is fixed by setting an offest to the reffenrens voltage in the software to calculate the voltage. ( Increased from 2.048 to 2.100) which made the messurment way more accurate. So maybe the voltage reference chip on the ADS1220 might be faulty, but i couldnt test it by now, im still waiting for my REF5020 (2048mV) and REF5040 (4096mV) voltage refference. When i got them i will try using the external voltage reference and see what i get.