ADS1115 error of reading from A0/A1 and A2/A3 having the same input voltage

[olivander]

Hi, guys.
I'm using ADS1115 to read an output voltage from a resistive divider, the voltage that must read is 1.5V.

I use the sketch (differential) from the adafruit library (Adafruit_ADS1X15.h).
In this sketch, I also added the code to read from A2/A3.

When I read the voltage from A0/A1 the voltage that i read is correct and very accurately. if i use the same cables pulling them out from A0/A1 and inserting them in the A2/A3 I read 1.67V

Can you tell me why there is a reading difference of 167mv?
between A0/A1 and A2/A3?
Thanks so much

[jremington]

Please read and follow the directions in the "How to get the best out of the forum" post.

Post your code in line, using code tags, and a schematic diagram of your wiring (hand drawn is best, avoid Fritzing).

[johnerrington]

How to make a schematic you can post.

When you submit a question on the forum you will usually need to add a schematic (circuit diagram). The tutorial that follows will explain some ways to create a useful image; however lets start by explaining how to add the image to your post. Its easy. Having created the image all you need to do is : Save the image somewhere convenient - maybe your desktop Start your post, and place the cursor (insertion point) where you want the image to be shown; [upload image to forum] 3: click th…

[patduino]

@olivander

Can you tell me why there is a reading difference of 167mv?

Not sure. That difference is approximately 3% which might be within expected tolerances between paths. If you aren't, you might want to take several measurements and average the values and see what you get.

You may also want to try different combinations of analog input paths. You were measuring using A0/A1 and A2/A3. What about trying A0/A3? or A1/A2? It would be interesting to see what you get.

Pat.

[Ron_Blain]

Less seeing your code or a schematic I can share this much. Using an ADS1115 with default gain, differential mode in the code I see a variation of a few mV at best. Most readings agree down to a millivolt or are the same. This with a 2 second delay between readings. Keep in mind the ADS1115 is not very fast.

Using a bench DMM, Agilent 34401A
4.9998 Volts Applied
CH 1 4.992 V
Ch 2 4.993 V

Working up from 0.0 volts to 5.0 volts in 1.0 volt increments both CH1 and CH2 agree within a few mV, never exceeding maybe 4.00 mV worst case. Here is an example:

Possibly a loose or poor connection? This would go better if as suggested you post your code and a good drawing.

Ron

[gilshultz]

Connect them all together and be sure the input resistance is less then about 5K. Posting a schematic would be a big help. The only sure way is to connect them together and read at the same time or as close together as possible. Also you probably have some noise that changes with lead dress.