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I am using an ADS1256 ADC chip (in the form of a Waveshare High-Precision AD/DA Board).

I am using a 5V reference voltage, but I can't seem to read full scale.

  • 0V reads out -344 bits (~0V)
  • 3.3V reads out 5559000 bits (~3.3V)
  • 5V reads out 6698000 bits (~4V)

Can anybody advise on how I can read full scale? I'm using PiPyADC.

Thanks.

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I am using a 5V reference voltage

The device is intended to be used with a 2.5 volt reference

If you are applying input signals in a single-ended way then this will cause other problems that you may be seeing dependent on what the PGA gain is and if you are using the input buffer.

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  • \$\begingroup\$ Thanks for your reply. The waveshare board gives me an option of 5V or 3.3V Vref so I'm assuming the board converts this to 2.5v? Please could you expand on the second part of your reply? Gains seem to be okay as up to ~4V the reading is correct.. \$\endgroup\$ – Tom Mar 18 at 10:27
  • \$\begingroup\$ If the ADC is "packaged" into an "unknown circuit" then this site is probably the wrong place to ask questions about it. Without knowing the circuit used, you are basically asking questions on commercial devices and these are off-topic. \$\endgroup\$ – Andy aka Mar 18 at 10:31
  • \$\begingroup\$ The schematic for the Waveshare board indicates that the 2.5V reference voltage is created by a LM285D-2.5.Considering the voltage may vary from 2.462 to 2.538 V not the best choice but well, it should do the trick somewhat. \$\endgroup\$ – Tom L. Mar 18 at 10:59
  • \$\begingroup\$ So, are you driving the inputs correctly i.e. differentially and, have you activated the internal buffer amplifier. If you have you may find that the inputs won't work properly as your common mode voltage rises too high. \$\endgroup\$ – Andy aka Mar 18 at 12:48

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