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Inconsistent measurements for 12, 14 and 16bit sampling

1282 Views - Created 13/12/2016

13/12/2016

Posted by:
jchevin

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Hi All,

I have a Pi 3 connected to an ADC Pi plus board. Currently it is sampling the output from a RTD and when measured with a multimeter, the output is stable, but when measured in the 12, 14 or 16 bit sampling modes we see fluctuations of +- 0.2v. If the sampling rate is changed to 18bit, this fluctuation ceases.

We have traced this issue down to the bit rate, and are confident of the output from the RTD. Has anyone has any issues sampling at lower bitrates that may explain our problems?

Thanks!

14/12/2016

Posted by:
andrew

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United Kingdom

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Hi

Which language version of the ADC library are you using for your project?  I will check the library to make sure there are no bugs when converting at the lower bit rates.

One possible cause for instability when sampling the ADC is if the power supply for the Raspberry Pi is unstable.  The MCP3424 ADC chips on the ADC Pi Plus have an internal voltage reference and smoothing capacitors but any major voltage drops on the power supply will cause the voltage reference to become unstable which will in turn show up as fluctuations on the sample readings.

If the power supply is unstable then one solution would be to use a second lower current power supply to power the ADC Pi Plus directly.  You can do this by connecting the second power supply to the 5V and GND pins on the ADC Pi Plus and cut the 5V trace on the PCB as shown by the red line on the photo below so it is disconnected from the 5V pin on the GPIO header.  This should eliminate any voltage drops or spikes on the Raspberry Pi power supply from effecting the ADC sampling.

Post image

AB Electronics UK Tech Support

19/12/2016

Posted by:
jchevin

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Hi Andrew,

We are using the python ADC library from the ABElectronics git. 

We have changed power supplies and double check the voltage stability with a multimeter and the power supplies seem fine to us.

We'll also try your modification to see if we can reduce the noise.

Thanks!

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