ADC reading inconsistent

[ESC]

Hello,

I have installed and am using Webiopi.  In using the REST API to poll an MCP3008 channel (that is hooked up to my raspberry pi) I notice that the results are inconsistent.  I know I am pushing 3.3 volts through channel 0 of the MCP but when I query the API sometimes I get 0.0 and sometimes I get 3.3.  When I watch that channel with my Multimeter I see no fluctuations. Any ideas ?

Thanks in advance!

esc

[Andreas Riegg]

Hi,

 

the MCP measures/"samples" the voltage in a very small time slot. If you have for any reasons a somewhat floating signal on the anaog input you will see that hardly with a multimeter as its will never show small "peaks" to 0 V whatever reason they may occur. Multimeters are too sluggish to show such peaks. You may see those using an oscilloscope, but probablly you don't have one as the most of us as that's too expensive.

 

You may try the following: Take some AA or AAA battery, connect the minus pole to analog GND and the plus pole to your MCP analog input and look for the result. It should be somthing in the range of 1v5 to 1v0. Typically, the voltage of the battery is very stable and you should get consistent readings. You should also check if analog GND and digital GND for the MCP are wired correct according to the data sheet of the MCP. If ypur results are now consistent, you have a problem with the 3v3 level. If your results are still inconsistent, you may have any other wiring error.

 

Andreas