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ad5235 A to B resistance measurement not stable with Ohm meter

Category: Hardware
Product Number: ad5235B-250
Software Version: none

Hello,

I'm am testing an ad5235B-250 digital pot.  I can write and read to the spi registers, and am powered by a clean 3.3V signal and the output pot is unloaded. 

When I take an Ohm meter and measure across A to B on either port, the resistance value changes.

I expect it to read the same value (~250K) every time as it's fully unloaded. 

For example, I program the port zero wiper to 0 using B10100000.  I can read this back and confirm the write worked. 

From a0 to b0 I get 187K (should be 250K).  a to wiper is 183K, b to wiper is 100.  So where did the 4K go?  

Then I program the port zero wiper to 512 using B10100000.  I can read this back and confirm the write worked. 

From a0 to b0 I get 208Kohm (should be 250K).  a to wiper is 105K, b to wiper is 108K.  So this is 203K, it should be 208K (should be 250K).

Why is the a to b reading changing with programming, and why does a to wiper to b add up to something different than a to b?

s there something I'm doing wrong to make this thing stable?

Is there some reason a benchtop meter would be messing things up?  Does it need a DC bias across the pot or something?

Thanks,

Dave.

Thread Notes

Parents Reply
  • Hi Dave,

    The leakage current from the device can affect the ohmmeter readings, adding unwanted errors. You can put an op-amp as a buffer and measure the voltage at the buffer output. Another method is to use a source-meter where a precision current source/sink is fed through one of the terminals with the other terminal held at GND or some other level and voltage is measured across the same terminals.

    Best Regards,

    Yokki

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