Hi,
What does the datasheet say?
It says it thas zero scale offset of max. 3LSB.
wiper resistance is 50 Ohms max.
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You didn´t say what resistor value your pot is, so I assume 10k.
Then the reistance step size is 10k / 255 = 39 ohms.
3LSB means 3 x 39 ohms = 117 ohms.
Now add the wiper resistance: 117 ohms + 50 ohms = 167 Ohms.
This should be the max. expectable resistance at zero position.
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The max expectable voltage is U_fullscale * 3 LSB / 255 LSB.
You didn´t say what voltage you applied to the resistor taps, so I assume 5V.
Then the max expectable output voltage with no load is 5V*3/255 = 59mV
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Now you say you have 250mV.
In worst case the difference voltage (250mV - 59mV) is caused by external current into the wiper.
--> I = U / R --> I = (250mV-59mV) / 167 Ohms = 1.1mA
So I expect you draw more than 1.1mA at the wiper.
(I expect a lot more)
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To verify this you should give us the (2) missing informations and a schematic (draft) of all your AD5290 circuit, including wiper load circuit.
Klaus