How to fix unstable ADC reading

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Wamor

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Hello,

I am having some problems with a ADC circuit on my PCB.
When I read the ADC samples this gives a very unstable result.
The ADC is connected by an opamp circuit which converts a 0 - 20mA current
to a 0 - 3.6V voltage which is fed into the ADC121S21 from National Semiconductor (see atached schematic) . The current I am measuring is about 1.5mA created by a LM317 current-source. When I connect a 4.5 digit multi-meter in series with the current-source I see that the ADC-results are getting even worse.
The current measured by the multi-meter does not fluctuate as much as measured by the ADC.
Does anybody has some idea what is wrong here?

Thank you and best regards,

Wamor
 

contact ohm might impact on voltage drop. try series with 0.1% resister rather than amp meter. than use meter measure voltage across it. reading fluctuate depand on load or instrumentation amp. refer to attachment
 

Hi,
How big is C142 at Op input & did you checked the Amp output (w. oscilloscope); how stabile, what kind of signal it has? Possible has some oscillation...
I dont read Op`s Type Nr, is it a RtoR output type?
K.
 

Hello Karesh,

Thank you for your response. C142 is not mounted yet. I will do some testing to find the correct value for this cap. Opamp-type is LM8261 from National and is a RtoR input and output-opamp.

I will do the measurements you suggested and let you know.

Hello Waicom,

Thank you for posting the document. I will read through it for some usefull tips.

Best regards,

Wamor
 

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