How to fix unstable ADC reading

[Wamor]

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

 

[waicon]

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

 

[karesz]

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.

 

[Wamor]

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