ADC lag at specific values 5

[mudandsnow]

Hello,

I have an odd issue and am welcome to any ideas. My ADC seems to lag at specific values and I would like to know why, or better yet, how to fix it. Lags are shown in attached pic.

Occurrence:
Unit 1 lags at ADC value 1638.
Unit 2 lags this at ADC value 1365.

Hardware:
Analog sensor ~10mV full scale
op amp - amplifies sensor output
ADC - ADS1110 reads output of op amp
MCU - reads ADC value
PC - reads and graphs MCU data

http://files.engineering.com/getfil...le=o2Flats-2-ADC1365-O2_19_4621-annotated.PNG

 

[IRstuff]

A simplified schematic might be in order. I'm not sure it's an op amp problem, per se, but not having the schematic makes it difficult. Unless you are running open loop on the op amp, you should not be seeing this. But, you've also ignored a previous request to disclose the op amp. It's more likely that someone has added something that sucks up extra current during zero crossings, which causes the distortion.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529

 

[mudandsnow]

I listed the ADC by accident when the op amp part# was requested.

This is the op amp:
OPA330

http://www.digikey.ca/product-detail/en/OPA330AIDBVT/296-25307-1-ND/2179367

Another thing to add is that the lag time is related to the rate of change of the signal. If the signal is changing quickly the lag doesn't last long, if it changes slowly, the lag lasts a while. Got the lag over 10 seconds with a signal that was changing very slowly.

 

[IRstuff]

That specific symptom is typical of a zero crossing problem, but there shouldn't be an issue with that particular op amp, assuming that it's being operated per guidelines.

Given the symptoms, it would appear that there's a back-to-back diode structure that's being current starved for some reason.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529

 

[mudandsnow]

I took some ohms out the resistor between the op amp output and the ADC input and that seems to have sorted it out. Thanks again for the help everyone.

 

[itsmoked]

So... what does this make the problem? Some sort of capacitive shunt inside the A2D? Violation of the A2D input impedance?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mudandsnow]

I still think it was related to op amp, not A/D because 2 different A/D showed the same results.