ZX24ae ADC jitter

pjc30943 · Post by **pjc30943** » 03 August 2007, 16:57 PM

Just got the ZX24ae from OakMicros; it's a nice device.
Here's a question on it:

Looking at a steady input going straight into an ADC pin (porta.5) of the zx24ae, I measure a solid voltage (at the solder joint on the zx42ae pcb) that fluctuates less than 1mV worst case. However, the getADC() result still varies by 3 counts; how can this be?

The power supply is solid as well to <=1mV.

Thoughts?

Post by **dkinzer** » 03 August 2007, 21:26 PM

The ADC readings can be affected by local noise generated by digital outputs changing, etc. The GetADC() function is a simple interface to the A/D converter that is easy to use but it does have shortcomings. To get optimal ADC results you may need to use one of the sleep modes to provide a quieter environment for the converter. The ADC section in the mega32 or mega644 datasheet describes ways to improve ADC accuracy.

mikep · Post by **mikep** » 03 August 2007, 22:27 PM

I retested a ZX-24ae on my test bench and got solid, consistent ADC readings.

Did you check your input signal and/or voltage source using a scope or a voltmeter? Not withstanding what Don has already suggested, you may have a noisy environment such as robot motors that is causing the problem.

pjc30943 · Post by **pjc30943** » 05 August 2007, 21:33 PM

Thanks for the thoughts guys.

As stated, the source is verified to be stable with a scope. There are no IO ops occurring, since this was just a test function; nor are there noise sources nearby.

I'll check into the mega644 datasheet, Don.

After I check a few more things I'll post back.