irregular ADC read-out

[kgeldhof]

Hello,

I downloaded the hands-on exercise PWM_ADC_SYN (synchronization of ADC with PWM) from the freescale website. As a test, I applied a constant voltage of 1.5V to the ADC input pin ANA0. However, successive ADC conversions (and scaling) give results that are scattered around the correct value. This means, the mean value of the measurements corresponds to the 1.5V, but there are deviations of approximately 10%. The applied voltage is constant (checked it with a scope). The internal VREFHI and VREFLO voltages (3.3V and GND respectively, used as reference voltages by the ADC) are also constant. What could be the reason of these bad read-outs?

Thanks in advance for any help.

Kristof Geldhof

[Alban]

Hello,

This post does not include the CodeWarrior version and the Device Part Number in the subject line or anywhere else.

It also does not seem to be a CodeWarrior question and would therefore be moved.

Please answer with the part number you are referring to.

Regards,
Alban.

[kgeldhof]

Apologies for the lack of information. I am using an evaluation module 56F8367EVM and CodeWarrior version 5.6.1.

Regards,
Kristof Geldhof

[trytohelp]

Hi,

Thanks for the details.

I suspect you're using CodeWarrior for DSC V8.0.

Be aware the 8.1.1 has been released.
This is a patch for the DSC8.0 (must be installed in first)
You can download this version on our web site via the link:

http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=01272600617882140B
You can have several versions installed on the same PC.

There is only one restriction: install them on separate folder.

Regarding the problem you meet with the example.

Do you got it from an application note AN ?

Can you please provide me the link you used to donwload it ?

Regards

Pascal

[kgeldhof]

Hello,

I applied a battery voltage to an input pin of the ADC and the ADC works fine with a 10 bit effective resolution. I would conclude that the bad quality of my measurements (see JPEG in attachment, 100 samples, 1 sample per 0.25ms PWM period) is caused by the lab-supply that I used to test the ADC, although my scope shows a nice flat line for the applied voltage...

By the way, for the synchronization of the ADC and PWM, I used following Freescale hands-on-exercise:

http://www.freescale.com/files/dsp/doc/reports_presentations/56800PWMADCSYNCH.pdf?fsrch=1
Thank you for the notice about the CodeWarrior 8.1.1 release.

Regards,
Kristof Geldhof

[trytohelp]

Hi,

Thanks for the details.

I think you should log this issue in the CRM system.

By this way this problem could be handle by a member of the DSC team.

See below details explaining how to do that.
For Technical Support Issues please use Freescale on line support web page.
- Go to following URL: http://www.freescale.com/TechSupport
- Click on Submit a Service Request. You will come to a login page.
- You can submit your request through the web from there.

Pascal