CW7.3 - EVM56F8367 - ADC does not measure negative voltages in differential mode?

[kgeldhof]

Hi,

I applied a bipolar test voltage (Fig. 'test_voltage.jpg' in attachment) at pin 1 of the ADC of the 56F8367 evaluation module. I configured the ADC to work in differential mode. Pin 0 was tied to the analog ground. The result of the ADC conversion (Fig. 'adc_result.jpg') reveals only one polarity of the voltage. How come?

Thanks in advance for any help.

Kristof Geldhof

test_voltage.jpg

adc_result.jpg

Message Edited by t.dowe on 2009-09-04 10:43 AM

[trytohelp]

Hi,

I checked with my colleague working for the hardware dpt.

See below their feedback:

For differential measurements, the digital result is proportional to the ratio of the difference in the inputs to the difference in the reference voltages.
so for 0V on both inputs the ADC result should be 2048 (or 2048*8) for +3.3V on second input the result is 0 or 4095 depend which input is grounded so each time you get value from 0-4095.
if you now set teh ADC offset register to 2047 (or 2047*8) then the sign bit in result registers will be set and you get negative result if using interger number

Pleare refer to the Peripheral manual.

            \DSC 8.0\Freescale_Documentation

the main info; ADC input does not accept negative voltage, it is cut by clamping diode and moreover the input can be damaged probably by higher voltage .

-3.3V probably does not destroy it, but I am not sure

If you still need more help for this issue, can you please log it in the Technical support system.
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.

By this way we could ask other team for help and Third party too (Processor Expert).

According to the info it seems you're using the DSC 7.3.
A new version is available: DSC 8.x.
You can download the DSC 8.0 on our web site via the link:
http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=01272600612247
There is a patch for the DSC8.0 (must be installed in first) DSC8.1.1
You can download it on our web site via the link:
 http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=01272600617882140B
The Special edition license is installed by default.
If you need another version of the license please contact your local distributor.
You can have several versions installed on the same PC.
There is only one restriction: install them on separate folder.

regards

Pascal