Hi,
My workplace has developed a subsystem based around the TMS570LS20216 MCU. When testing on our hardware, I am finding that when I attempt a self test on the ADC modules, my Vu, Vn and Vd are all approximately equal to each other for ADC input channels that are connected to a device (in our case, a differential chip). They are approximately equal to the voltage being applied to the input of each ADC channel from the differential chips. Because of this, Vu is sometimes slightly less than Vn or Vd and Vd is sometimes slightly greater than Vn or Vu and as a result, we fail the self-test.
I found this post that has some relevance to the problem I am experiencing: http://e2e.ti.com/support/microcontrollers/hercules/f/312/t/200346.aspx
I have also tested our ADC self-test code on a TMS570 MCU Development Kit, specifically on the ADC channels connected to the temperature and light sensors, and see the correct results (Vu slightly less than AD_REFHI, Vd slightly greater than AD_REFLO and Vn in the middle), so I am convinced there is likely a hardware issue with our subsystem board.
I am wondering if there is a minimum output impedance for the driving buffer on the ADC input channels, in order to achieve appropriate ADC self-test results? As I understand it, the lower the impedance of the driving buffer, the less effect AD_REFHI and ADREF_LO have on Vin that is converted by the ADC core.
Please advise on any suggestions or requests for additional information.
Thanks,
Mike