PXS3020 / MPC567x series micro controller ADC Reference

[rameshmarichamy]

Hi,

Am using PXS3020 / MPC567x micro controller series in one of my project. I am using the ADC inbuilt to the chip.

The ADC needs a reference voltage of 3.3V or 5V. I have connected to 3.3V rail with proper filtering.As I have connected 3.3V +/-5%whenever the input supply changes, it affects my ADC reference and my analog input sensing changes.

Does the chip has any internal voltage reference option, so that i can use it.?

Normally ADC references are used to fix the voltage threshold and saturation limits of ADC. But the chip specifies 3.3V or 5V as the recommended reference inputs. If by chance am using a precision reference generator,possibly 2.5V will it affect my ADC functionality?. Please revert ASAP.

Regards,

Ramesh.M

[davidtosenovjan]

Voltage reference is only external over VDD_HV_ADRx/VSS_HV_ADRx pads and you can use precise reference but all references supplies must be at the same supply level - either 3.3V or 5.0V. Unfortunately 2.5V is not allowed.

[aero72]

Hi,

eQADC has additional internal channels including a local bandgap reference (1.22V), and it can also measure its VRH and VRL reference voltages, plus 25%,50% and 75% of the difference between those voltages. You should use these values to calibrate the gain and offset of the ADC if you require better accuracy - there is an application note to tell you how this should be done. For the 5674F, take a look at section 26.5 of reference manual v7 for the full channel list.

You could also connect an external precision bandgap to one of the standard ADC channels to compare its value with the internally generated ones, but if you're really concerned about tolerance in VRH/VRL, the normal way to reduce that to a minimum is to use a separate tracking voltage supply as a reference, thereby separating your core voltage supply and ADC supply, but keeping a small absolute difference between them.

Hope that helps.

[davidtosenovjan]

Thanks for the response but customer’s question is apparently targeted to PXS3020/MPC5675K device that does not use eQADC module but SAR ADC. This ADC version does not have any kind of software calibration.