LPC MicrocontrollersのトピックRe: Reference used on LPC4370 ADCHS 12-bit converter.

Reference used on LPC4370 ADCHS 12-bit converter.

jordsan — Wed, 22 Feb 2017 20:07:05 GMT

In looking at the 12-bit ADC on the LPC4370, it is not clear what is the reference voltage used for the conversion. What is the min/max of the voltage reference? What is the accuracy of the voltage reference? I have looked both on the user manual and the datasheet.

Thanks.

Re: Reference used on LPC4370 ADCHS 12-bit converter.

soledad — Wed, 22 Feb 2017 20:43:42 GMT

Hi Jorge,

VDDA is the analog power and also voltage reference VREF for both ADCs.

Have a great day,
Sol

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------

Re: Reference used on LPC4370 ADCHS 12-bit converter.

jordsan — Wed, 22 Feb 2017 22:17:07 GMT

Hi Soledad,

Thank you for the fast reply. Looking at the LPC4370 datasheet it would seem VDDA is only the reference for the 10-bit ADCs.

The user manual seems to imply again that VDDA is only the reference for the 10-bit ADC.

The 10-bit ADC0/1 chapter in the user manual hints at VDDA being the reference for both of the 10-bit ADCs (ADC0 and ADC1). Difficult to assume that it refers to all three ADCs (ADC0/1/HS).

Doing a search for "reference" in the 12-bit ADC chapter a mention of an ADC band gap reference is found.

Unfortunately there is no mention of the characteristics of this ADC band gap reference anywhere in the datasheet or the user manual. There is mention of the characteristics of a band gap in the datasheet.

It would seem this reference is nowhere near good enough for a 12-bit ADC. It shows an absolute variation of ±21.5mV around its typical value of 0.6425V. This translates on ±88.064 LSBs of variation on this reference (0.0215/(1/4096)). If this is really the reference for the 12-bit ADC, is it possible to connect an external reference for the 12-bit ADC? Otherwise this 12-bit converter is really more akin to a 6-bit converter (12-(log(88.0624)/log(2)) = 12 - 6.46 = 5.54 bits

Or is the idea to measure this band gap using the 10-bit ADC in order to then correct the 12-bit conversion? Which after writing all this, if there is no other reference possible for the 12-bit ADC, I venture guess that was the intent of the designers.

Thanks.