TMS320F28335 ADC input Range

Hi Shihab,

The input range will a fixed range of 0 to 3.0V, regardless of the reference voltage selected (I think there are two other fixed options other than 2.048V).  So 3.0V will correspond to code 4095 and 0V will correspond to code 0.

Thanks Devin for the reply!

Can you provide little bit more information regarding the internal to the ADC. Whether 3V will be attenuated to Vref.

Or Vref will be amplified internally? For doing an accuracy analysis can you provide the tolerance of the internal gain circuits?

Thanks,

Shihab.

Shihab,

I am honestly not sure for this particular ADC design.  Either way, I think you should find all the necessary information in the below table to do an accuracy analysis for your application.     

Shihab,

  Actually both the ADC input and the internal reference voltages are attenuated to a common range.  We don't provide any further details on the reference range or accuracy but as Devin pointed out, we do specify the maximum gain error over the recommended operating conditions, which should be enough for your accuracy analysis.

regards, Joe

Hello Joe,

Does this mean that the ADC input voltage is limited to 0 to 2.048V , if external reference of 2.048V is used?

Regards,

Midhun

Hi Midhun,

  No, what Devin said is absolutely correct, the full scale range of the ADC is always 0-3V no matter which reference mode is selected.  Shihab had asked whether the reference voltage is scaled up to the 3V range or whether the ADC input voltage is scaled down to the reference range, and I was just trying to clarify that both the ADC input and the reference are scaled appropriately to a common range.

I hope this clears it up for you.

regards, Joe

Hi,

How the 3V input will be converted w.r.t 2.048V ref?

I think you mean that 0V is 0x0000 and 2.048V would become 0xFFF0, but anything between 2.048V and 3.0V will also result in the digital value 0xFFF0.

The input range of the analog front end is 0-3V3, but your full-scale reference is your reference voltage which results in 0xFFF0. So 0xFFF0 is the result for 2.048V (or higher) analog input voltage on a reference of 2.048V.

And 0xFFF0 is the result for 3.0V (or higher) analog input voltage on a reference of 3.0V.

Hopes this is your answer.

Hi Tjarco,

  The specified full scale input range of 0-3V is the full conversion range in all reference modes, not simply the operating range of the front end of the ADC.  Ignoring effects of gain, offset, and linearity errors, 0V will convert to 0x0000 and 3.0V will convert to 0xFFF0 (which corresponds to 4095) with all voltages in between linearly distributed between those codes, so 2.048V will convert to 0xAEC0 (corresponding to 2796).  This is true for all reference modes.

  For clarity sake I want to specify that in the above examples, 0xFFF0 and 0xAEC0 are what would be seen in the ADC results registers which are left shifted, the ADC mirror registers will right shift these values to 0x0FFF and 0x0AEC respectively.

I hope this clears it up,

Joe

Hi Joe,

O Yes the mirror registers.. I don't use them ;)  I process them in IQ16.

Tjar

Hello Joe,

Thank you for clarifying on the ADC input range and the reference voltage.

One more clarification is needed on the reference voltage selection, what is the impact on the reference voltage accuracy to the ADC conversion.

Regards,

Midhun

Hi Midhun,

   Inaccuracies of the internal reference will manifest primarily as gain error.  Furthermore, this gain error can drift over temperature.  We do not specify error due to the reference separately from other sources of error in the converter, rather simply guarantee the maximum error seen from all internal sources of error over the full recommended operating conditions.  Selecting different internal reference modes will not change the accuracy, however if external reference mode is used, care must be taken to add the error from the external reference to the error specified for the F28335 ADC. 

Regards,

Joe