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TMS320F2806x ADC external reference.

Toshio Ushikubo
Mastermind 9620 points
Other Parts Discussed in Thread: LM4040-N

Hello,

 

Q1:

TMS320F2806x has following ADC specification on the datasheet page 88.

                  

 Please advise the accuracy of external reference to guarantee “Overall gain error with external reference: -40LSB min to 40LSB max”.

 

Q2:

If I use 3.0-V LM4040-N(voltage tolerance is +/-15mV max@25C, +/-45mV max@Full Temp.), 45mV match 61.4LSB. So I think Overall gain error is equal to in case of internal reference and no need to use external reference.

 

Please advise how to calculate Overall gain error using external reference accuracy when I use external reference.

 

Best Regards.

  • 0
    TI__Guru 60865 points
    Hi Ushikubo-san,
    (1) The DS specification for external reference assumes an ideal external reference. Whatever error exists in the external reference should be added to this specification. If you had an external reference with 20LSBs of error worst case, you could directly add the spec's and get 40 + 20 = 60 LSBs. This will probably be very pessimistic, so you may want to add the error powers instead of the error magnitudes => total errror = sqrt(40^2 + 20^2) = 45 LSBs.

    (2) I wouldn't reccomend using this particular external reference over internal reference mode, unless you really need a 2.5V range instead of the internal reference range of 3.3V.
    There are other reasons to use an external reference other than raw accuracy though. For example, if you have other data converters in your system (maybe an external DAC chip or a sigma-delta ADC), the system error can be lower if they track together, even if the absolute error is increased.