How to choose Reference for ADC

Hi everyone,

I'm wondering how to select a reference IC when I design ADC sometimes.
For expample, looking at AD7918, 10bit-ADC external referene and the gain error itself is 0.5LSB MAX.
Reference accuracy is said to be less than 1% according to the comment of datasheet.

Ive attached a file on this mail.Please refere to it.
But I'm wondering that the accuracy of reference is 1% sounds larger.
Because the resolution of 10bit is equivalent to 0.1% and the gain error of this device is only 0.5LSB(0.05%).
Please give me advice on the coneection between the gain error and the accuracy of reference voltage.
(In this case, reference noise and tempareture coefficient are not considered to make it simple)
And one more thing, I want to know how to estimate source current of ADC refinput to choose right reference IC.

Tarzan

AD7918_Reference.pdf
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  • 0
    •  Analog Employees 
    on Jun 29, 2011 2:00 PM

    Hi Tarzan,

    During the Gain Error test, tester is connected to the reference and measure its value. Using this value of reference, the ideal first and last transitions are calculated and compared with the test result therefre reference tolerance/variance/tempco cannot influence Gain specifications simply because tester takes into account actual reference value. As I said test specification focus on the Gain Error figure of the converter and the reference voltage error needs to be calibrated out. Of course reference tollerance affects DC accuracy of the converter however the Gain Error is not. If you like to calculate total unajusted error of the converter (TUE) you should take into account, Offset, Gain, INL, Reference tolerance and Reference tempco. Combination of these parameters defines absolute error of the converter.

    This part is specified to operate with a 2.5 V +/-1 % external reference and the AC/DC specifications on the datasheet are determined using this value. If you operate the AD7918 with reference other than 2.5 V you may observe incorrect conversion resutls. The most critical situation is with the reference voltage higher than 2.5 V. The reason behind this is that the comparator input will go outside of the rails for certain values of the input voltage and this will cause errors in the DAC due to lost charge when diodes turn on in the usual way. Therefore it is not recommended to oparete this device with the reference voltage outside the specification.

    Best Regards,

    Jakub.

Reply
  • 0
    •  Analog Employees 
    on Jun 29, 2011 2:00 PM

    Hi Tarzan,

    During the Gain Error test, tester is connected to the reference and measure its value. Using this value of reference, the ideal first and last transitions are calculated and compared with the test result therefre reference tolerance/variance/tempco cannot influence Gain specifications simply because tester takes into account actual reference value. As I said test specification focus on the Gain Error figure of the converter and the reference voltage error needs to be calibrated out. Of course reference tollerance affects DC accuracy of the converter however the Gain Error is not. If you like to calculate total unajusted error of the converter (TUE) you should take into account, Offset, Gain, INL, Reference tolerance and Reference tempco. Combination of these parameters defines absolute error of the converter.

    This part is specified to operate with a 2.5 V +/-1 % external reference and the AC/DC specifications on the datasheet are determined using this value. If you operate the AD7918 with reference other than 2.5 V you may observe incorrect conversion resutls. The most critical situation is with the reference voltage higher than 2.5 V. The reason behind this is that the comparator input will go outside of the rails for certain values of the input voltage and this will cause errors in the DAC due to lost charge when diodes turn on in the usual way. Therefore it is not recommended to oparete this device with the reference voltage outside the specification.

    Best Regards,

    Jakub.

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