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

Hi Tarzan,

The AD7918 device can be operated with 2.5 V +/-1% reference only. Such restriction is driven by the internal comparator in the part which starts to operate outisde its normal operating region when the reference is not withing datasheet limits. This situation can result in incorrect or inconsistent conversion results.

It should be noted that the reference variation doesn't influence the Gain Error specification as the reference voltage is measured and calibrated out during this test. Therefore the Gain Error specification is error of the AD7918 itself and not the reference source. Please be however aware that Offset Error, Gain Error, Linearity Error and Reference Error (absolute error, tempco) defines absolute error of the converter.

In terms of chosing the reference source for ADC. The AD7918 datasheet indicates suitable reference sources for this device as well as the REFin input impedance therefore it is relatively easy to calculate required current. At 1 MSPS the REFin input impedance is equal to 36 kOhm therefore reference input current is in tens of uA region.

Best Regards,

Jakub.