Electronic – Voltage reference output impedance calculation

adcvoltage-reference

I'm using AD7190 to sample 4ch single-ended signal. The datasheet suggests ADR431 or ADR421 as voltage reference for AD7190. However, an article on Analog Dialogue gives a way to calculate whether an additional output buffer is needed for voltage reference.

The article gives an example:
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So, I did calculation myself for ADR435 and AD7190.

The output impedance of ADR435 is calculated in the article above.

The AD7190 is 24bit ADC and the Iref is 7uA/V, which gives its Ro_max=(5V/2^25)/(7uA*5)=0.00425 Ohm.

The Ro_max(AD7190) < Ro(ADR435).

Is that means a output buffer is needed or my calculation is wrong?

Thanks.

The link of the article is down below.

https://www.analog.com/en/analog-dialogue/articles/precision-successive-approximation-adcs.html

Best Answer

The ref has 3.5µVpp noise (on 2.5V that's 19.5 bits).

This noise is higher than voltage drop due to ADC current (7uA*5)*0.075R = 2.62µV.

However, note your ADC is a sigma delta, so it doesn't draw current pulses from the reference voltage, unlike the SAR ADC mentioned in your article. So the article doesn't apply for this ADC.

Also the ADC datasheet says:

Average Reference Input Current 7 μA/V typ

Average Reference Input Current Drift ±0.03 nA/V/°C typ

This implies the reference current is constant, which makes sense for a sigma delta ADC, it's just going to end up on a comparator input.

So your reference current is just going to cause a constant DC offset which will be negligible compared to the tolerance on your reference voltage.

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