AD4000 reference voltage and power consumption

Hi Mohi,

In this comment I'll respond to the query on the AD4000 REF input drive requirements (i.e. buffered vs. un-buffered voltage reference).

The short answer is "it depends" and it is a function of the sample rate of the AD4000 and the accuracy/error tolerance of your system. This article is a good way to familiarize yourself with the nature of SAR ADC REF input as a dynamic current load and requirements on the end-system: https://www.analog.com/en/analog-dialogue/articles/precision-successive-approximation-adcs.html

The ADA4807-1 on the EVAL-AD4000FMCZ board was selected due to that amplifiers low noise and low output impedance, plus it's ability to maintain stability with a 10uF load (i.e. the capacitor next to the REF pin). It's simply one of the best options for maintaining really high accuracy of the VREF voltage even while running the AD4000 at the full 2 MSPS.

Question: What sample rate do you plan to operate the AD4000 at?

When selecting which device will be driving the SAR ADC REF pin (i.e. voltage reference directly driving vs. buffering it with an op amp), the main consideration should be, how much error in the VREF voltage can be tolerated for the application, and how fast of a sample rate needs to be used.

From the article provided you'll notice that SAR ADC REF pins behave like transient current loads, where they draw a spike of current for a short period of time, as opposed to being a fixed current over time. Basically whenever the ADC performs a conversion, it needs to pull charge from the REF input for the SAR ADC bit trials, and that's what you'll see in those plots.

If you would prefer to drive the REF pin directly with a voltage reference device, then I would recommend either the ADR4550 or ADR435 family of voltage references, as they have quite good load regulation specifications.

Thanks,

Tyler

Hi Tyler,

The current draw by REF pin of AD4000 is 100uA at maximum, so ADR4550 seems to be a good fit.

It can supply 10mA of current and has very low output noise.

Thanks for your analysis and pointing to the literature.

-Mohi

Hi Tyler,

One last question: Is there any performance concern if we use ADR4540 (4.096Vref or any other reference) instead?

The input to AD4000 will be 0 to 4V.

Thanks,

-Mohi

Hi Mohi,

From a settling/accuracy perspective, the ADR4540 should behave just as well as the ADR4550.

Note that the dynamic range/SNR will be slightly less for a 4.096V vs. a 5V VREF, but since your signal is only 0 to 4V anyway, that won't be a problem. Input referred noise will be the same between VREF values anyway.

Thanks,

Tyler

Thanks, Tyler.

Thanks, Tyler.