AD4000 reference voltage and power consumption

Hello,

A couple of questions about AD4000 design:

It is a 1.8V device; however, the current consumption is not mentioned anywhere in the DS (or I am not looking hard enough). On page 6 of datasheet, power dissipation numbers are given for different sampling rates. Can one assume this to be power consumption (vs power dissipation) numbers also?

In user guide for AD4000 UG-1042 a buffered Vref ADA4807-1 is used. Can it be replaced with an alternate like ADR381? If not, what might be a good candidate for reference voltage?

Thanks

Parents
    •  Analog Employees 
    on Mar 18, 2021 1:48 PM

    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

Reply
    •  Analog Employees 
    on Mar 18, 2021 1:48 PM

    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

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