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Original question:

ADS8691: Extra electronics

ADS8691: Passive Component Selection

Stephen Poulsen
Prodigy 140 points
Part Number: ADS8691
Other Parts Discussed in Thread: REF5040, ADS8681

I have a number of question regarding the proper implementation of the ADS8691. 

1) The current project contains 4 ADCs which for consistency need to share a reference. It appears that I can either create a separate RefSignal (REF5040) and bus it to each of the inputs, OR I can use the internal Reference from one of the 4 ADCs and bus it to the other 3. Is there a recommended best practice?

2) The spec Sheet for the ADS8691 includes an example layout. The layout includes a few resistor pads with connected resistors. Values are not given. How should these values be calculated?

3) On the same layout, there is an options section separated by a diode. is this section an external input, or something else. I am trying to determine if it may be useful for testing, or something else. 

4) The spec sheet indicates the voltage requirements for the ADC, but i am unable to locate details on the power consumption, or current draw for either the Analog, or digital supplies. Nor how those may vary with the voltage sources. How is this to be determined? 

  • 0
    TI__Mastermind 35520 points

    Hello Stephen,

    Welcome to E2E forum.

    1. Either external voltage reference or sharing internal voltage reference on one ADC can be used because ADS8691's internal VREF is a precision voltage reference with higher accuracy and lower drift(4ppm/C typ and 7ppm/C max)  also there is a integrated reference buffer which is very important when sharing a voltage reference with multi-ADCs.

    2. For front-end resistors of RC filter to ADC input, the resistor value will determine your cut-off frequency of filter, ADS8691 integrates PGA and ADC driver so you don't need to worry to design a front-end circuit to drive ADC properly and settle your input signal during limited acquisition time, but larger resistor will lead to a higher system gain error combined with constant 1Mohm input impedance on ADS8691, also will have a higher noise and drift issue, please refer to this app note:

    www.ti.com/.../getliterature.tsp

    3. This layout example comes from ADS8681EVM board, the diode's footprint is one option for the user to design and test external overvoltage transient input protection to ADS8681 family including ADS8691. Actually ADS8691 has internal OVP circuit which can protect the ADC from +/-20V input signal or +/-11V input signal if AVDD is not available. If your input signal or transient event may be higher than +/-20V, an external clamp diode is necessary to protect the ADC, also external protection circuit is a good practical design as a secondary protection even if your system doesn't have higher voltage input signal or transient event. Please let me know if you need more information for clamp diode protection design.

    4. The current consumption on power supplies are specified on Electrical Characteristics table in page 7 of ADS8691 datasheet, for example, AVDD analog power supply current is 7mA typ/8.75mA max with external voltage reference at highest sampling rate 1Msps, please note that the consumed current on AVDD is proportional to the sampling rate, and the lower sampling rate, the lower current consumption.

    I hope these information have answered your questions and are helpful for you, thanks for selecting TI precision ADCs.

    Best regards

    Dale Li

  • 0 in reply to Dale Li
    Prodigy 140 points
    Referring to Item #2.
    " the resistor value will determine your cut-off frequency of filter", If I want to measure DC voltages, then Do I not need any input resistor, since I do not want to cut off any of the low end frequencies?
  • 0 in reply to Stephen Poulsen
    TI__Mastermind 35520 points
    Hello Stephen,
    Since ADS8691 has integrated a 2nd-order low-pass filter, so you may measure your DC signal directly, but if there is an unexpected signal (interference or noise component) in your useful signal or system also its frequency is less than 15kHz, you will need a external RC filter to filter it.

    Best regards
    Dale