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ADS8900B: What is the specialty of this part over ADS86xx series?

Venkatesh P17
Intellectual 895 points
Part Number: ADS8900B
Other Parts Discussed in Thread: ADS8671, , REF5040, ADS8691, THS4551

Hi TI Team,

The part ADS86xx integrated analog front end, internal reference, SPI interface with 66Mhz.

for one of my design I have chosen ADS8900B because of the resolution. And another design I need only 13-bit. For footprint point of view I may choose this same part for this application also but in the cost sense it is quit high. So it is advisable to use ADS8900B or can I go with ADS8671? 

What are the limitations over ADS8671 over ADS8900b?

Can I configure the ADS8671 as the following?

Here the block is I created in dummy just for representation. Please comment your suggestion.

Thanks,

  • 0
    TI__Mastermind 24940 points
    Hello,
    The ADS8671 and the ADS8900B are not equivalent parts. The former is a single ended input device, while the latter is a differential input. The ADS8900B is a high precision device with other integrated features; to maintain the performance of the device a discrete input driving circuit would be better suited. If you need a high resolution device, then the ADS8900B family is a great choice.
    The schematic shows the device ADS8671 set up correctly. The REFIO pin needs to be decoupled with REFGND, as well as the DVDD decoupled to GND with minimum 10uF and 1uF caps. I would suggest connecting the RESET to a controller to have the ability to reset the device if needed. What will your input signal be/sourced? It seems you will be sampling the same input with the ADS8671 and ADS8900B. What is the application of the design?

    Regards, Cynthia
  • 0 in reply to Cynthia
    Intellectual 895 points
    Hi Cynthia,

    Thank you for your response.

    Are you sure that I don't need any Buffer before the ADC? Shall I directly give signal to be measured to ADC?
    For reference, I plan to use REF5040.

    You suggested that to use 10uF and 1uF in DVDD, can you please explain why to use this combination? Can I use either one or both?

    The main objectives of my application is to measure the current to the load and voltage across the load measuring and monitoring. The current to load changes upto 1MHz.
    I am using ADS8900B for current measurement because I need more resolution(3.5A with 0.01mA resolution = 350000 steps)[current sense resistor value is ok, because the I_avg is less]. So I chosen ADS8900B part. For voltage measurement the requirement is as I mentioned in previous question.
    For current measurement I use current sense resistor drop and have to calculate equivalent current calculation in my controller.

    Thanks,
  • 0 in reply to Venkatesh P17
    TI__Mastermind 36240 points
    Hi Venkatesh,

    The input impedance of ADS8671 is 1Meg. This will only add ~3uA of error current to the measurement. Assuming your full scale range is 3.5A and using a 14b converter, your resolution is 214uA, so this error is much less than the resolution of the ADC. You do not need a buffer in this case.

    It is suggested to use two capacitors, the larger capacitor is there to handle most of the load change, but the smaller one is needed to supply the high frequency load currents.

    Also, the ADS8671 does have an internal reference, but you can use an external ref as well if needed. Also, please note that the input voltage needs to be less than +/-20V. Since you are measuring fast current transients, there may be high voltage transients that occur due to parasitic inductance in the wiring and connectors. If this is the case, you will need to add some type of external overvoltage protection. Please refer to section to section 7.3.3. of the datasheet.

    Take a look at the evaluation board schematics for a complete circuit with all of the recommended passive components.
    www.ti.com/.../sbau252.pdf

    Regards,
    Keith N.
  • 0 in reply to Keith Nicholas
    Intellectual 895 points

    Hi Keith,

    Thank you.

    For current measurement ADS8900B is must because of the resolution. So I decided to use this. 

    And also I was thinking to use ADS8691 when once you suggested in another post. but I needed 10uA resolution in 3.5A full scale. So I chosen ADS8900B. But this requires front end, buffer, reference and also need a buffer to voltage divider circuit from Vref to Vocm pin of THS4551. So the cost increased but I hope it will give better performane

    But For voltage measurement 13-bit is enough so I think like why should spend with ADS8900B? Because it is costly and need to design analog front end and buffer, these also drew some cost and place consuming. So I referred the part ADS8671.

    I will check with my load and parasitic inductance to whether it will affect my design.

    Thanks,