Need help deciphering/circuit analyzing this ADC setup

Discussion in 'The Projects Forum' started by Mach3Maelstrom, Feb 14, 2013.

  1. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
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    I have a design that used to use one type of ADC, but for design reasons I must switch to a related ADC of similar pin layout (I already have the bare boards).

    The problem: My design worked flawlessly with the old ADC, but I have problems with reading data from the new ADC and I need to analyze why.

    Old ADC, ADS7820: Link to datasheet
    New ADC, ADS8504: Link to datasheet

    Summary of major differences between the two chips:
    • ADS7820 uses straight binary logic for its 12-bit output, ADS8504 uses two's complement.
    • ADS7820 can only take analog values between 0V - 5V. ADS8504 can take analog values between -10V - 10V.
    • My analog signal will ALWAYS be between 0V and 2.5V, so both differences above shouldn't make much of a difference

    For ADS7820 (pic found on page 9 of the datasheet):
    [​IMG]

    The purpose of that potentiometer-circuit is to give REF a 2.5V reference, so I installed a 2.5V source there direct to pin 3 (REF) instead. The analog signal would feed into pin 1 directly. This circuit would work flawlessly in my design as it portrays the 12-bits of information to my CPLD.


    For ADS8504 (pic found on page 14 of the datasheet):
    [​IMG]

    From what I understand, the datasheet mandates that I install a voltage divider (Offset) to pin 1. It still wants 2.5V on REF, so I want to still use my 2.5V reference to provide that accurately.

    Two things I can't understand are:
    1) Why is this voltage divider needed? The datasheet states it provides overvoltage protection but, as I stated above, I never go over 2.5V.
    2) What voltage are they suggesting the net Offset (between the left pot & the 33.2K resistor) to be? This isn't clear to me in the datasheet. Hooking this directly into the 2.5V reference won't work as per my bench experiments.

    Any help would be greatly appreciated. Thanks in advance!
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    1.) It isn't looking at the next figure to the one you linked(Figure 24b) shows the ADC without the offset. All this does is allow for tuning of the reference. Leaving nothing, save for a capacitor to ground, attached to the REF pin causes the device to use it's own 2.5V reference. (pages 14-15)

    2.) The Offset is used to calibrate your device, so there can't be any specific information about what to set it to.(see page 14-Calibration)
     
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  3. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
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    0
    tshuck - Thanks for your response!

    In case I wasn't clear in my first question, by "voltage divider" I meant the 200R/33.2K voltage divider configuration, not the 50K pot. The first ADC uses the analog signal "straight up", whereas the second ADC uses this voltage divider off of pin 1. I just wanted to know why this is done.

    I recognize that I can use the internal 2.5V reference, though I fear this may result in less accurate readings than using an external 2.5V voltage reference.

    Do you think I'm able to keep my external 2.5V reference on pin 3 to ensure accuracy and still follow the schematic for connecting the voltage divider to pin 4 (CAP)?

    For everyone else's reference, this is what tshuck and I are referring to on the datasheet. I left it out in the initial question because I'm using an external 2.5V source on pin 3 and felt (b) was thus not an option.

    [​IMG]
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Well, like I said, this is used to make small adjustments in the reference. This is only for calibrating the reference to offset any error...

    This may not be the case, look as the specs for your external reference and compare that to those found on page 14:
    If you can beat that, do it!

    I will respond with the datasheet:
    As far as I can determine, you should be good.
     
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  5. Mach3Maelstrom

    Thread Starter New Member

    Jun 19, 2012
    10
    0
    Thanks tshuck. I didn't know how offset or gain adjustments affected the end code, so having read over that since my last post, I better understand what you were explaining.

    My 2.5V reference has a 3 ppm/°C drift, so I'll stick with that reference while adding the offset calibration circuit on pin 1.

    Thanks for all your help!
     
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