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problems with ADS823

Oscar Seijo
Prodigy 30 points

Hi there.

I am having problems with the ADS823 converter. I used the recommended circuit shown in the Fig. 2. of the datasheet but I am only getting one bit conversion output. I mean, every out pin gives the same output voltage. When the input signal has more than 2,5 V the output is "1111111111", and when it is less than 2,5 V the output is "0000000000". I think the problem is related to the ByB and ByT PINs, cause I let both of them open. However, the datasheet does not give enough information about their mission, or I am not understanding it.

Could be my problem related to those pins? Should I bypass and connect them to ground? What is the function of those pins?

Thank you very much for your attention.

Óscar

  • 0
    TI__Mastermind 29220 points

    Hi,

    The data sheet says on page 11 that ByB and ByT are brought out to pins on the device only so that an external bypass capacitor can be conencted to them to help keep the signal node clean.  These pins are not to be used for anything else.  Figure 6 shows that the REFT and REFB are generated by the device  (if in internal reference mode) and the signals ByB, ByT, and CM are derived from the REFT and REFB.    Not having a bypass cap on ByT and ByB would not cause the issue you are seeing but rather just might cause a little loss of performace if the caps were not there.  Grounding these pins or doing anything else with them would cause problems for the internal reference.

    Make sure you are actually in internal reference mode with the pin INT/EXT grounded.   if this pin is not grounded then your circuit would have to supply the 3.5V and 1.5V needed for REFT and REFB.   Otherwise, the full scale range might be nothing and you might see the result you are seeing.  Take a good high-impedance voltmeter and check that you have the INT/EXT pin low and that the pins REFT, ByT, CM, ByB, and REFB are 3.3V, 3V, 2.5V, 2V, and 1.5V respectively.

    Unless your design is supposed to be using external reference, and then check that the voltage you are supplying to REFT and REFB is 3.5V and 1.5V.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 30 points
    I wont be able to check the PINs voltage until the next week, but I swear that I saw the VCM voltage (fig. 2.) was 2.5 V in my pcb . The INT/EXT pin is also grounded in the design file. Unless I had forgotten to tin that pin...
    I will write next week if that did not resolve my problem.

    Thank you for your attention :)

    Óscar.
  • 0 in reply to Richard Prentice
    Prodigy 30 points
    Hello again,

    I have cheked those PINS, and I have the outputs you said, except ByT and ByB that are 2.5 V. I have also tined a bypass capacitor on ByT and ByB. The pin INT/EXT is correctly grounded.

    Im sorry I was wrong, my circuit converts, but only when the input value is higher than 2.5 V. For example, if my input is a sine_wave, the voltage in the IN PIN of the ADS823 is 2.5V+sin_wave. When sin_wave<0 V, all the output are 0, when it is >0 V, it works correctly.

    Besides, sometimes the ADS stops working, and I have to unplug and plug it again to make it work again. May I am short-circuiting anything?

    Thank you for your attention,

    Óscar.