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ADS1601 INTERNAL REFERENCE ISSUE

WhiteHorse
Genius 4430 points
Other Parts Discussed in Thread: ADS1601

e2e,

We have a customer using the ADS1601 with the following problem:

We’re using the ADS1601. It is set up for the internal reference, i.e. /REFEN (pin 13) is low (pulled low by the internal pull-down). I attached the schematic.

 That should make the reference about 3 V, but it’s more like 0.9 V. I’m measuring about 3 V on REFP, and 2.1 V on REFN. It varies from one part to the next, but they are similar.

 Other than that, the ADC works, but it makes the full-scale range rather small.   The ADC works and produces reliable data; it’s just the reference that’s wrong.

What could be causing the reference voltage to be wrong.  I have attached part of their schematic.  

 

Thanks for your help.

John Wiemeyer 2068.ADS1601 Schematic.pdf

 

  • 0
    TI__Mastermind 23235 points

    John -

    Have they tried pulling the REFEN pin low? (ie not relying on the internal pull-down)

    They also might want to check the decoupling capacitors for leaks/bad cap.  It sounds like one could be resistive; they can just pull them off the board and measure the VREFP/N pins to see if the voltages are correct.

  • 0 in reply to Greg Hupp
    Genius 4430 points

    Hi Greg,

    They tried pulling REFEN pin low externally and saw no chnage.  They tried pulling it high and the reference output when to 0V. 

    I asked them to remove the capacitors and finally lift the pins from the PWB to ensure it isn't the problem.  

    Thanks for your help.

    John Wiemeyer 

  • 0 in reply to WhiteHorse
    Genius 4430 points

    Hi Ggreg,

    The customer found the problem. 

    I’m using a value of 267K for the bias resistor (pin 8 to GND). That’s appropriate for operating at a low clock rate, but if you read the fine print, they point out that the internal reference voltages go goofy if you use a bias resistor higher than the standard 60K, so you should only do that when you’re using an external reference.

    The solution is to use 60K for that resistor. I tried it on one circuit, and it works fine.

    Regards,

    John Wiemeyer