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UCD9240 documentation inconsistencies

EDGAR BROWN
Intellectual 450 points
Other Parts Discussed in Thread: UCD9240

I have completed one design with the UCD9240 (80 pin version) but I still have some design issues due to datasheet inconsistencies. Namely:

What to do with the ADCREF pin? Depending on what documentation I follow this pin:

- should be left floating when unused.

- should be bipassed with a .1uF capacitor to gnd.

- can be used for an external 1.6V reference.

What is this pin purpose, what should I do with it?

 

What design criteria should we use for V33FB? Depending on the document this pin should have

- 10mA _typical_ design current (no minimum)

- 10mA _maximum_ design current (no minimum), with 5mA recommended as a guideline.

After following design guidelines, using 4mA as a target resulting in a resistance range from 300Ω to 1KΩ , during testing I had to further reduce it to 220Ω to get a reasonable voltage out (within 5%), with a very high dependency in this resistor value. Is this output even regulated in any real sense? (The documentations is not clear on this point either.)

  • 0
    Intellectual 450 points

    After talking to TI and some experimentation:

    - The ADCREF signal is not connected at all in the smaller packages (it is pin 80 in the one we are using), so it does not _need_ to be connected to anything. However, if it is left floating it _could_ couple some external noise into the measurement circuitry, therefore it does not hurt to bypass it to ground with a small capacitor. How much of an issue this could be for you completely depends in the environment the IC is in. So how much do you need such capacitive bypassing is relative. (I was not able to measure any voltage on that pin, so I am not sure there is any real use for it.)

    - Our issue with the V33FB current was related to a different problem, the IC reference was out of spec due to insufficient capacitive bypassing of the analog supply. The 4mA target design value worked properly after fixing that issue.