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ADS930 Tri State Outputs.

martin b
Intellectual 560 points
Other Parts Discussed in Thread: ADS930

Hi All.

 

I am in the process of using the ADS930 ADC, and I would like to be able to tristate the digital outputs, or put them in a high impedance state, in order to also be able to write to an LCD screen that will be connected to the same digital I/O lines.

I plan initially to sample for some milliseconds, and then process the results, before starting to collect more samples. Whilst not collecting data I plan on putting the digital outputs in the high impedance state, and at this point update the LCD.

 My question relates to how the data sheet is worded, which is as follows.

“The digital outputs of

the ADS930 can be set to a high impedance state by driving

the OE (pin 16) with a logic “HI”. Normal operation is

achieved with pin 16 “LO” or Floating due to internal pulldown

resistors. This function is provided for testability

purposes but is not recommended to be used dynamically.”

Does this mean that the high impedance state feature is not recommended in operation, and in fact this ADC cannot put its digital outputs in high impedance state?

 

Kind Regards

Martin

 

  • 0
    Intellectual 560 points

    Hi All.

     

    Hmmm, It' may be obvious to the pro that my question may be trivial, but is it so trivial that it does not deserve a reply.  How about it Richard, or anyone else,...please remind me on what forums are for?

     

    martin

  • 0 in reply to martin b
    TI__Mastermind 29220 points

    Hi,

    I tried to reply from my Blackberry that i am looking for the answer for you, but when my post did not show up I looked at what 'TO: TI E2E Community" pointed to and the address was noreply at e2e.ti.com.  Hmmm.  Sorry about that.

    You're right, i should have replied thatI was working on it.  This is an older device and these sometimes require a bit of searching.   I hope to reply shortly with what I find out for you.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    TI__Mastermind 29220 points

    Hi,

    I have been searching for clear information on what the author of that data sheet meant by that note that it is not recommended to switch OE dynamically, and am finding information scarce on an older device like this.  The outputs do go high-impedance with the deassertion of OE output enable.  There are timing specs on page three of the datasheet for enable and disable time, and this whole family of ADS9xx devices have the OE pin and three state drivers. 

    Since that paragraph starts off with a description of the pipeline latency, the author may have been trying to say simply that you can't enable or disable on a cycle by cycle basis and expect to get some samples from the pipeline and not others.  Or this note may have been copied from an earlier datasheet and not relevant here at all, as I notice that the similar ADS931datasheet does not have this statement about dynamic use of the OE at all.

    On some older devices the enable time and disable time were not carefully characterized, as this OE feature was not expected to need precise timing for frequent enabling or disabling, but since this datasheet does have the enable and disable time specified with maximums, then i think the OE was meant to be used on this device.

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Intellectual 560 points

    Hi Richard.

    Don't know what “noreply at e2e.ti.com” is about, but you got there in the end.

    Thanks for your help, it’s important for me to assemble a circuit near enough to correct the first time, as incomplete or inaccurate data sheets make this very difficult.

    Based on what you have said I’m really glad I did not empty my recycle bin, and was able to get my original schematic circuit back. The chip may be an oldie, but it looks good to me!!

     

    thanks again, martin