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DDC114

Stewart Forbes
Prodigy 60 points
Other Parts Discussed in Thread: DDC114We are experiencing some odd behaviour on one of our products, which uses the DDC114. If the units have been powered up for some time ( say more than 4 hours ) and we run a burst of acquisitions ( the DDC114s are being run in non-continuous mode ), quite often we see an unexpectedly high level in the data returned ( in the region of ~ 140,000 ). This is not transitory, as might be expected by a pre-charged integration capacitor. This level is present on some or all of the three channels we are using on each of the two converters in our system ( 6 channels in total ): NB. Channel 4 on each converter is left open circuit. We have subsequently discovered that the effect appears to get worse when we use the RANGE pins to set the smallest integration capacitor. As such, I have a couple of specific questions :- • What should we do with the unused input ( AI4 ) on each of the two converters ? • I see in Rev 2 of the DDC114 datasheet ( Last Sentence, Paragraph 1, Page 9 from the TI Web site, also seen in a couple of diagrams... ), that if we chain two converters on a serial link then we should do the following:- “…DIN and [NOT] DIN are only used when multiple converters are cascaded, and otherwise should both be tied to DGND…”. Besides the fact that this seems odd ( both DIN and the inverted input tied to the same point ), it is also mentioned on the revision information on Page 28. So the question is, what should we be doing here ? • Finally, have you ever come across any situations where the DDC114 provides what I can only describe as an erroneous stream of readings in the 140,000 range and if so, what caused it ? I look forward to hearing your thoughts.
  • 0
    TI__Guru 65190 points

    Stewart,

    Sorry for the delayed response. When you see the high output, if you take a few captures, does the problem go away? How have you been fixing the issue?

    As far as the unused inputs: The analog inputs on these devices are high-impedance so you should leave them floating if you are not using them.

    The complementary DINs should both be tied low as per page 39 of the DDC11X product development kit which includes the DDC114 device http://www.ti.com/lit/ug/slau234a/slau234a.pdf

    I have not seen the behavior you mentioned but maybe we can come up with a solution based on your answers.

    Regards,

    -Adam Sidelsky

  • 0 in reply to Adam Sidelsky
    Prodigy 60 points

    Adam,

    thank you for the reply - Your help with this is much appreciated.

    We have found that the problem persists evens after a burst of ( >20 ) acquisitions. We have however found that the problem goes away if we reconfigure the Range setting on the DDC114, which is our current work around. For example, if we change from Range 0 to Range 1 and then back to Range 0, the problem goes away ( if the machine is left on for several hours, the problem might return )...

    Please let me know if you need any more detail.

    I look forward to hearing from you.

    Kind Regards,

    Stewart

  • 0 in reply to Stewart Forbes
    TI__Guru 65190 points

    Stewart,

    Are you simply pulling the range 0 bit low and pulling the range 1 bit high? Are you sure that you are not resetting the device at all, changing the CLK speeds or CONV timing when this range selection is changing? In principle all that the range change does is disconnect the range 0 cap and then connect the range 1 cap, then reconnect the range 0 cap.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 60 points

    Adam,

    in answer to your question, we start with :-

         RANGE 2 is 0

         RANGE 1 is 0

         RANGE 0 is 1

    then we change the following :-

         RANGE 2 is 0     ->     0

         RANGE 1 is 0     ->     1

         RANGE 0 is 1     ->     0

    and the fault clears.

    We then set it back to :-

         RANGE 2 is 0     ->     0

         RANGE 1 is 1     ->     0

         RANGE 0 is 0     ->     1

    and the fault remains clear for some hours ( we have seen it return after > 4 hours ).

    We do not change CLK speed or apply a (N)RESET and we do not change the acquisition timing.

    Please let me know if you need any further information.

    Kind Regards,

    Stewart

    Table 1. Range Selection of the DDC114

    RANGE2 RANGE1 RANGE0 CF

    (pF, typ)

    INPUT RANGE

    (pC, typ)

    0 0 0 3 −0.048 to 12

    0 0 1 12.5 –0.2 to 50

    0 1 0 25 –0.4 to 100

    0 1 1 37.5 –0.6 to 150

    1 0 0 50 –0.8 to 200

    1 0 1 62.5 –0.1 to 250

    1 1 0 75 –1.2 to 300

    1 1 1 87.5 –1.4 to 350

  • 0 in reply to Stewart Forbes
    TI__Guru 65190 points

    Stewart,

    During your "idle" time (the few hours before you try bursting acquisitions again), what state are your CONV, CLK, VREF, and power pins?  

    Regards,

    -Adam

  • 0 in reply to Stewart Forbes
    TI__Guru 65190 points

    Stewart,

    Any update here?

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Prodigy 60 points

    Adam,

    sorry, I've been tied up on another job.

    As soon as I am able, I'll check the above and give you some feedback.

    Thank you for following up on this question.

    Kind Regards,

    Stewart