• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 479 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DDC232 reference and negative current input

Josep Segura
Prodigy 10 points

Other Parts Discussed in Thread: DDC232, DDC112, DDC264

Hello,

We are building demonstration boards for our customers containing a large area image sensor with 200 output channels. Signals are currents which are negative when seen from the readout circuit side. DDC232 and similar must have positive current input.

The only method I found from your literature is to add an offset to be summed with the input to allow bipolar signal compliance (application note "Creating a Bipolar Input Range for the DDC112"). Such technique, however, is quite difficult to implement for every input as the system has 200 channels. Noise also increases.

My question is: looking at the DDC232 pinout,  VREF has 2 input pins (A6, B6). Do they correspond separately to VREF for Cf reset (in charge amplifier) and VREF for ADC? My idea would be that setting VREF for feedback capacitors to 0V would allow fully range negative input.

Thank you very much for your answer.

Josep

  • 0
    TI__Expert 4680 points

    Hi Josep,

    Sorry for late answer but I am waiting confirmation from design. In the mean time, I'll say that I don't think the 2 pins are for the two blocks, as you described them. I think they are tight together internally, and duplicated just to reduce series inductance.... but let me get confirmation...

    Also, even if they were separated, usually things are internally much more complicated than they look on the DS, so, I think it is a long shot.

    Overall, you are right on the assessment of the situation. I also think it would be at the very least, noisy. Is there a way to change the polarity on the sensor? I guess that is the first thing you tried but probably all anodes are tight together and can't be done. Is that right? Got to think more on this... How do they deal with this usually? Feel free to write me at e-bartolome1@ti.com and we can discuss further...

    Regards,
    Eduardo

  • 0 in reply to Eduardo Bartolome
    TI__Expert 4680 points

    Hi Josep,


    After thinking a bit more on this, actually I think that adding the resistors may not be so bad from a performance point of view. I agree that it will take board space. It will also add parasitic capacitance which will increase noise, but may be acceptable (may be a fraction of that on the photodiode?).

    Bottom line, we use this kind of connection to measure linearity (to inject current in through a resistor, see DDC264 EVM) and although we observe a tiny degradation on noise, it may be acceptable if nothing else is possible (?).

    Regads,

    Eduardo

  • 0 in reply to Eduardo Bartolome
    TI__Expert 4680 points

    Just to update other folks on the latest discussion...

    Got answer from one of the designers that said that it is not possible to support a negative current to the DDC for other reasons as well as the 2 ref being used in other places. He mentioned they will send more details later but I basically don't think this is going to work.

    Adding to the discussion, he recommended the external resistors to be very high to minimize noise from them, therefore, the voltage applied needs to be relatively high. There will be a need, after that, to calibrate for errors between resistors but that seems doable.

    Will update in the future any other info I can get...

    Best regards,

    Eduardo