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DDC316: How to collect current data

Jonathan Navor
Genius 9055 points
Part Number: DDC316
Other Parts Discussed in Thread: DDC264, , DDC232, DDC112

Hi Expert, 

We need your help to suggest a device for our customer. Here is the full detail below. The customer would be interrested on this product to measure current from multielectrodes of the position sensitive photodetector. 

My project is the application of different novel contact configurations on two-dimensional position sensitive photodetector to improve position estimation accuracy. I attach a drawing sketch to simply illstruate how this device work. 
PSD device.pdf
Generally speaking, when a laser hit a point on the 2D photodiode, the photocurrent will be divided unevenly to contacts along the perimeter. The hitting point position is a function of current from these multiple outputs , so that when a laser hit a random point, the position can be predicted by these current outputs. For my project, I need to collect current from multiple output up to 16 simultaneously. 
Do you have any suggestion for me? I also want to know what is the minimum current value that your product can read, because according to my calculation, the current can be as low as 1uA.
Thank you in advance for the support.
Best regards, 
Jonathan
  • 0
    TI__Mastermind 19815 points

    Hi,

    Does customer roughly know the range of the current and how long they want to integrate the current?

    And, Could customer clarify whether they will connect the photodiode in the same bias setting as shown in datasheet page 10, figure 5 and assuming current is flowing into the analog input?

    Thanks

  • 0
    TI__Expert 4590 points

    Hi,

    Actually 1uA is about the maximum current that our DDCs can read. Basically look at the full-scale of the device (say 150pC for the DDC264) and the minimum integration time (166us on that device) and you will get the maximum current that the given device can take without saturating. In this case 150pC/160us=~0.9uA.

    DDC316 is not much different (12pC/10us=1.2uA)

    If too much current is an issue, they'll have to adjust it at the source (lower the intensity) or sensor level (add optical attenuator), or have a resistor divider at the output of the sensor before the DDC. Using one sensor output connected to several DDC inputs (splitting the current across inputs) is also likely to work (don't recall doing the test though).

    Other than that, to select what DDC to use (assuming you want to stay with DDCs), you would go by number of channels and speed, as I think they were doing already...

    Regards,

    Edu

  • 0 in reply to ChienChun Yang
    Prodigy 50 points

    Thanks for your reply. The total photocurrent generated by laser is about 250uA. The dark current is around 0.25uA uniformly distributed across the whole plane. Depending on the number of electrodes and where the laser hit device, I think each electrode may receive current varying from 0.1 to 250uA. Since I haven't tested the device, I am not sure if this range is accurate.

    As for the bias setting, a voltage source will let the device under reverse bias, and current from each electrode will flow into analog input. 

    As my major is materials science, I don't quite know the electracial knowledge. It's not easy for me to read the datasheet.

  • 0 in reply to Eduardo Bartolome
    Prodigy 50 points

    Thanks for your reply. The total photocurrent generated by laser is about 250uA. The dark current is around 0.25uA uniformly distributed across the whole plane. Depending on the number of electrodes and where the laser hit device, I think each electrode may receive current varying from 0.1 to 250uA. Since I haven't tested the device, I am not sure if this range is accurate.

    As you say the maximum current that DDCs can read is only 1uA, apparently DDC is not a good choice for my purpose. I wonder if you have any other products that are suitable for my project?

  • 0 in reply to Jinyi Liu
    TI__Expert 4590 points

    I am not aware of anything else in TI with transimpedance amplifier functionality and the DDC level of integration that would support higher input current range. Basically it sounds to me that you are going to have to implement this with a discrete amplifier (an array of them) but I am not an expert on that part of the portfolio.

    Regards,

    Eduardo 

  • 0 in reply to Eduardo Bartolome
    Prodigy 50 points

    Hi Edu,

    I would like to confirm with you that if DDC232CK has maximum current input 7uA (350pC/50us=7uA in noncontinuous mode)?

    Does noncontinuous mode mean that only discontinuous current value can be collected in this mode( for my project, there are a few seconds interval between each collection point, which I think is suitable for noncontinuous mode?)

    Thanks

  • 0 in reply to Jinyi Liu
    TI__Mastermind 19815 points

    Hi,

    Yes, maximum current input 7uA (350pC/50us=7uA in noncontinuous mode).

    By referring DDC232 datasheet with Figure 10 and Table 8. State Descriptions, in the noncontinuous mode, the states/processes of the chip goes roughly like

    Figure 13&16 - 

    Integrate A side(State 3), Integrate B side(State4), wait(the input switches are opened)(State 1 then 2), release, Integrate A side(State 5) 

    or

    Figure 14&15 -

    wait(the input switches are opened)(State 8 then 7), Integrate B side(State 6), Integrate A side(State5), wait, release, Integrate B side(state 6)

    i.e.

    regardless whether the input current is continuous or not, it can only integrate current non-continuously as during the wait state, the inputs switches are opened.

    Thanks.

  • 0 in reply to ChienChun Yang
    Prodigy 50 points

    Thank you very much. DDC232 is what I need.  I was also told that an evm is needed to work with DDC232 together. However, I didn't see DDC232EVM in its page.

  • 0 in reply to Jinyi Liu
    TI__Mastermind 19815 points

    Hi,

    Correct, we don't have EVM for DDC232.

    You could try DDC11xVM to evaluate, it has noncontinuous mode with Non-Continuous Mode integration time as low as 50us. 

    https://www.ti.com/tool/DDC11XEVM-PDK

    Thanks.

  • 0 in reply to ChienChun Yang
    Prodigy 50 points

    I wonder if DDC11xVM can evaluate 32 channels at the same time? as it is designed for DDC112 and114.

  • 0 in reply to Jinyi Liu
    TI__Mastermind 19815 points

    Hi,

    They don't have 32 channels. They have either 2 or 4 channels.

    But, maybe you can use it to evaluate 2 to 4 channels first, and once you experiment&test enough to have more understanding, then you could start developing your own PCBA for the 32 channels chip e.g. DDC232.

    Thanks