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LMH32401: LMH32401 APD bias voltage

Part Number: LMH32401

Hi TI support team,

at the moment we are planning a new design for optical detections systems. The new part LMH32401 seems to be very interesting for our apltication but there is one thing that is not clear to us. In the datasheet you find the follwing description:

Does that mean using PD with opposite biasing where the cathode has a high positve voltage and Anode ist connected to the input of TIA will not work or is there a way to make it work?  After checking quite a lot of APD power supplies I could not find any which directly brings out a negative APD- voltage for biasing. They will bring out a postive voltage with reference to GND. Maybe I did not have the right keywords.

So questions is: What circuit do you recommend for optimal biasing?

Any hint, suggest or help is very welcomed.

Thanks in advance.

  • Hello Henning,

    It seems like the picture wasnt properly posted. Could you clarify what part of the datasheet you are referring to?

    Best,

    Hasan Babiker

  • Hi Hasan,

    thanks for informing about the problem.

    I refer to to section 7.1 on page 14 of LM32401' datasheet stating that the anode of the PD/APD has to be connected to a negative voltage source to work properly with LM32401 device.

    As already said parts seems to very interesting but maybe our application will yield for a APD with positive power supply at the cathode. It would be great if the part can also handle that.

    Any help, hint or idea is very welcomed.

    Thanks in advance,

    Christian

  • Hi Henning,

    no, the LMH32401 must source the photocurrent at the "IN" pin.

    Generating a negative bias voltage for the photo diode should not present a problem. It's no rocket science. The anode's potential merely needs to be lower than the "default input bias voltage" of 2.47V. What bias voltage do you intend to apply?

    Kai

  • Hi Kai,

    thanks for the quick answer.

    For  a photo diode without using a reverse biasing voltage you are right. But we want to use an APD in photoconductive mode so we have to bring diode up to a voltage that can be more or less 90V to work properly.

    If I take a closer look at any of the available compact APD power supplies which gives the chance of fine adjustment of the output voltage which is important for best performance, i could not find any which will bring a negative voltage reference to the input ground.

    The only chance to get a negative voltage would be an isolated power supply to change output polarity and connect to diode.

    Or is this a kind of misunderstanding which means that Vbias in the datasheet describtion is related to the LMH32401 characteristics but not with respect to the connected APD?

    Maybe anybody can help me to get the things right.

    Christian

  • Hi Chriatian,

    Vbias is the bias voltage applied to the photo diode. See also section 7.3.2 and section 8.2.2 of datasheet LMH32401!

    Do you know these appnotes?

    application-note-apd.pdf

    application-note-apd-array.pdf

    Kai

  • Hi Kai,

    okay, that it is no misunderstanding. I just wanted to double check this.

    Then we have to chekout the optimal and space saving way to get the negative ADP power supply.

    Christian