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Original question:

LMH32401: How to connect negative voltage and APD with LMH32401

LMH32401: The photo diodes used to test LMH32401

Helen chen
Mastermind 40080 points
Part Number: LMH32401

In the LMH32401 EVM UG, on page 4, it wrote:

"the input trace can be used to connect a common three-terminal photo-diode in a TO package. The photo-diode can
be mounted over the edge of the board where the cathode can be connected to the input of the amplifier
and the case and anode connections can be soldered to any of the three traces on the bottom of the
board."

I log into the first sensor website, but didn't found any negative biased APD. could you please share the datasheet of the "three-terminal photo-diode in a TO package"  with us, thanks!

  • 0
    TI__Mastermind 22270 points

    Hello Helen,

       This statement on page 4 in the LMH32401RGTEVM User guide describes the option of mounting any three-terminal photo-diode of your choice that comes in a TO-package. This option is given to easily be able to test photodiodes with the EVM, rather than only having the option to input a voltage source to the input of the EVM. Ideally you would connect the photodiode's cathode to the trace leading to the input of the amplifier, while the anode would be connected to the external bias using any of the three options on the EVM (A, B, C). Boost converters can be used for applications that include APD biasing; but this is not what is mounted to the EVM. Instead, in this case, these can be used as an external bias to an APD. Usually for experimental/testing purposes, you can just use a precise power supply for quick negative biasing of the photodiode. An example of a three-terminal photodiode would be this: Example1 and Example2 , but you can find others using a third party distributor depending on desired specs (capacitance, dark current, active area, bandwidth, etc). A diode with a different package would work as well, but will require some unique attention to soldering and mounting. 

    Thank you,

    Sima

  • 0 in reply to SimaJalaleddine
    TI__Mastermind 40080 points

    Thank you Sima for the detailed explanation. 

    I checked the APD datasheet, they are positive voltage biased APD, do you ever used some negative voltage biased APD in the test? Because seems LMH32401 is mainly for the negative voltage biased APD current measurement.

  • 0 in reply to Helen chen
    Guru 47485 points

    Well Helen, 

    A diode is a diode, this is just how the manufacturer draws their example circuit. 

    But if you connect the cathode to the amplifier input, with a negative bias on the anode, you will get a current flowing down towards the bias causing a positive going output for the amplifier, 

    You can take this exact diode and connect it as shown in this document, 

    www.ti.com/.../sboa337.pdf