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LMH32401: LMH32401

Part Number: LMH32401
Other Parts Discussed in Thread: TINA-TI

Hi,

How can I design or configure the LMH32401 to work with photodiode (PD) configurations as a current source?

Do you have a practical circuit?

  • Hello Siamack,

      The datasheet for the LMH32401 shows the ideal functional diagram starting on page 20: 

      There is also a section on testing the device with a voltage source: 

      What are your photodiode characteristics (rise time, capacitance, and current range)?

    Thank you,

    Sima 

  • Hello,

    Thank you very much for your reply.

    I use the photodiode otir149si-d.

    Rise time: ?

    Capacitance: typ: 2pF, Max 10pF

    reverse dark current: typ: 2nA, Max 10nA

     

    Please check the attached picture from the datasheet.

  • Hello Siamack,

      Thank you for the information. For the capacitance range of 2pF with a max of 10pF, this is characterized in the datasheet which will determine the bandwidth of the system:

      The linear input range of the LMH32401 device is approximately 65 µA in the high-gain configuration and 650 µA in the low-gain configuration. The spectral response of the photodiode will determine the range (Photosensitivity at wavelength and power).

    Thank you,
    Sima 

  • Hello Sima,

    Thank you for the information, do you have a concrete and practical circuit that uses the LMH32401 except what datasheet provided?

    Br,

  • Hello Siamack,

      The LMH32401 is a specialized device, and the circuit provided is most recommended setup. What will be the specific application?

    Thank you,
    Sima 

  • Hello Sima,

    Thank you very much for your answer. I just need to design a transimpedance amplifier for light detection. I need the configuration of LMH32401, and the important part is the input and output compatibility of LMH32401. The input is already known, but what the output, is there any limitation?

    Br

  • Hello Siamack,

      This device is usually driving/interface an ADC, Comparator to TDC, or a RF transformer due to the benefit of the differential output. Limitation is mainly due to output loading and output swing. The device can not exceed around 27mA of output current and its output voltage range is between 0.36 to 2.9V. 

       Below is an example interfacing with an ADC (50 ohm resistors are not necessary for low frequencies and placement of both devices in relation to each other):

        

      Also, we have an EVM available for the device at this link which features a transformer to convert the differential output of the LMH32401 to a 50 ohm single ended connection. 

    Thank you,
    Sima 

  • Hello Sima,

    Thanks!!

  • Hello Sima,

    What happens if the capacitance of the photodiode is greater than the input capacitance of the LMH32401?

    And if the minimum output current of the photodiodes is higher than the maximum input current of the LMH32401?

    Thanks in advance!

    BR

  • Hello Siamack,

       If the diode capacitance is greater than specified in datasheet, there might be a increased chance of instability depending on how large of an input capacitance seen at the input of the amplifier. This is because the amplifier compensated internally. 

       And, if the input current is greater than the specified linear range for each gain configuration, then the amplifier will saturate which increases the recovery time. The end result is a broadening of the output pulse leading to blind zones in the system response. To protect against this condition, the LMH32401 features an integrated clamp that absorbs and diverts the excess current to the positive supply (VDD1) when the amplifier detects its nodes entering a saturated condition. The integrated clamp minimizes the pulse extension to less than a few ns for input pulses up to 100 mA. When the amplifier is in low-power mode, the clamp circuitry is still active, thereby protecting the TIA input. The LMH32401 features two internal clamps, a fast recovery clamp and a soft clamp. The fast recovery clamp is the active clamp when the photodiode is sinking a photocurrent. The soft clamp is the active clamp when the photodiode is sourcing a photocurrent. 

    Thank you,
    Sima

  • Hello Sima,

    I wonder if the photodiode output current is higher than the range of input current of LMH32401, the TIA will go into saturation, what we can do to solve this problem if we want to keep our photodiode?

    The next question is: Can we simulate LMH32401 in LTSpice?

    Thank you in advance!

    BR,

    Siamack

  • Hello Siamack,

       What is your input current range? 

       We test LMH32401 in Tina-TI and PSpice for TI. Usually devices that work in PSpice can be inserted into LTSpice via the netlist/library file available at this link under Design & Development tab with zip folder titled LMH32401 for PSpice Library (Rev. B), but we have not tested this method. 

    Thank you,

    Sima 

  • Hello Sima,

    Good question, and I'm still not sure because the supplier is not sure about the output current of photodiode.. But let me look into this more.

  • Hello Kai,

      Thank you for testing and verifying the LMH32401 in third-party simulators! 

    Hello Siamack,

       Sounds good; feel free to update the thread when supplier provides this information. Otherwise, if the input current range is entirely out of the usable range of the device, then is AC coupling (ambient light cancellation) a required design choice for your application?

    Thank you,
    Sima