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LMH34400: The output issue for LMH34400

Zhang Liang
Genius 4385 points
Part Number: LMH34400
Other Parts Discussed in Thread: LMH32401

Tool/software:

Hi,

We use the LMH34400 for the different photometric testing, there is a significant difference in the signal output of LMH34400 between the high reflection (white) objects and low reflection (black) objects

( including pulse width and rising edge),  which affects the subsequent TDC timing, please advise how to solve this issue, thanks.

The waveform of white object:

The waveform of black object:

The schematic diagram:

  • 0
    Genius 4385 points

    Hi,

    Please kindly review the issue and provide your comment for this case, thanks.

  • 0 in reply to Zhang Liang
    Genius 4385 points

    Hi,

    We refer to the datasheet the Iin=1mA waveform as below picture, and please help to provide the commend for below issue:

    1. Why the black and white reflective plate has the different pulse width and rising edge, is this normal or not;

    2. The minimum testing accuracy is in the nanosecond level, and the 1ns light transmission distance time is calculated by 15cm, 

        and the black and white reflective plate has the 1 ns time difference, so please advsie if the LMH34400 is meets our precision request or not?

  • 0 in reply to Zhang Liang
    TI__Mastermind 29920 points

    Hello Zhang,

      Thank you for your patience on this issue. I reviewed your question and additional reply, and below is our feedback:

    1. This type of behavior is normal since the laser diode output current/type will differ with different amount of lights received by the diode. Linked here is a short but informative blog showing the same type of diode response types with more vs less light.

    2. The figure shown in reply is overloaded pulse response when amplifier is fed a large input current, and has to clamp + recover. This would occur in high reflection objects in which the diode will most likely output its maximum current, but not in the low reflection object.

      For this device, the overloaded pulse response at 10mA with pulse duration of 10ns is around 18ns. At 1mA, this would be faster as shown in figure. I was able to find the diode's datasheet from part number in your attached schematic, and I believe the device is able to output a peak maximum of 0.25mA. In that case, the device would recover a bit quicker than shown in the 1mA case.

      If you require a faster response for your design based on your 1ns requirement, I would suggest looking at the LMH32401. This device is also an integrated 450MHz TIA with a selectable gain of 2kOhm and 20kOhm -- with selectable gain, it could be useful for avoiding high saturation leading to faster recovery times. Rise time for the device is specified at 0.8ns - 1.3ns compared to LMH34400's 1.5ns. Overload pulse response looks similar between the two due to the technology of the over current protection clamp used internal to both devices. However, the 2kOhm selectable gain can be used to avoid overloading the device if possible in your design.

    Thank you,
    Sima