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Original question:

LMH34400: Is it OK for LMH3440 being a TIA to convert small current of photo diode to voltage

LMH34400: Is it normal for LMH34400 always outputs 0.3V?

Jerry Wu
Prodigy 110 points
Part Number: LMH34400
Other Parts Discussed in Thread: LMH6611,

Hi TI,

Just get the LMH34400 + LMH6611 PCBA from menufactory, somes questions as below

1. Is it normal for LMH34400 always outputs 0.3V even there is no input 

2. Is R25 = 49.9R OK for LMH6611 ?

3. Whether C50*1uF) is necessary for LMH6611 ?

4. what difference is between using LMH34400's pin 6 IDC_EN = 1 and IDC_EN = 0

  • 0
    TI__Genius 15131 points

    Hello Jerry,

    I am looking into your questions today & tomorrow.  

    Thank you.

    Best,

    Alec

  • 0
    TI__Expert 6050 points

    Hi Jerry,

    To answer your questions:

    1. According to the datasheet, if there is zero input current the output voltage should be 1.0V. Can you provide some more information on how are you measuring the output voltage of the LMH34400?
    2. I do not see any issues with R25 having a value of 50 ohms
    3. C50 is acting as an AC coupling capacitor which will block any DC signal
    4. When IDC_EN of the LMH34400 is set to 0V, the ambient light cancellation feature is enabled. When IDC_EN is set to 3.3V, the ambient light cancellation feature is disabled. This feature is intended to offset the effect of DC current from ambient light. More information on this feature can be found in Section 7.4.1 of the datasheet

    I have also attached a simulation circuit if you would like to do some analysis to see the behavior of the circuit. If you have any further questions, please let me know.

    LMH34400andLMH6611_E2E.TSC

    Thanks,

    Nick