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LMH6321 loading

Mukund Jha
Prodigy 70 points
Other Parts Discussed in Thread: LMH6321

Hi,

I am using LMH6321 to drive my trigger signal of 8V , i need to drive around 30 m 50 ohm cable with load of 50 ohm. if i use the sample circuit as suggested in datasheet, R4 (50 ohm) resistance is used in series to compensate the capacitance of coaxial cable but  output voltage decreases to half in this configuration. What should i do to keep the output voltage same as input voltage and drive it to 30m also.

Regards,

Mukund Jha

  • 0
    TI__Genius 13935 points
    Hello Jha,

    If the 30m cable is terminated with a 50 Ohm load it is possible to remove the termination resistor. This will result in a singly terminated cable. Single termination can be sufficient if implemented carefully. However, with a singly terminated cable, any mismatch, anywhere along the cable will generate reflections. Any traveling wave that hits the amplifier end of the cable will now reflect and end up at the load.

    If you need an 8V pulse, can you increase the supply on the amplifier to allow for a gain of 2?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 70 points

    Thanks Loren,

    Do you mean Power supply voltage, i have ±15V supply and input voltage is 8V pulse signal and need to send same level signal to load.

    Regards,

    Mukund

  • 0 in reply to Mukund Jha
    TI__Genius 13935 points
    Hello Mukund,

    You can remove R4, that will remove the gain loss. It will also remove the benefit of double termination. You must decide if this solution will work.

    Otherwise, you will need an amplifier with a gain of 2 to overcome the loss in the termination resistor.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 70 points

    Hi Loren,

    I just show the LMH6321 evm schematic and it shows it works in noninverting mode and gain can be set by setting RF, RG and RT. I was trying to simulate lmh6321 with a gain of 2 but gain is still one. schematic is attached. Reference schematic from lmh6321 evm is also attached.

  • 0 in reply to Mukund Jha
    TI__Genius 13935 points

    Hi Mukund,
    As shown in Figure 1 above it takes two amplifiers to have a total gain of 2. The LMH6321 is a unity gain buffer, it will only ever have a gain of 1.

    To further clarify:  The OPA1 in Figure 1 provides the gain, the LMH6321 is used as an output buffer for the composite circuit.  If you want to use this circut  you need to use the whole circuit. 


    Regards,
    Loren

  • 0 in reply to Loren Siebert 1
    Prodigy 70 points

    thanks Loren,

    Can you please clarify,

    1. Since I am using dual supplies I do not need to use OPA2.

    Regards,

    Mukund

  • 0 in reply to Mukund Jha
    TI__Genius 13935 points
    Hello Mukund,

    You are correct, using split supplies you do not need OPA2. Have you been able to simulate the circuit? Is it working as expected? Have you decided on single termination at a gain of 1 vs double termination and a gain of 2?

    Regards,
    Loren