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LMH6704, LMH6702 output impedance

Alex Sorokin
Prodigy 20 points
Other Parts Discussed in Thread: LMH6703, LMH6702, LMH6704

Is there any data on output impedance vs frequency for LMH670x?

The datasheets don't seem to have any graphs.

What I'm looking for is a buffer that has low input impedance (<1.6 pF, preferably 1 pF), noise <5 nV/sqrt(Hz), 500 MHz BW with 1 Vpk-pk pulses or better, and output impedance of only a few ohms up to 100 MHz & reasonably flat (for easy termination in 50 ohm system). Clean pulse response with little overshoot is required, and settling time to 0.1% <10 ns (faster is better, but overshoot has to be at most few %).

  • 0
    TI__Genius 13935 points
    Hi Alex,

    The information you are looking for is in Figure 18 and Figure 8 of the LMH6702 datasheet. The LMH6703 is based on the same core and will be the same for output resistance and similar for settling time.

    If you are using 1/2 ounce copper on your PCB be aware that the PCB parasitics will be comparable or greater than the amplifier for any significant trace length. These very high speed amplifiers will be sensitive to board layout in general. It is especially important to remove ground metal from under the output trace and the inverting input trace.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 20 points

    Thanks, Loren. I did not notice the Rout curve in Figure 18 (the graph looks nothing like a typical Rout plot). So I can assume that LMH6704 behaves similarly as well?

    Does anyone happen to have data for Rout at >100 MHz? I'd need to choose a terminating resistor, and my upper frequency is about 500 MHz. I'll be using the chip to drive ~10 cm long 50Ω or 30Ω stripline, with a bunch of buffered mux switches hanging on it.

  • 0 in reply to Alex Sorokin
    TI__Genius 13935 points
    Hi Alex,

    It's best to use the DC value termination resistor. The Rout as shown in the graph is inductive and it does not serve to match the load very well. The resistive termination will absorb reflections where an inductor would not.

    Regards,
    Loren