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BUF802: Input impedance

Taketo Sato
Genius 10205 points
Part Number: BUF802

Tool/software:

Hello team,

I have a question about input impedance information for BUF802.

its datasheet EC table says the input impedance is about 50Gohm//2.4pF at 100MHz. However figure 6-21 shows much different value. Is figure 6-21 information really correct? 

Best regards,

  • +1
    TI__Genius 13210 points

    Hi Taketo,

    The value in the EC table is captured at DC. The reason you see this drop in input impedance across frequency is due to the 2.4pF input capacitance of the device. Page 23 in the datasheet highlights this idea and ways to account for this at the higher frequencies.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Genius 10205 points

    Hello Ignacio-san,

    Understood, thank you.

    What is meaning of "f=100MHz" test condition for 50Gohm//2.4pF? I just curious.

    Best regards,

  • +1 in reply to Taketo Sato
    TI__Genius 13210 points

    Hi Taketo-san,

    That is a good question, this test condition is saying that this 50Gohm//2.4pF specification is valid at that frequency. If you look at the Figure 6-21, you can see this 50Gohm//2.4pF condition stays true in the region where the graph is linear before it starts really dipping in the 1GHz range. From 100MHz and below, this impedance stays true. Above 100MHz, the specification does start to change and 50Gohm//2.4pF is not true anymore.

    Best Regards,

    Ignacio