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TI IC LMH 6559

Abhay Negi
Prodigy 40 points
Other Parts Discussed in Thread: LMH6559, SN7407

Hi !

I want to use a high speed buffer IC LMH6559 which is being driven by a comparator (at input). However, I'm not sure about a typical circuit given in the datasheet (pic enclosed). I'm driving a comparator to drive the Buffer which in turn drives a BJT.  

Although the buffer has a high input impedance of 200 kohm but the typical schematic has a 50 ohm resistance at the input. In case one uses this circuit, wouldn't it effectively reduce the input impedance of buffer to very low value (around 50 ohm) ?

Can I connect output of comparator directly to input of buffer and skip the 50 ohm resistor, 100 nF cap and 10 kohm resistor at the input ? 

  • 0
    TI__Genius 13935 points
    Hello Abhay,

    If you want high impedance you can remove the 50 Ohm resistor. It is for use with 50 Ohm test equipment.

    A comparator output is a digital signal. The LMH6559 is an analog buffer. Is there a reason you did not select a digital buffer?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    TI__Mastermind 22825 points
    Hello,
    To add Loren's comments:
    The 100nF cap and the 10 kohm resistors are to make sure the input / output swing is well centered around Vcc/2, if you can deal with AC coupling the signal. Assuming you use the LMH6559 with 5V single supply (Vcc and ground), then the input / output voltage range is 1.2V to 3.88V (RL = 100ohm) at room temperature.
    If your comparator swing is 0 to 5V (for example), then you would have to somehow accept the reduced swing of 2.7V when you use Vcc=5V single supply, or run the LMH6559 at a higher supply voltage (Vcc= 10V allows 6.4Vpp swing).
    If you want the comparator / LMH6559 interface to be DC coupled (no 100nF AC coupling cap), and your comparator swings 0-5V, you could use unsymmetrical supplies for the LMH6559 and Direct-couple the signal (i.e. V- = -2.5V, V+ = 7V). In the DC coupled case, you do not need the 10k biasing resistors of this image.

    Regards,
    Hooman
  • 0 in reply to Hooman Hashemi
    Prodigy 40 points
    Thanks Loren. I think SN7407 will be a better choice for my application.
    Thanks Hooman for the deatils.