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LMH6642: can I use LMH6642MF as an open-loop comparator?

Part Number: LMH6642
Other Parts Discussed in Thread: TLV1805-Q1

Dear Sir/Madam,

 

Can i check if i can use LMH6642MF as an open-loop comparator?

(Some op-amps generate weird behavior when used as open-loop comparator)


 

Best regards,

kpk

  • Hello KpK,

    The first clue is that the Abs Max input differential range is only ±2.5V, so it has input clamps.

    High speed op-amps can be extra problematic when used as a comparator.

    This is a bipolar device, and generally, bipolar device output stages do NOT like to have the outputs jammed into the rails The supply current will increase as the outputs try to drive hard into the rails (because there is real base current being driven in the output stages).

    The output devices themselves can even oscillate - even when "jammed" against the rail.

    You would be much better off using a proper comparaotr.

    What are your supply and input conditions?
  • hi Paul,

    Here is the application circuit. any recommendation on the P/N for comparator? 

    Best regards,

    kpk

  • Hi Poh,

    with a supply voltage of 3V you would violate the common mode input voltage range.

    Kai
  • Hello KpK,

    My rule of thumb is that the absolute minimum prop delay of the comparator should be 40% of the input signal period.

    1.6MHz is 625ns, so you would want a comparator of at least 250ns or less (less is better).

    Is the 3Vpp square wave centered around a bias voltage? Or is it 0 to 3V?

    The 12V is a sticking point, as most of the "high speed" comparators are 5V max supply. Can you run from a 5V supply?

    Do you need a push-pull output? Or can you use a open collector with pull-up? Single or dual?

    From what you show above, assuming the input square wave actually goes 0 to 3V, you need a R-R input, 12V comparator with a speed of <250ns.

    Right now the only device that juuust meets these requirements is the newly released TLV1805-Q1. It is sampling now and will be released mid-December. The commercial (non-Q1 version) should release in January time frame.

    If you can switch to a 5V max supply, there are more choices available.