[sdiy] In search of a reasonably precise comparator output

Scott Gravenhorst music.maker at gte.net
Wed Jun 20 18:34:35 CEST 2018

It would be helpful if we knew what was being driven by the comparator.  Logic inputs
don't require absolute 0.000 volts for a low logic level nor absolute Vcc for a high logic
level.  Each logic type will have it's own specification for that.  Is this a zero volt
and five volt source for some analog process - or is it digital?

Guy McCusker <guy.mccusker at gmail.com> wrote:
>Thank you.
>I can readily stick a solid 5V reference on the pull-up of an
>open-collector or open-drain comparator, but I am feeling stupid about what
>happens for the low output voltage. The LM339 data sheet lists a V_OL of
>150 to 400mV. That's a reasonable way above 0V.
>Should I just live with it and trim it somewhere else in the circuit?
>On Wed, 20 Jun 2018 at 17:02, Pete Hartman <pete.hartman at gmail.com> wrote:
>> On Wed, Jun 20, 2018 at 10:30 AM, Lloyd Abbott <lloydabbott94 at gmail.com>
>> wrote:
>>> What rails do you have available ? Have you considered a comparator IC
>>> (e.g. LM339) rather than using general purpose op-amps ?
>>> If you have 0V, +5V and +15V rails, with an LM339 you can reference the
>>> comparator input side to the 0V and +15V rails and have the output side
>>> referenced to 0V and +5V. The datasheet has an example circuit using one
>>> pullup resistor and one cap per comparator.
>> I was going to suggest this as well.  The 339 and its family are all open
>> collector outputs, so you can use a pull up to any voltage you like, a rail
>> or a reference voltage or whatever to get the swing you need and if
>> necessary convert levels.

-- ScottG
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22

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