[sdiy] In search of a reasonably precise comparator output

[Guy McCusker]

It's an analog thing: what I really want to do is take an analog
voltage X and return X-5 if if X > 5, and just X otherwise.

I did mention this in the first message, but right at the end, and in
parentheses :)




On Wed, Jun 20, 2018 at 5:34 PM, Scott Gravenhorst <music.maker at gte.net> wrote:
>
> 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:
>>--===============1895041468437579696==
>>
>>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?
>>
>>Guy.
>>
>>
>>
>>
>>On Wed, 20 Jun 2018 at 17:02, Pete Hartman <pete.hartman at gmail.com> wrote:
>>http://pi3c.lan/cgi/mailbody.cgi?MD4696.MSG=+Get+&MAILBOX=chordman&DATE=20_06_2018_04_00_20
>>> 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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