# [sdiy] In search of a reasonably precise comparator output

Lloyd Abbott lloydabbott94 at gmail.com
Thu Jun 21 10:22:55 CEST 2018

```If you're worried about the LM339's V_OL, the datasheet mentions, "The V_OL
is resistive and scales with the output current." The 150mV to 400mV
specified is measured with I_OL = 4mA, if you can happily reduce the output
current down an order of magnitude or two from 4mA, then you're dealing
with much lower offset.

On Wed, Jun 20, 2018 at 10:30 PM, Guy McCusker <guy.mccusker at gmail.com>
wrote:

> Thanks everybody for the suggestions. I am sort of relieved that there
> is not one completely obvious way to do this but a range of
> possibilities -- I guess I just have to choose one, or experiment a
> bit.
>
> The only one I won't make use of in this case is the idea of doing
> this digitally, but only for the not very good reason that the overall
> circuit I am aiming for would be soooo much easier to do digitally
> (and better too I imagine) but I am trying to create an analog
> solution for the fun and pain of it.
>
> Thanks again!
>
>
>
>
>
> On Wed, Jun 20, 2018 at 8:58 PM,  <d1ffeq at teknik.io> wrote:
> > So it's a voltage modulo function. All I can think of is a comparator
> controlling an analog switch that switches between original and -5V biased
> inputs. But this solution is not what I'd call simple. Maybe some sort of
> A/D converter would be simpler.
> >
> > June 20, 2018 8:27 PM, "Guy McCusker" <guy.mccusker at gmail.com> wrote:
> >
> >> 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
> >>>> 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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> >>> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
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