[sdiy] In search of a reasonably precise comparator output

d1ffeq at teknik.io d1ffeq at teknik.io
Wed Jun 20 21:58:37 CEST 2018

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 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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