[sdiy] In search of a reasonably precise comparator output

[Guy McCusker]

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 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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