Re: 8 switches


John Popelish wrote:
Oops. I need to fix the picture to put Vcm above V2sw.

> Nice analysis with practical considerations (though, if I were really
> going to go with a 3 volt battery supply, I think I would go with the
> low voltage version of the dual comparator). I understand why you are
> pushing the noise margin of Vhi to stay under the common mode voltage
> limit, but why do you not center Vlo between GND and V1sw? It seems
> to me that this reduces the tolerance limit for the lower resistor in
> the reference string. Is there some advantage of raising this voltage
> that I missed?
>
> Perhaps a better solution to the common mode limit would be to reduce
> the common switch resistor a bit more. Something like 4.7k helps
> quite a bit. It lowers V1sw (=.959 vs. 1.2 or down 20%) more than it
> does V2sw-V1sw (=.495 vs. .50 or down 1%). The 1% drop in V2sw-V1sw
> is more than compensated for by making the whole swing usable.
>
> The list changes to:
>
> Vcc---------------3.000V
>
> Vcm---------------1.5V
>
> V2sw--------------1.454V
>
> Vhi---------------1.206V
>
> V1sw--------------0.959V
>
> Vlo---------------0.480V
>
> GND---------------0V
>
> This provides .248 volts margin on both sides Vhi, while operating
> below Vcm for V2sw. Vlo still has .480 volts margin on each side.
>
> Vcm won't interfer till the battery falls to 2.41, assuming the
> comparator still works at all.
>
> Of course, if the circuit will run from a 9 volt battery, then I would
> go back to the 6.8k common resistor and have huge noise margin and
> maximum resistor tolerance.
>
> Are we having too much fun with this simple circuit, yet?

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