Re: Comparators vs. op amps

On Oct 1, 6:07 pm, bill.slo...@xxxxxxxx wrote:
On Oct 2, 1:02 am, John Larkin



<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, 01 Oct 2007 09:49:42 GMT, Jan Panteltje

<pNaonStpealm...@xxxxxxxxx> wrote:
On a sunny day (Mon, 01 Oct 2007 06:34:22 +0200) it happened Rene Tschaggelar
<n...@xxxxxxxx> wrote in <470078de$...@xxxxxxxxxxxxxxx>:

While a comparator has the full, or almost the full
supply range as input range, some OpAmps do not like
the differential input voltage being too large.

Rene

OK, now I have heard 2 theories (Bill's and yours).
I disagree with Bill Sloman on the comparator usually having more gain stages.
Maybe same number or even less of stages with more gain.

I know of one comparator that has back-to-back clamp diodes on its
inputs! But opamps and comparators use similar processes, so have
similar constraints.

This is a bit misleading. Op amps are constrained by the requirement
that that have to be stable when used with a fair bit of negative
feedback, which limits the number of stages of voltage gain that they
can use. Comparators aren't used with negative feedback, so the
designer can add extra stages of voltage gain - which does add extra
propagation delay, silicon area and so forth, so they don't go for
many more stages.

Many fast comparators have very limited input
differential voltage specs and often have weird common-mode rules.

Comparators tend to have lower gains, because they only need gains in
the ballpark of 1000, just enough to guarantee a solid output when the
inputs differ by more than the offset spec. And they are designed for
speed, another reason to keep the gain and the number of stages low.

But the balance between delay per stage and gain per stage does seem
to optimise around three stages of voltage gain.

--
Bill Sloman, Nijmegen

I'd have to do some digging, but there is a classic equation that
relates the speed of the comparator to the number of stages. Generally
more stages are faster if you are using the comparator in its "linear"
region, i.e. close to zero volts. It is not a situation where you are
propagating logic, but rather you have a number of stages moving
linearly a the same time, each one amplifying the other. If you want
more bandwidth, you use less gain per stage, but more stages. This way
written up generally in strobed comparator design, when you off-set
cancel each comparator stage in one phase, then compare in the next
clock phase.

Of course, now I made a fine mess for myself since I need to dig a
reference on this.

.

Relevant Pages

  • Re: Comparators vs. op amps
    ... the differential input voltage being too large. ... I disagree with Bill Sloman on the comparator usually having more gain stages. ...
    (sci.electronics.design)
  • Re: Comparators vs. op amps
    ... the differential input voltage being too large. ... I disagree with Bill Sloman on the comparator usually having more gain stages. ...
    (sci.electronics.design)
  • Re: Comparators vs. op amps
    ... the differential input voltage being too large. ... I disagree with Bill Sloman on the comparator usually having more gain stages. ... amp since the uA709, which have - at most - two stages of voltage ...
    (sci.electronics.design)
  • Re: Comparators vs. op amps
    ... the differential input voltage being too large. ... I disagree with Bill Sloman on the comparator usually having more gain stages. ...
    (sci.electronics.design)
  • Re: Comparators vs. op amps
    ... the differential input voltage being too large. ... I disagree with Bill Sloman on the comparator usually having more gain stages. ... But opamps and comparators use similar processes, ... inputs differ by more than the offset spec. ...
    (sci.electronics.design)