Re: Hysteresis Comparator Methods

On Thu, 04 Jan 2007 03:35:04 GMT, myrealaddress@xxxxxxxxx (D from BC)
wrote:

"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote
in message
news:5honp2hq336nqmeo13aam7uple440n0qsb@xxxxxxxxxx
On Wed, 03 Jan 2007 09:15:38 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Wed, 03 Jan 2007 07:57:54 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

[snip]
This can be a lot better, because there's no delay coming
back around
into a positive-feedback node, and because the comparator
prop delays
are effectively pipelined... everything flows left-to-right.
The
comparators still have to be well behaved and well
characterized,
specifically to have fairly symmetric rising and falling edge
prop
delays. But this configuration has fewer screwup modes.

The other nice thing here is that the two comparator
references can be
programmed with DACs, allowing thresholds and hysteresis to
be under
software control. I use this configuration in my tachometer
conditioning modules, and it's very flexible.

John


Then there's the optimum method I devised in my MC1650/51
design in
the mid '60's ;-)

...Jim Thompson


Hysteresis internal to ICs can be a lot better, because you
can
localize the positive feedback into a very tight, single-pole
loop.
The HC14 type schmitts are apparently immune from teasing,
too.

Opamps often make better comparators than comparators, because
they
have one dominant pole and don't store as much information as
the
usual fast-multistage comparator topology.

It's funny that the common comparator-plus-hysteresis circuit
is
almost universally taught as being a clever thing, taught by
people
who don't understand it.

John


One thing I don't understand is why you Old Blokes bolloxed on
about
comparators with hysteresis and then gave us comparators with
hysterectomies...... and we still have to deal with them.

DNA
-------------------
Great! Looks like this cct. is a "sports car" of hysteretic
compartors.

I'm now more suspicious of app notes. Trust nothing. :)

Got curious and looked up the MC1650.. Dual A/D converter??..

I didn't name it. Back in those days Motorola had a department that
did data sheets. I think the name derived from the 64-wide version I
made for WPAFB for a flash converter.

Huhhh...Still have alot to learn.. :) Internally it's in the
design "ball park" of interest.

Yep. My 1965 speed numbers compare favorably with today's designs.


Op amps make better comparators!! Looks like I picked up a
myth..In my internet travels, I recall reading that comparators
are best in comparator applications..

I think it's a myth too.

I haven't yet compared op-amp/comp internals and specs.
Just blindly running on rumour..
It's a good thing I don't design life support systems :)

My next move is to take a look at the various IC's containing a
combo of op amps (or comparators) and a D latch to make a
hysteretic comparator.
CMOS would be nice..

Thanks for the verification..
D
------

I've done several comparator plus D-latch (or even S-R) versions in
CMOS.

But they're custom ASIC's.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
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