What is the benefit with comparators that have a Hysteresis

[Hammy]

What is the benefit with comparators that have a Hysteresis pin?

I'm using one the LTC1440 and I don't see any benefit you still
require two resistors for Hysteresis. Is the Hysteresis pin tied
internally to the output (positive feedback)?

If I wanted to use this comparator in an inverting mode can I still
use the suggested method in the application note for Hysteresis
(pg.11)? Or should I use the standard method a resistive divider off
the output back to the reference?

LTC1440 Datasheet (Hysteresis is on pg.11)

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1463,P1172,D2764

Hysteresis for inverting comparator

http://i32.tinypic.com/29bozeu.png

Thanks

 

[John Larkin]

On Sun, 18 May 2008 19:20:39 GMT, Hammy <spamme@hotmail.com> wrote:

What is the benefit with comparators that have a Hysteresis pin?

I'm using one the LTC1440 and I don't see any benefit you still
require two resistors for Hysteresis. Is the Hysteresis pin tied
internally to the output (positive feedback)?

If I wanted to use this comparator in an inverting mode can I still
use the suggested method in the application note for Hysteresis
(pg.11)? Or should I use the standard method a resistive divider off
the output back to the reference?

LTC1440 Datasheet (Hysteresis is on pg.11)

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1463,P1172,D2764

Hysteresis for inverting comparator

http://i32.tinypic.com/29bozeu.png

Thanks

External hysteresis, like in your second link, has lots of problems.
Fast noisy inputs can blow through the comparator, and make multiple
output edges, before the positive feedback arrives. Comparators with
internal hysteresis usually do it right.

It's amazing how often the 2-resistor external thing is used and
documented, even though it's inherently flawed.

John

 

[Hammy]

On Sun, 18 May 2008 13:54:46 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

External hysteresis, like in your second link, has lots of problems.
Fast noisy inputs can blow through the comparator, and make multiple
output edges, before the positive feedback arrives. Comparators with
internal hysteresis usually do it right.

It's amazing how often the 2-resistor external thing is used and
documented, even though it's inherently flawed.

John

Thanks for the reply John :

I'm still a bit confused (surprise .

Are you saying that the sole porpose the Hyst pin is there to save on
propagation time? The LTC1440 response time is measured in uS so would
this still be beneficial at these speeds?

 

[ĺnřn˙mřu§]

On Sun, 18 May 2008 19:20:39 GMT, Hammy <spamme@hotmail.com> wrote:

What is the benefit with comparators that have a Hysteresis pin?

I'm using one the LTC1440 and I don't see any benefit you still
require two resistors for Hysteresis. Is the Hysteresis pin tied
internally to the output (positive feedback)?

If I wanted to use this comparator in an inverting mode can I still
use the suggested method in the application note for Hysteresis
(pg.11)? Or should I use the standard method a resistive divider off
the output back to the reference?

LTC1440 Datasheet (Hysteresis is on pg.11)

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1002,C1463,P1172,D2764

Hysteresis for inverting comparator

http://i32.tinypic.com/29bozeu.png

Thanks
This pin can only apply up to 100mV +/- 15% of Hysteresis.

Its faster in the since that its only swinging 100mV around.
Most of the time I will measure the Hysteresis to see just how much is
really needed. This is completely dependent upon the noise at the
input to the comparator.
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