can a comparator pull down to a negative rail

[panfilero]

I would like to use a comparator so that it's output is either open drain (or collector) or it's pulled down to its negative rail (Let's say -5V for my application) I've looked at a couple of comparators, and they have +Vin, -Vin, and GND pins... but from looking at the datasheets it doesn't look like they can output their negative rail... am I right about this? if i am, then what's the point of the -Vin input?

I was looking at the LT1011
http://cds.linear.com/docs/en/datasheet/1011afe.pdf

and the LM319
http://www.ti.com/lit/ds/snosbj2a/snosbj2a.pdf

much thanks!

 

[panfilero]

What Jim said: "ground" is the reference for the output -- this is handy

when your analog circuit is running off of differential supplies and your

digital circuit is referenced to "0V".



Be sure to check the common-mode range of the inputs -- they may not go

down to the V- rail (or, for that matter, up to the V+ rail). If you

need that, you must go looking for a rail-rail input comparator, or you

must rearrange your input circuit to suit what you plan on using.



--

My liberal friends think I'm a conservative kook.

My conservative friends think I'm a liberal kook.

Why am I not happy that they have found common ground?



Tim Wescott, Communications, Control, Circuits & Software

http://www.wescottdesign.com

ahh ok, so I could set up my rails at +/-10V and tie my ground to -5V and should be good to go then... great.

- thanks

 

[Tim Williams]

Comparators like the LM319 (LM311, ...) would be better called
"uncommitted transistor output". Like the TL494 outputs, if you're
familiar with it. You can use it for pull-up (OUT to +V, output from
"GND") or pull-down (normal), or whatever your imagination can come up
with.

Other ones, made for specific logic levels (LM360, TTL level, comes to
mind), may be more particular about voltages. In the latter case, for
example, obviously you need the analog supplies to be more than the analog
input voltages (so you could do +/-5V for a [-3, 3V] range, say), and the
output has to be referenced to whatever it connects to (i.e., +5V and GND,
or insert-logic-level-here). Consider: TTL, CMOS, LVDS, ECL...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com

I would like to use a comparator so that it's output is either open drain
(or collector) or it's pulled down to its negative rail (Let's say -5V for
my application) I've looked at a couple of comparators, and they have
+Vin, -Vin, and GND pins... but from looking at the datasheets it doesn't
look like they can output their negative rail... am I right about this? if
i am, then what's the point of the -Vin input?

I was looking at the LT1011
http://cds.linear.com/docs/en/datasheet/1011afe.pdf

and the LM319
http://www.ti.com/lit/ds/snosbj2a/snosbj2a.pdf

much thanks!