can a comparator pull down to a negative rail

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?

You can just connect the "gnd" pin to -5.

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?

From p2 of the data sheet...

Supply Voltage (Pin 8 to Pin 4).....................36V Output to Negative Supply (Pin 7 to Pin 4) LT1011AC, LT1011C...................................40V LT1011AI, LT1011I...................................40V LT1011AM, LT1011M (OBSOLETE)........................50V Ground to Negative Supply (Pin 1 to Pin 4)..........30V

In both devices, "GND" is somewhat of a misnomer.

In the LT1011, it's the potential that the output pulls down to, it can be set equal to V-

Likewise in the LM319, though with slightly different range bounds. ...Jim Thompson

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.

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

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:

I was looking at the LT1011

and the LM319

much thanks!