op amp input ranges

I'd like to know how close to the positive rail you can go with the
inputs.
I was looking at the LT1013 in particular, but op amp datasheets in
general don't seem to adress the question explicitly.
Using an op amp as a comparator in a circuit with a single supply, I
need to use the inputs to within a tenth of a volt of the positive
rail.
I had a 741 in the junk box, so I checked and it worked as a comparator
with the inputs 0.1 volt from the positive rail. Is this true of the
LT1013 or other op amps, and how can l tell before ordering?

 

Why use an op amp that requires output offset adjustment? If you want
an old op amp, use the LM1458. It too is a jelly bean (cheap) component.

 

"kell"

** See page 3, http://www.farnell.com/datasheets/41645.pdf

The "input voltage range" is given as (up to) + 3.8 volts with a +5 volt
supply.





......... Phil

 

Usually the datasheet gives an "input voltage range" specified in terms of
Vcc and Vee; sometimes both a safe range (guaranteed not to blow up the
chip) and a specified range (guaranteed to work as specified).

For instance, looking at the TI LT1013 datasheet (it just happened to be the
first one on Google), I see absolute max input range of Vee-5V to Vcc. The
specified common-mode input range may be less than the safe range; for
instance, on the same datasheet I see common-mode input voltage range
specified from -15V to +13.5V, with +/-15V supplies, at 25C. So that's from
Vee, to 1.5V below Vcc. What they're saying is that if you get closer than
two diode drops to the positive rail, you won't hurt anything but they won't
guarantee proper operation - for instance, it might work okay with less than
a certain amount of source impedance.

Some opamps can get close to both rails, some to the positive rail, some to
the negative. It depends on the circuit topology, and there are tradeoffs.
So-called "single-supply" opamps, IME, generally let you get close to the
negative rail but not to the positive rail.

 

You need to check the *input common mode voltage*on the data sheet.

Graham

 

A 741 does not *require* input offset adjustment. Leave those pins open if
you don't need to use them.

Graham