input range on op-amp

[[email protected]]

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp
designed to work on +/- 15 volts into service in this singly-supply
application and expect its negative input to withstand the negative 12
volts.

 

[Lostgallifreyan]

[email protected] wrote in

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp
designed to work on +/- 15 volts into service in this singly-supply
application and expect its negative input to withstand the negative 12
volts.

I think many op-amps and comparators can accept a small overshoot beyond
their supply rails. If the intermittent low is short, and you don't need to
know the value, and maybe only need to detect whether it goes below zero,
you could clamp it on the input using a germanium or Schottkey diode.
Anything that has around 0.3 V forward drop. I don't know how well the
comparator will detect the difference between that and the actual zero, but
often you'll get a better comparotr using a normal op-amp, just stick a
100K or 1M resistor between output and non-inverting input to accelerate
the slew on the output for clean transitions.

 

[John Popelish]

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp
designed to work on +/- 15 volts into service in this singly-supply
application and expect its negative input to withstand the negative 12
volts.

Without a negative supply, no. The input voltage range has
no idea what voltage you are calling ground. It only reacts
to the its input voltage relative to its supply voltages.

If the negative supply is available, you could use a 2903
and adapt its output signal to the logic range you need. If
a negative supply is available, you also have lots of other
choices of comparators that have separate output stages that
do this, internally.

 

[PeteS]

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp
designed to work on +/- 15 volts into service in this singly-supply
application and expect its negative input to withstand the negative 12
volts.

You don't say what you are comparing (what level event is being captured).

Are you trying to capture the negative pulse? Where is the output of the
comparator going?

If you are comparing a voltage within the normal range of the
comparator, then it's a simple matter to clamp the input so the negative
pulse does not result in a negative pulse at the device pin(s).

A little more information would help.

Cheers

PeteS

 

[Jamie]

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like the
2903 can't take that, so I was wondering if I could press an op-amp
designed to work on +/- 15 volts into service in this singly-supply
application and expect its negative input to withstand the negative 12
volts.

have you thought of using a clamping diode on the input?

 

[Ban]

I have a circuit that needs a comparator where the negative input will
go 12 volts below ground intermittently. Standard comparators like
the 2903 can't take that, so I was wondering if I could press an
op-amp designed to work on +/- 15 volts into service in this
singly-supply application and expect its negative input to withstand
the negative 12 volts.

I do not recommend an opamp, there might be more caveats than using a
comparator. You do *not* need a negative voltage. A simple resistor of 33k
followed by a schottky-diode to gnd will clamp the voltage to save levels,
that are within the -.3V common range of the 2903. Even if you want to
measure the negative voltage, you can put up a summing comparator without
negative supply. Just look in the datasheet. To prevent spurious oscillation
you might want to add some hysteresis and need anyway those input resistors.
The On-semi datasheet LM339 has as an example a zero x-ing detector.