LM833 As A comparator

[Sara]

Dear friends

I used lm833 as a comparator because of it's high slew rate but the
problem is one of the
inputs comes from a detector- an analog signal which amplitude is 0 to
9V-.
The minimum is 0 but when I connect it to the input pin of opamp it
comes up about 0.5V -DC is added by opamp-and the operation of
comparing with about 0.5V intrfaces big problems .My circuit is a very
primary one with no resistances in the output.
Any idea how can I remove the DC from the opamp pin?

Thanks in advance
Sara

 

[vasile]

Dear friends

I used lm833 as a comparator because of it's high slew rate but the
problem is one of the
inputs comes from a detector- an analog signal which amplitude is 0 to
9V-.
The minimum is 0 but when I connect it to the input pin of opamp it
comes up about 0.5V -DC is added by opamp-and the operation of
comparing with about 0.5V intrfaces big problems .My circuit is a very
primary one with no resistances in the output.
Any idea how can I remove the DC from the opamp pin?

Any comparator configuration (inverting or noninverting) require input
resistors, one to the signal to be compared and the other to the
reference voltage. Also you must have a positive feedback resistor to
increase the switching speed and add a small hysteris on the input.

Vasile

 

[[email protected]]

Dear friends

I used lm833 as a comparator because of it's high slew rate but the
problem is one of the
inputs comes from a detector- an analog signal which amplitude is 0 to
9V-.
The minimum is 0 but when I connect it to the input pin of opamp it
comes up about 0.5V -DC is added by opamp-and the operation of
comparing with about 0.5V intrfaces big problems .My circuit is a very
primary one with no resistances in the output.
Any idea how can I remove the DC from the opamp pin?

The input common range of the LM833 typically only goes down to about
1V above the negative rail. They don't guarantee it below 3V above the
neagtive rail.

http://cache.national.com/ds/LM/LM833.pdf

If you must use an op amp as a comparator - a very bad idea if you
don't know what you are doing - use a single supply op amp. For single
supply op amps the input voltage range usually includes the negative
rail.

Have a look at the LM393 dual comparator.

http://cache.national.com/ds/LM/LM193.pdf

 

[Jon]

Dear friends

I used lm833 as a comparator because of it's high slew rate but the
problem is one of the

Sara,
If an op-amp saturates, its input offset voltage is not gauaranteed.
When using an op-amp as a comparator, it's best to do the following:
..
Connect a reference voltage to the reference input (Inv or Non inv,
depending on which input the input signal is connected to.) This
reference must be higher than the minimum allowable common mode input.
I suspect that this is where your 0.5 VDC is coming from.
..
Use a smal amount of positive feedback to guarantee that the op-amp
does not try to stay in its linear range.
..
Regards,
Jon