I have a circuit with a 1458 analog op amp that is powered with a single polarity supply of 12 volts. The first stage (pin 3) of the circuit has incoming pulses that go abruptly from about 1 volt to near the 12 volt supply voltage. The first stage of the same amp also has a potentiometer that provides a vref low input to pin 2. The op amp is used with open loop gain. When I turn up the voltage to vref low (pin 2)gradually from 0 upward, as one might expect the output signal at pin one starts to mirror the pulses of the input at pin 3, but at around 2volts, the output of pin 1 stays high, leaving only a small, fickle range of voltage that can be applied to pin 2 and still have the output signal "mirror" the input at pin 3. It seems as though there should be a wider range that the output "mirrors" the input. Any ideas?
Rich

 

Rich,

Is your oscilloscope synchronized with the input signal? I can imagine if the oscilloscope were triggered internally on the output, you could miss the signal.

Try a low frequency triangle wave, 1k or so, something without that fast rise time to make sure the circuit is performing as it should. Then you can go back to the square wave or pulses, and see how to improve your measuring technique.

 

Originally posted by richbrune@Oct 28 2005, 06:20 PM
I have a circuit with a 1458 analog op amp that is powered with a single polarity supply of 12 volts. The first stage (pin 3) of the circuit has incoming pulses that go abruptly from about 1 volt to near the 12 volt supply voltage. The first stage of the same amp also has a potentiometer that provides a vref low input to pin 2. The op amp is used with open loop gain. When I turn up the voltage to vref low (pin 2)gradually from 0 upward, as one might expect the output signal at pin one starts to mirror the pulses of the input at pin 3, but at around 2volts, the output of pin 1 stays high, leaving only a small, fickle range of voltage that can be applied to pin 2 and still have the output signal "mirror" the input at pin 3. It seems as though there should be a wider range that the output "mirrors" the input. Any ideas?
Rich

[post=11312]Quoted post[/post]​

Hi,
The 1458 is an old opamp and not one of the new rail-to-rail types. If you look in the datasheet, you'll se that the input range is specified 2-3V from each rail. I would suppose that 'things' happens if you input voltages close to the +12V supply. Try to decrease the input range and see if the opamp is behaving then. 3-9V is inside the minimum specified range.

TOK