I'm a junior electronics engineer and have a question about a window comparator circuit I've built:
The op amps have a tight reference voltage on their non-inverting terminals. U1A pin 3 is set to 5.025V with precision resistors and U1B pin 5 is set to 5V.
The inverting pins of both op-amps are tied to a 0-10V input signal that varies depending on an external system.
The outputs of both op amps are tied to the LED of an opto isolator (U1A output = Anode of the opto-isolator, U1B output = Cathode of the opto-isolator). When the input of the external system providing the 0-10V signal is in the window of 5V - 5.025V, the opto-isolator will be active as U1A pin 1 will be high and U1B pin 7 will be low therefore pulling the opto-isolator to ground.
I originally tried using 2 x OPA27 op amps instead of a dual package MC1458. When I initially tested the circuit, the opto-isolator would not switch and the voltage on the inverting terminals of the OPA27 devices started to "follow" the 0-10V signal (by this I mean as the voltage on the inverting terminals swung linearly between 0-10V, the non-inverting terminals no longer remained at their fixed precision voltages as defined by the resistor ladder).
My question is why did the OPA27 not work in a window comparator configuration while the MC1458 has no problem? The MC1458 is not a comparator. It is, much like the OPA27, a general purpose op-amp. They both have similar Common Mode Rejection but I can't seem to understand why the MC1458 is better for the job (and ultimately, why the MC1458 worked but the OPA27 did not).
Any help would be greatly appreciated - Apologies in advance if anything similar has been asked or if this is a really rookie question.