While using op amp as a comparator in instrumentation, the open loop can be used to compare the two voltages. Therefore, depending on the difference between the input voltage value and the reference voltage value, the output Vout will be equal to the maximum high value or minimum low value (input voltage value will be greater than or less than the reference voltage value few by micro volts).
The reference voltage is fed to non-inverting input terminal of the op amp and variable voltage is fed to inverting input terminal of the op amp. Consider the op amp comparator circuit diagram shown in the figure, if the voltage fed to pin 2 is greater than the reference voltage fed to pin 3, then the output voltage becomes low and it is marginally greater than –Vs. If the voltage fed to pin 2 is less than the reference voltage fed to pin 3, then the output voltage becomes high and it is marginally less than +Vs.