In the typical comparator circuit with feedback hysteresis as shown in this example:



The voltage divider formed by R1 and R2 at the positive input to the comparator sets the nominal threshold for the comparison. Resistor R4 is driven in parallel with R1 or R2 depending upon if the output of the comparator is high or low. This has the effect of raising the threshold voltage if the output is high or lowers it if the output is low.

The factors mentioned in the EDN design idea article can have an effect on the accuracy of the threshold voltages achieved in this conventional manner. For push-pull type output on the comparator the V_OH and the V_OL of the comparator will change from part to part and with varying conditions of the load on the comparator thus having an effect on the actual threshold voltages achieved. For open collector or open drain type comparators the V_OL and the output pullup resistor (R3) value affect the threshold voltages achieved. In this case loading variations can have an even greater impact when the output is high.

There are certainly many many designs where this scheme is really rather quite suitable. Careful selection of the comparator, output style used and feedback resistor size relative to the load impedance can have a big impact on how much the mentioned factors will have on the accuracy of the threshold voltages that are generated. Along with that there are certainly various applications where the threshold voltage variations in the circuit have little impact on the intended circuit functionality.

Now that said there are another family of applications where the accuracy of the threshold voltages can be very important. In these cases a design as presented in the EDN article can be used. There are also other ways that can be used as well by using accurate voltage references and/or resistor dividers to produce the threshold voltages and then use separate analogue switches to flip the threshold selection between the appropriate levels. In this latter case the comparator output can be used to run the analogue switch control pins but now it is a digital signal and not an pseudo "analogue" signal as in the circuit topology in the diagram above.

Michael Karas