Window comparator design

John's answer is certainly a good one and you should take it under consideration, but depending on the level of sophistication needed you might want to consider a digital solution using an A-D with a good reference. Using comparators will definitely be cheaper, but if you already have a microcontroller in your circuit, adding an A-D and then making the comparison in software may be a bit more flexible. I've seen this used for self-monitoring in some safety critical controllers I work with.

Chris

I read in sci.electronics.design that Dave wrote (in ) about 'Window comparator design', on Thu, 24 Feb 2005:

No, you need comparators. Don't use an op-amp as a comparator unless you really have no choice. A single quad comparator, probably even the vintage (simple and cheap) LM339, will do what you want, unless your numbers have a lot of trailing zeroes, i.e. you need high precision.

The main design point is getting the three reference voltages correct and very stable. There are numerous ways to do it, but I find that a current source feeding a resistor chain is simple and reliable.

If you have a 1 mA current source, either a precision device or a discrete bipolar with temperature-compensated biasing using an LED (free circuit function indicator!), then your three voltages can be set up with two 100 ohm resistors and two others, of as close a tolerance as you wish. ASSuMEing, of course, that you have a DC supply available suitably in excess of 8.5 V (you might squeeze in with 10 V but 12 V would be better). If not, it gets just a little more complicated, because you have to precision-attenuate the input signal down to something compatible with the DC supply voltage.