The circuit starts with a resistor ladder. Each resistor has the same value so the voltage dropped across each one is the same.
Simple voltage follower OP Amp circuits buufer the voltages.Each of the buffered voltages are connected to a window detector. The
windows detector is made up of two voltage comparators. One comparator detects the lower threshold and the other the upper. The
open collector outputs of the pair are 'wired OR' together. This means that the output is only high when both comparator outputs are high.
The lower threshold comparator is set to have it's output high when the input signal that is common to all the stages is above the voltage from the
ladder. The upper threshold is set to have it's output high when the input is lower than the upper threshold value. The upper threshold
value is adjustable from the current tap to two steps higher. This allows the window to be adjusted from 0 to two steps wide.