9.1 Comparator_A

I concentrate on the newer Comparator_A+ module in this section but the principles apply equally well to Comparator_A.

An analog comparator compares the voltages on its two input terminals, V ₊ and V _?. Its output is high if V ₊ > V _? and low if V ₊ < V _?. Thus it provides a basic bridge between the analog and digital domains and acts like a 1-bit ADC. From an analog perspective it looks something like an operational amplifier (op-amp) without the usual negative feedback. In this case the high open-loop gain ensures that the output is almost always saturated at either the positive or negative supply rail. (Real op-amps are never used like this but you may have seen the same behavior due to a bad joint in the feedback circuit.) The internal design of a comparator is rather different from that of an op-amp and is optimized to switch the output between high and low as rapidly as possible, taking about 100 200ns in Comparator_A+. This is assisted by a little positive feedback, which also gives a small hysteresis of around 1 mV.