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Using op amps and comparators: What the digital guys can teach us

My caller last week was in a real bind. The design was almost done and he had run out of room. He needed help. The system needed a comparator function and the circuit board was full. After going over the schematic diagram I agreed that one comparator should finish the design.

I continued to look at the schematic and asked about “U3-B,” a part shown on the bill of materials, but I could not find this op amp function in the schematic. Yes, he had that spare op amp but he needed a comparator.

I commented that there were more similarities than differences between op amps and comparators. Now that a large number of systems are using a single 5V supply, and the op amps have rail-to-rail input and output capability, the differences are often minimal. Except for the missing propagation delay specification an op amp, operating open loop, might be considered an over specified, or “alternately specified” comparator. In fact, it is not uncommon for the rail-to-rail output stages on op amps to swing closer to the supply rail than some comparators.

On the input side of the device the specifications are very similar. Voltage offset, offset drift, offset current and bias current parameters are the same for both devices. Common mode voltage range and common mode rejection also follow the same pattern for both devices.

One major difference between an op amp and a comparator is that op amps are compensated to be stable when operated at or above a given gain. Keep in mind that, in general, speed increases with increased quiescent current; therefore, at a given quiescent current an uncompensated comparator will have a much higher slew rate than a stabilized op amp. A side effect of the compensation is that the op amp may take longer to come out of saturation than a comparator.

Differences also appear at the output stage. In terms of output circuit topology, comparators may have either one of two configurations. Some comparators have open collector or open drain output stages, the balance have push-pull outputs. Those with the push-pull output stage may have symmetric rise and fall times while the open collector types can be collector “OR'ed”. This flexibility is offset by the fact that the rise time is set by the pull-up resistor, and any associated load capacitance, and is slower than the fall time. Op amps always have push-pull type output stages but that does not guarantee the output slew rate is symmetric.

Since both devices have differential input stages either can be arranged with hysteresis. This stops the chatter associated with slow moving input signals. In the digital arena this circuit is often called a Schmitt trigger.

Thanks for reading. Comments Please.
klein_bill@ti.com

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