Comparator

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In electronics, a comparator is a device which compares two voltages or currents and switches its output to indicate which is larger. More generally, the term is also used to refer to a device that compares two items of data.

A standard op-amp without negative feedback can be used as a comparator, as indicated in the following diagram.

When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by ± V_S) this action can be written:

V_out = V_S sgn(V₊ − V₋)

where sgn(x) is the signum function. Generally, the positive and negative supplies V_S will not match absolute value:

V_out <= V_S+ when (V₊ > V_-) else V_S- when (V₊ < V_-).

Equality of input values is very difficult to achieve in practice. The speed at which the change in output results from a change in input (often called the slew rate in operational amplifiers) is typically in the order of 10ns to 100ns, but can be as slow as a few tens of μs.

A dedicated voltage comparator chip, such as the LM339, is designed to interface directly to digital logic (for example TTL or CMOS). The output is a binary state, and it is often used to interface real world signals to digital circuitry (see analog to digital converter). The LM339 accomplishes this with an open collector output. When the inverting input is higher, the output of the comparator is connected to the negative power supply. When the noninverting input is higher, the output is floating (has a very high impedance to ground). With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5 and can be interfaced to TTL logic:

V_out <= V_cc when (V₊ > V_-) else 0.

A dedicated voltage comparator will generally be faster than a general-purpose op-amp pressed into service as a comparator. A dedicated voltage comparator may also contain additional features such as an accurate, internal voltage reference and adjustable hysteresis.

It is incorrect to consider a comparator as a device with a differential (bipolar) input and a logic (0/V_cc) output as the inputs of real comparators are not isolated. This means that not only their difference affects the output but also their voltages must not exceed the power voltage range: V_S- ≤ V₊,V_- ≤ V_S+. In the case of TTL/CMOS logic output comparators negative inputs are not allowed: 0 ≤ V₊,V_- ≤ V_cc.

When comparing a noisy signal to a threshold, the comparator may switch rapidly from state to state as the signal crosses the threshold. If this is unwanted, a Schmitt trigger can be used to provide hysteresis and a cleaner output signal.