Comparator

In electronics, a comparator is a device which compares two voltages or currents and switches its output to indicate which is larger.

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, an adjustable hysteresis and a clock gated input.

Input voltage range

The input voltages must not exceed the power voltage range : $V_{S-}\leq V_{+},V_{-}\leq V_{S+}$

In the case of TTL/CMOS logic output comparators, negative inputs are not allowed: $0\leq V_{+},V_{-}\leq V_{cc}$

Op-amp implementation of voltage comparator

A simple op-amp comparator.

A standard op-amp operating without negative feedback can be used as a comparator. 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:

$V_{out}=V\cdot sgn(V_{+}-V_{-})$

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

$V_{out}\leq V_{S+}$ 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.

Dedicated voltage comparator chips

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). If one of the voltages is fixed, for example because a DC adjustment is possible in a device earlier in the signal path, a comparator is just a cascade of amplifiers. When the voltages are nearly equal, the output voltage will not fall into one of the logic levels, thus analog signals will enter the digital domain with unpredictable results. To make this range as small as possible the cascade is long and high gain, that is bipolar transistors instead of field effect transistors are used, except sometimes for the first stage. For high speed the input impedance of the stages is made low. This already reduces the saturation of the slow, large P-N junction of the bipolar transistors, which would otherwise lead to long recovery times. Fast that is small Schottky diodes as in binary logic are applied to improve matters even further. Also like in binary logic the speed is not as high as if the amplifiers would be used for analog signals. Slew rate has no meaning for these devices. For the application in flash ADCs after each amplifier the signal can be fanned out over 8 ports matched to the voltage and current gain and resistors are used as level-shifters.

The LM339 accomplishes this with an open collector output. When the inverting input is at a higher voltage than the non inverting input, the output of the comparator is connected to the negative power supply. When the non inverting input is higher than the inverting input, the output is floating (has a very high impedance to ground).

Inputs	Output
− > +	Negative
+ > −	Floating

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:

$V_{\mathrm {out} }\leq V_{\mathrm {CC} }$ when $V_{+}\geq V_{-}$ else $0$ .

Precautions in use

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, positive feedback forming a Schmitt trigger can be used to provide hysteresis and a cleaner output signal.

Applications

Comparators as detectors

Null detectors

Comparators are a type of amplifier, which is designed distinctively for null comparison measurements. A comparator is a very high gain amplifier with well-balanced inputs and controlled output limits. The comparator, as its name suggests, compares the two input voltages, determining which is larger. The inputs are an unknown voltage, and a reference voltage, usually referred to as v_u and v_r. The reference voltage is generally connected to the non-inverting input (+), while vu is usually connected to the inverting input (-). (The inputs are labeled according to the sign of the output when that input is greater than the other.) The output is given as either a positive or negative limit, for example +/-12V.

A null detector is one that functions to identify when a given value is zero. In this case, the idea is to detect when there is no difference between in the input voltages. This gives the identity of the unknown voltage, since the reference voltage is known.

When using a comparator as a null detector, there are limits as to the accuracy of the zero value measurable. Zero output is given when the magnitude of the difference in the voltages multiplied by the gain of the amplifier is less than the voltage limits. For example, if the gain of the amplifier is 10⁶, and the voltage limits are +/-6V, then no output will be given if the difference in the voltages is less than 6uV. One could refer to this as a sort of uncertainty in the measurement.

Zero-crossing detectors

For this type of detector, a comparator is used to detect each time an ac pulse changes polarity. The output of the comparator changes state each time the pulse changes its polarity. That is, the output is HI for a positive pulse and LO for a negative pulse. The comparator also amplifies and squares the input signal.

neuron simulators, triggers, logic, sensor control

External links

IC Comparator reference page at home.cogeco.ca