Comparator

Open collector output

Some comparators (e.g. LM339) use open collector output to help interface to different logic families. When the inverting input is at a higher voltage than the non inverting input, the output of the comparator connects to the negative power supply. When the non inverting input is higher than the inverting input, the output is high impedance, so the output voltage in this state can be set by an external pull-up resistor to a different voltage supply.

Power and Speed

While in general comparators are "fast", their circuits are not immune to the classic speed-power tradeoff. High speed comparators use transistors with larger aspect ratios and hence also consume more power.^[8] Depending on the application, select either a comparator with high speed or one that saves power. For example, nano-powered comparators in space-saving chip-scale packages (UCSP), DFN or SC70 packages such as MAX9027,^[9] LTC1540,^[10] LPV7215,^[11] MAX9060,^[12] and MCP6541,^[13] are ideal for ultra-low-power, portable applications. Likewise if a comparator is needed to implement a relaxation oscillator circuit to create a high speed clock signal then comparators having few Nanoseconds of propagation delay may be suitable. ADCMP572 (CML output),^[14] LMH7220 (LVDS Output),^[15] MAX999 (CMOS output / TTL output),^[16] LT1719 (CMOS output / TTL output),^[17] MAX9010 (TTL output),^[18] and MAX9601 (PECL output),^[19] are examples of some good high speed comparators.

Hysteresis

A comparator normally changes its output state when the voltage between its inputs crosses through approximately zero volts. Small voltage fluctuations due to noise, always present on the inputs, can cause undesirable rapid changes between the two output states when the input voltage difference is near zero volts. To prevent this output oscillation, a small hysteresis of a few millivolts is integrated into many modern comparators.^[20] For example, the LTC6702,^[21] MAX9021,^[22] and MAX9031,^[23] have internal hysteresis desensitizing them from input noise. In place of one switching point, hysteresis introduces two: one for rising voltages, and one for falling voltages. The difference between the higher-level trip value (VTRIP+) and the lower-level trip value (VTRIP-) equals the hysteresis voltage (VHYST).

If the comparator does not have internal hysteresis or if the input noise is greater than the internal hysteresis then an external hysteresis network can be built using positive feedback from the output to the non-inverting input of the comparator. The resulting Schmitt trigger circuit gives additional noise immunity and a cleaner output signal. Some comparators such as LMP7300,^[24] LTC1540,^[10] MAX931,^[25] MAX971,^[26] and ADCMP341,^[27] also provide the hysteresis control through a separate hysteresis pin. These comparators make it possible to add a programmable hysteresis without feedback or complicated equations. Using a dedicated hysteresis pin is also convenient if the source impedance is high since the inputs are isolated from the hysteresis network.^[28] When hysteresis is added then a comparator cannot resolve signals within the hysteresis band.

Output type

Because comparators have only two output states, their outputs are either near zero or near the supply voltage. Bipolar rail-to-rail comparators have a common-emitter output that produces a small voltage drop between the output and each rail. That drop is equal to the collector-to-emitter voltage of a saturated transistor. When output currents are light, output voltages of CMOS rail-to-rail comparators, which rely on a saturated MOSFET, range closer to the rail voltages than their bipolar counterparts.^[29]

On the basis of outputs, comparators can also be classified as open-drain or push–pull. Comparators with an open-drain output stage use an external pull-up resistor to a positive supply that defines the logic high level. Open-drain comparators are more suitable for mixed-voltage system design. Since the output has high impedance for logic high level, open-drain comparators can also be used to connect multiple comparators to a single bus. Push–pull output does not need a pull-up resistor and can also source current, unlike an open-drain output.^[30]

Internal reference

The most frequent application for comparators is the comparison between a voltage and a stable reference. TL431 is widely used for this purpose. Most comparator manufacturers also offer comparators in which a reference voltage is integrated on to the chip. Combining the reference and comparator in one chip not only saves space, but also draws less supply current than a comparator with an external reference.^[29] ICs with wide range of references are available such as MAX9062 (200 mV reference),^[12] LT6700 (400 mV reference),^[31] ADCMP350 (600 mV reference),^[32] MAX9025 (1.236 V reference),^[9] MAX9040 (2.048 V reference),^[33] TLV3012 (1.24 V reference),^[34] and TSM109 (2.5 V reference).^[35]

Continuous versus clocked

A continuous comparator will output either a "1" or a "0" any time a high or low signal is applied to its input and will change quickly when the inputs are updated. However, many applications only require comparator outputs at certain instances, such as in A/D converters and memory. By only strobing a comparator at certain intervals, higher accuracy and lower power can be achieved with a clocked (or dynamic) comparator structure, also called a latched comparator. Often latched comparators employ strong positive feedback for a "regeneration phase" when a clock is high, and have a "reset phase" when the clock is low.^[36] This is in contrast to a continuous comparator, which can only employ weak positive feedback since there is no reset period.

Null detectors

A null detector identifies when a given value is zero. Comparators are ideal for null detection comparison measurements, since they are equivalent to a very high gain amplifier with well-balanced inputs and controlled output limits. The null detector circuit compares two input voltages: an unknown voltage and a reference voltage, usually referred to as v_u and v_r. The reference voltage is usually on the non-inverting input (+), while the unknown voltage is usually on the inverting input (−). (A circuit diagram would display the inputs according to their sign with respect to the output when a particular input is greater than the other.) Unless the inputs are nearly equal (see below), the output is either positive or negative, for example ±12 V. In the case of a null detector the aim is to detect when the input voltages are nearly equal, which gives the value of the unknown voltage since the reference voltage is known.^[37]

When using a comparator as a null detector, accuracy is limited; an output of zero is given whenever the magnitude of the voltage difference multiplied by the gain of the amplifier is within the voltage limits. For example, if the gain is 10⁶, and the voltage limits are ±6 V, then an output of zero will be given if the voltage difference is less than 6 μV. One could refer to this as a fundamental uncertainty in the measurement.^[38]

Zero-crossing detectors

For this type of detector, a comparator detects 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 (high) for a positive pulse and LO (low) for a negative pulse squares the input signal.^[39]

Relaxation oscillator

A comparator can be used to build a relaxation oscillator. It uses both positive and negative feedback. The positive feedback is a Schmitt trigger configuration. Alone, the trigger is a bistable multivibrator. However, the slow negative feedback added to the trigger by the RC circuit causes the circuit to oscillate automatically. That is, the addition of the RC circuit turns the hysteretic bistable multivibrator into an astable multivibrator.^[40]

Level shifter

This circuit requires only a single comparator with an open-drain output as in the LM393,^[41] TLV3011,^[42] or MAX9028.^[9] The circuit provides great flexibility in choosing the voltages to be translated by using a suitable pull up voltage. It also allows the translation of bipolar ±5 V logic to unipolar 3 V logic by using a comparator like the MAX972.^[26][29]

Analog-to-digital converters

When a voltage comparator is used to determine whether an input voltage is above or below a particular threshold (known as the threshold voltage^[43]), it is essentially performing a one-bit quantization. This function is used in nearly all types of analog to digital converters (including flash, successive approximation, delta-sigma modulation and dual-slope) in combination with other circuitry to achieve a multi-bit quantization.^[44]

Window detectors

Comparators can also be used as window detectors. In a window detector, a comparator is used to compare two voltages and determine whether a given input voltage is under voltage or over voltage.

Absolute-value detectors

Comparators can be used to create absolute-value detectors. In an absolute-value detector, two comparators and a digital logic gate are used to compare the absolute values of two voltages.^[45]