comparator

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

Input voltage range

The input voltages must not exceed the power voltage range:

In the case of TTL/CMOS logic output comparators, negative inputs are not allowed:

Op-amp implementation of voltage comparator

A simple op-amp comparator

An operational amplifier has a well balanced difference input and a very high gain. The parallels in the characteristics allows the op-amps to serve as comparators in some functions.^[1]

A standard op-amp operating in open loop configuration (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:

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

when else when

In practice, using an operational amplifier as a comparator presents several disadvantages as compared to using a dedicated comparator:^[2]

1. Opamps are designed to operate in the linear mode with negative feedback. Hence, an opamp typically has a lengthy recovery time from saturation. Almost all opamps have an internal compensation capacitor which imposes slew rate limitations for high frequency signals. Consequently an opamp makes a sloppy comparator with propagation delays that can be as slow as tens of microseconds.
2. Since opamps do not have any internal hysteresis an external hysteresis network is always necessary for slow moving input signals.
3. The quiescent current specification of an opamp is valid only when the feedback is active. Some opamps show an increased quiescent current when the inputs are not equal.
4. A comparator is designed to produce well limited output voltages that easily interface with digital logic. Compatibility with digital logic must be verified while using an opamp as a comparator.

Dedicated voltage comparator chips

Several voltage comparator ICs

A dedicated voltage comparator will generally be faster than a general-purpose operational amplifier (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.

A dedicated voltage comparator chip such as LM339 is designed to interface with a digital logic interface (to a TTL or a CMOS). The output is a binary state often used to interface real world signals to digital circuitry (see analog to digital converter). If there is a fixed voltage source from, for example, a DC adjustable device in the signal path, a comparator is just the equivalent of 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 amplifier cascade is high gain. The circuit consists of mainly Bipolar transistors except perhaps in the beginning stage which will likely be field effect transistors. For very high frequencies, the input impedance of the stages is low. This reduces the saturation of the slow, large P-N junction bipolar transistors that would otherwise lead to long recovery times. Fast small Schottky diodes, like those found in binary logic designs, improve the performance significanly though the performance still lags that of circuits with amplifiers using analog signals. Slew rate has no meaning for these devices. For applications in flash ADCs the distributed signal across 8 ports matches the voltage and current gain after each amplifier, and resistors then behave 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 connects 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).

With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5 and can interface with TTL logic:

when else 0.

Key specifications

While it is easy to understand the basic task of a comparator, that is, comparing two voltages or currents, several parameters must be considered while selecting a suitable comparator:

Speed and power

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.^[3] 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, LTC1540, LPV7215, MAX9060 and MCP6541 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 nano seconds of propagation delay may be suitable. ADCMP572 (CML output), LMH7220 (LVDS Output), MAX999 (CMOS output / TTL output), LT1719 (CMOS output / TTL output), MAX9010 (TTL output), and MAX9601 (PECL output) 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.^[4] For example, the LTC6702, MAX9021 and MAX9031 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, LTC1540, MAX931, MAX971and ADCMP341 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.^[5] 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 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 rails than their bipolar counterparts.^[6]

On the basis of outputs, comparators can also be classified as open drain or push–pull. Comparators with an open-drain output stage use a 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 is high impedance for logic level high, open drain comparators can also be used to connect multiple comparators on to a single bus. Push pull output does not need a pull up resistor and can also source current unlike an open drain output.

Internal reference

The most frequent application for comparators is the comparison between a voltage and a stable reference. 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.^[6] ICs with wide range of references are available such as MAX9062(200 mV reference), LT6700(400 mV reference), ADCMP350(600mV reference), MAX9025(1.236V reference), MAX9040(2.048V reference), TLV3012(1.24V reference) and TSM109(2.5V reference).

Applications

Null detectors

A null detector is one that functions to identify when a given value is zero. Comparators can be a type of amplifier distinctively for null comparison measurements. It is the equivalent to a very high gain amplifier with well-balanced inputs and controlled output limits. The circuit compares the two input voltages, determining the larger. The inputs are an unknown voltage and a reference voltage, usually referred to as v_u and v_r. A reference voltage is generally on the non-inverting input (+), while vu 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.) The output is either positive negative, for example +/-12V. 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 6μV. One could refer to this as a sort of uncertainty in the measurement.^[7]

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. The comparator also amplifies and squares the input signal.^[8]

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.^[9]

Level shifter

This circuit requires only a single comparator with an open-drain output as in the LM393, TLV3011 or MAX9028. 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 ±5V logic to unipolar 3V logic by using a comparator like the MAX972.^[6]

Analog to Digital Converters

When a comparator performs the function of telling if an input voltage is above or below a given threshold, it is essentially performing a 1-bit quantization. This function is used in nearly all analog to digital converters (such as Flash, Pipeline, SAR, Delta Sigma, Folding, Interpolating, Dual-slope and others) in combination with other devices to achieve a multi-bit quantization. ^[10]

References

1. ^ Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 5.
2. ^ Ron Mancini, "Designing with comparators," EDN, March 29, 2001.
3. ^ Rogenmoser, R.; Kaeslin, H, "The impact of transistor sizing on power efficiency in submicron CMOS circuits," Solid-State Circuits, IEEE Journal of Volume 32, Issue 7, Jul 1997 Page(s):1142–1145.
4. ^ Ron Mancini, "Adding Hysteresis to comparators," EDN, May 3, 2001.
5. ^ AN3616, Maxim Integrated Products, Adding Extra Hysteresis to Comparators.
6. ^ ^{a b c} AN886, Maxim Integrated Products, Selecting the Right Comparator.
7. ^ Electronics and Instrumentation for Scientists. Malmstadt, Enke, and Crouch, The Benjamin/Cummings Publishing Co., In., 1981, pp.108–110.
8. ^ Electronics and Instrumentation for Scientists. Malmstadt, Enke, and Crouch, The Benjamin/Cummings Publishing Co., In., 1981, p.230.
9. ^ Paul Horowitz and Winfield Hill: The Art of Electronics, Cambridge University Press, Second edition, Cambridge 1989, pp.284–285.
10. ^ Phillip Allen and Douglas Holberg: CMOS Analog Circuit Design, Oxford University Press, Second edition, Oxford 2002.

See also

• Digital comparator
• Current comparator
• Constant fraction discriminator
• Flash ADC
• Sorting network

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".

External links

Look up comparator in Wiktionary, the free dictionary.

• IC Comparator reference page at home.cogeco.ca