Comparator

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For other uses, see Comparator (disambiguation).

In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals V_+\, and V_-\, and one binary digital output V_o\,. The output is ideally

V_o = 
\begin{cases} 
1,  & \mbox{if }V_+ > V_-  \\
0,  & \mbox{if }V_+ < V_- 
\end{cases}

A comparator consists of a specialized high-gain differential amplifier. They are commonly used in devices that measure and digitize analog signals, such as analog-to-digital converters (ADCs), as well as relaxation oscillators.

Differential Voltage[edit]

The differential voltages must stay within the limits specified by the manufacturer. Early integrated comparators, like the LM111 family, and certain high-speed comparators like the LM119 family, require differential voltage ranges substantially lower than the power supply voltages (±15 V vs. 36 V).[1] Rail-to-rail comparators allow any differential voltages within the power supply range. When powered from a bipolar (dual rail) supply,

V_{S-} \le V_+,V_- \le V_{S+}

or, when powered from a unipolar TTL/CMOS power supply:

0 \le V_+,V_- \le V_{cc}

Specific rail-to-rail comparators with p-n-p input transistors, like the LM139 family, allow input potential to drop 0.3 volts below the negative supply rail, but do not allow it to rise above the positive rail.[2] Specific ultra-fast comparators, like the LMH7322, allow input signal to swing below the negative rail and above the positive rail, although by a narrow margin of only 0.2 V.[3] Differential input voltage (the voltage between two inputs) of a modern rail-to-rail comparator is usually limited only by the full swing of power supply.

Op-amp voltage comparator[edit]

A simple op-amp comparator

An operational amplifier (op-amp) has a well balanced difference input and a very high gain. This parallels the characteristics of comparators and can be substituted in applications with low-performance requirements.[4]

In theory, a standard op-amp operating in open-loop configuration (without negative feedback) may be used as a low-performance 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 the output to saturate at the highest positive voltage it can output. When the non-inverting input (V+) drops below the inverting input (V-), the output saturates at the most negative voltage it can output. The op-amp's output voltage is limited by the supply voltage. An op-amp operating in a linear mode with negative feedback, using a balanced, split-voltage power supply, (powered by ± VS) has its transfer function typically written as: V_{out}=A_{o} (V_1 - V_2). However, this equation may not be applicable to a comparator circuit which is non-linear and operates open-loop (no negative feedback)

In practice, using an operational amplifier as a comparator presents several disadvantages as compared to using a dedicated comparator:[5]

  1. Op-amps are designed to operate in the linear mode with negative feedback. Hence, an op-amp typically has a lengthy recovery time from saturation. Almost all op-amps have an internal compensation capacitor which imposes slew rate limitations for high frequency signals. Consequently an op-amp makes a sloppy comparator with propagation delays that can be as long as tens of microseconds.
  2. Since op-amps 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 op-amp is valid only when the feedback is active. Some op-amps 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 op-amp as a comparator.
  5. Some multiple-section op-amps may exhibit extreme channel-channel interaction when used as comparators.
  6. Many op-amps have back to back diodes between their inputs. Op-amp inputs usually follow each other so this is fine. But comparator inputs are not usually the same. The diodes can cause unexpected current through inputs.

Working[edit]

Several voltage comparator ICs

A dedicated voltage comparator will generally be faster than a general-purpose operational amplifier 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. 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 significantly 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 eight 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). The gain of op amp as comparator is given by this equation V(out)=V(in)*A

Inputs Output
− > + Grounded
+ > − Floating

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

V_\mathrm{out} \le V_\mathrm{CC} when V_+ \ge V_- else 0.

Key specifications[edit]

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[edit]

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.[6] 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[edit]

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.[7] 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, MAX971 and 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.[8] When hysteresis is added then a comparator cannot resolve signals within the hysteresis band.

Output type[edit]

A Low Power CMOS Clocked Comparator

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.[9]

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 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[edit]

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.[9] ICs with wide range of references are available such as MAX9062(200 mV reference), LT6700(400 mV reference), ADCMP350 (600 mV reference), MAX9025 (1.236 V reference), MAX9040 (2.048 V reference), TLV3012 (1.24 V reference) and TSM109 (2.5 V reference).

Continuous versus clocked[edit]

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.[10] This is in contrast to a continuous comparator, which can only employ weak positive feedback since there is no reset period.

Applications[edit]

Null detectors[edit]

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 vu and vr. 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 or negative, for example ±12 V. 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 ±6 V, then no output will be given if the difference in the voltages is less than 6 μV. One could refer to this as a sort of uncertainty in the measurement.[11]

Zero-crossing detectors[edit]

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.[12]

Relaxation oscillator[edit]

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.[13]

Level shifter[edit]

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 ±5 V logic to unipolar 3 V logic by using a comparator like the MAX972.[9]

Analog-to-digital converters[edit]

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, successive approximation, delta-sigma modulation, folding, interpolating, dual-slope and others) in combination with other devices to achieve a multi-bit quantization.[14]

Window detectors[edit]

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

See also[edit]

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

References[edit]

  1. ^ LM111/LM211/LM311 datasheet. Texas Instruments. August 2003. Retrieved 2014-07-02.
  2. ^ LM139/LM239/LM339/LM2901/LM3302 datasheet. Texas Instruments. August 2012. Retrieved 2014-07-02.
  3. ^ LMH7322 datasheet. Texas Instruments. March 2013. Retrieved 2014-07-02.
  4. ^ Malmstadt, Enke and Crouch, Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Company, Inc., 1981, ISBN 0-8053-6917-1, Chapter 5.
  5. ^ Ron Mancini, "Designing with comparators," EDN, March 29, 2001.
  6. ^ 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.
  7. ^ Ron Mancini, "Adding Hysteresis to comparators," EDN, May 3, 2001.
  8. ^ AN3616, Maxim Integrated Products, Adding Extra Hysteresis to Comparators.
  9. ^ a b c AN886, Maxim Integrated Products, Selecting the Right Comparator.
  10. ^ Pedro M. Figueiredo, João C. Vital (2009). Offset Reduction Techniques in High-Speed Analog-to-Digital Converters: Analysis, Design and Tradeoffs. Springer. pp. 54–62. ISBN 978-1-4020-9715-7. 
  11. ^ Malmstadt, Howard V.; Enke, Christie G.; Crouch, Stanley R. (January 1981), Electronics and Instrumentation for Scientists, The Benjamin/Cummings Publishing Co, pp. 108–110, ISBN 0-8053-6917-1 
  12. ^ Electronics and Instrumentation for Scientists. Malmstadt, Enke, and Crouch, The Benjamin/Cummings Publishing Co., In., 1981, p.230.
  13. ^ Paul Horowitz and Winfield Hill: The Art of Electronics, Cambridge University Press, Second edition, Cambridge 1989, pp.284–285.
  14. ^ Phillip Allen and Douglas Holberg: CMOS Analog Circuit Design, Oxford University Press, Second edition, Oxford 2002.

External links[edit]