Editing Comparator (section)

==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.<ref>{{cite journal |last1=Rogenmoser |first1=R. |last2=Kaeslin |first2=H. |title=The impact of transistor sizing on power efficiency in submicron CMOS circuits |journal=IEEE Journal of Solid-State Circuits |volume=32 |issue=7 |date=July 1997 |pages=1142–1145|doi=10.1109/4.597307 |bibcode=1997IJSSC..32.1142R |s2cid=15703793 }}</ref> 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,<ref name=qv_pk/4268>{{cite web |title=MAX9025, MAX9026, MAX9027, MAX9028: UCSP, 1.8V, Nanopower, Beyond-the-Rails Comparators With/Without Reference |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4268 |url-status=dead |archive-date=2008-05-04 |archive-url=https://web.archive.org/web/20080504155318/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4268}}</ref> LTC1540,<ref name=LTC1540>{{cite web |title=LTC1540 - Nanopower Comparator with Reference |publisher=Linear Technology |url=http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1002,C1463,P1593 |archive-url=https://web.archive.org/web/20110103043546/http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1002,C1463,P1593 |archive-date=2011-01-03 |url-status=dead}}</ref> LPV7215,<ref>{{cite web |title=LPV7215 - Micropower, CMOS Input, RRIO, 1.8V, Push-Pull Output Comparator from the PowerWise® Family |publisher=National Semiconductor Corporation |url=http://www.national.com/pf/LP/LPV7215.html |archive-url=https://web.archive.org/web/20090503133424/http://www.national.com/pf/LP/LPV7215.html |archive-date=2009-05-03 |url-status=dead}}</ref> MAX9060,<ref name=qv_pk/5823>{{cite web |title=MAX9060, MAX9061, MAX9062, MAX9063, MAX9064: Ultra-Small, Low-Power Single Comparators in 4-Bump UCSP and 5-SOT23 |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5823 |url-status=dead |archive-date=2008-05-17 |archive-url=https://web.archive.org/web/20080517010639/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5823}}</ref> and MCP6541,<ref>{{cite web |title=MCP6541: In Production |publisher=Microchip Technology Inc. |url=http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010414 |archive-url=https://web.archive.org/web/20140213033924/http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010414 |archive-date=2014-02-13 |url-status=dead}}</ref> 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),<ref>{{cite web |title=ADCMP572: Ultrafast 3.3 V Single-Supply Comparator w/CML Output Drivers |publisher=Analog Devices, Inc. |url=https://www.analog.com/en/products/adcmp572.html}}</ref> LMH7220 (LVDS Output),<ref>{{cite web |title=LMH7220: High Speed Comparator with LVDS Output |publisher=Texas Instruments |url=http://www.ti.com/product/lmh7220}}</ref> MAX999 (CMOS output / TTL output),<ref>{{cite web |title=MAX961, MAX962, MAX963, MAX964, MAX997, MAX999: Single/Dual/Quad, Ultra-High-Speed, +3V/+5V, Beyond-the-Rails Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1481 |url-status=dead |archive-url=https://web.archive.org/web/20100414060742/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1481 |archive-date=2010-04-14}}</ref> LT1719 (CMOS output / TTL output),<ref>{{cite web |title=LT1719 - 4.5ns Single/Dual Supply 3V/5V Comparator with Rail-to-Rail Output |publisher=Linear Technology |url=http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1004,C1012,P1817 |archive-url=https://web.archive.org/web/20110102135645/http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1004,C1012,P1817 |archive-date=2011-01-02 |url-status=dead}}</ref> MAX9010 (TTL output),<ref>{{cite web |title=MAX9010, MAX9011, MAX9012, MAX9013: SC70, 5ns, Low-Power, Single-Supply, Precision TTL Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2490/t/al |url-status=dead |archive-url=https://web.archive.org/web/20091228173145/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2490/t/al |archive-date=2009-12-28}}</ref> and MAX9601 (PECL output),<ref>{{cite web |title=MAX9600, MAX9601, MAX9602: Dual ECL and Dual/Quad PECL, 500ps, Ultra-High-Speed Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3400/t/al |url-status=dead |archive-url=https://web.archive.org/web/20100328173649/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3400/t/al |archive-date=2010-03-28}}</ref> 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.<ref>{{cite web |author=Ron Mancini |title=Adding Hysteresis to comparators |url=https://www.edn.com/adding-hysteresis-to-comparators/ |website=EDN |date=May 3, 2001<!--Correct date from https://web.archive.org/web/20050221054157/http://edn.com/article/CA84881.html-->}}</ref> 
For example, the LTC6702,<ref>{{cite web |title=LTC6702 - Tiny Micropower, Low Voltage Dual Comparators |publisher=Linear Technology |url=http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1004,C1012,P38930 |archive-url=https://web.archive.org/web/20110102140904/http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1154,C1004,C1012,P38930 |archive-date=2011-01-02 |url-status=dead}}</ref> MAX9021,<ref>{{cite web |title=MAX9021, MAX9022, MAX9024: Micropower, Ultra-Small, Single/Dual/Quad Single-Supply Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2411/t/al |archive-url=https://web.archive.org/web/20090330140406/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2411/t/al |archive-date=2009-03-30 |url-status=dead}}</ref> and MAX9031,<ref>{{cite web |title=MAX9030, MAX9031, MAX9032, MAX9034: Low-Cost, Ultra-Small, Single/Dual/Quad Single-Supply Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2363/t/al |archive-url=https://web.archive.org/web/20090331134510/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2363/t/al |archive-date=2009-03-31 |url-status=dead}}</ref> 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,<ref>{{cite web |title=LMP7300 - Micropower Precision Comparator and Precision Reference with Adjustable Hysteresis from the PowerWise® Family |publisher=National Semiconductor Corporation |url=http://www.national.com/pf/LM/LMP7300.html |url-status=dead |archive-date=2009-05-03 |archive-url=https://web.archive.org/web/20090503133351/http://www.national.com/pf/LM/LMP7300.html}}</ref> LTC1540,<ref name=LTC1540/> MAX931,<ref>{{cite web |title=MAX931, MAX932, MAX933, MAX934: Ultra-Low-Power, Low-Cost Comparators with 2% Reference |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1219 |url-status=dead |archive-url=https://web.archive.org/web/20100330215239/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1219 |archive-date=2010-03-30}}</ref> MAX971,<ref name=qv_pk/1279>{{cite web |title=MAX971, MAX972, MAX973, MAX974, MAX981, MAX982, MAX983, MAX984: Ultra-Low-Power, Open-Drain, Single/Dual-Supply Comparators |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1279/t/al |url-status=dead |archive-url=https://web.archive.org/web/20090330185135/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1279/t/al |archive-date=2009-03-30}}</ref> and ADCMP341,<ref>{{cite web |title=ADCMP341: Dual 0.275% Comparator and Reference with Programmable Hysteresis |publisher=Analog Devices, Inc. |url=http://www.analog.com/en/power-management/battery-management/adcmp341/products/product.html |archive-url=https://web.archive.org/web/20090815095219/http://www.analog.com/en/power-management/battery-management/adcmp341/products/product.html |archive-date=2009-08-15 |url-status=dead}}</ref> 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.<ref>{{cite web |id=AN3616 |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/appnotes.cfm/an_pk/3616/ |title=Adding Extra Hysteresis to Comparators |url-status=dead |archive-url=https://web.archive.org/web/20080509160549/http://www.maxim-ic.com/appnotes.cfm/an_pk/3616/ |archive-date=2008-05-09}}</ref> When hysteresis is added then a comparator cannot resolve signals within the hysteresis band.

===Output type===
[[File:Dynamic Comparator.png|thumb|right|upright=1.35|A low-power CMOS clocked comparator]]

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.<ref name="test">{{cite web |id=AN886 |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/appnotes.cfm/an_pk/886/ |title=Selecting the Right Comparator |url-status=dead |archive-url=https://web.archive.org/web/20080501144538/http://www.maxim-ic.com/appnotes.cfm/an_pk/886/ |archive-date=2008-05-01}}</ref>

On the basis of outputs, comparators can also be classified as [[open-drain]] or [[push–pull output|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.

===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.<ref name="test"/> ICs with wide range of references are available such as MAX9062 (200 mV reference),<ref name=qv_pk/5823/> LT6700 (400 mV reference),<ref>{{cite web |title=LT6700 - Micropower, Low Voltage, Dual Comparator with 400mV Reference |publisher=Linear Technology |url=http://www.linear.com/product/LT6700 |archive-url=https://web.archive.org/web/20160518160612/http://www.linear.com/product/LT6700 |archive-date=2016-05-18 |url-status=dead}}</ref> ADCMP350 (600&nbsp;mV reference),<ref>{{cite web |title=ADCMP350: Comparator & 0.6V Reference in 4-SC70 w/ Open-Drain Active-Low Output |publisher=Analog Devices, Inc. |url=https://www.analog.com/en/products/adcmp350.html#product-overview |access-date=2023-07-01}}</ref> MAX9025 (1.236&nbsp;V reference),<ref name=qv_pk/4268/> MAX9040 (2.048&nbsp;V reference),<ref>{{cite web |title=MAX9039, MAX9040, MAX9041, MAX9042, MAX9042A, MAX9042B, MAX9043, MAX9043A, MAX9050, MAX9051, MAX9052, MAX9052A, MAX9052B, MAX9053, MAX9053A, MAX9053B: Micropower, Single-Supply, UCSP/SOT23 Comparator + Precision Reference ICs |publisher=Maxim Integrated Products |url=http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2122/t/al |url-status=dead |archive-url=https://web.archive.org/web/20091221015430/http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2122/t/al |archive-date=2009-12-21}}</ref> TLV3012 (1.24&nbsp;V reference),<ref>{{cite web |title=TLV3012: Low-power comparator with reference (push-pull) |publisher=Texas Instrument |url=https://www.ti.com/product/TLV3012}}</ref> and TSM109 (2.5&nbsp;V reference).<ref>{{cite web |title=TSM109/A: DUAL COMPARATOR AND VOLTAGE REFERENCE |publisher=STMicroelectronics |url=https://pdf.datasheetcatalog.com/datasheet/stmicroelectronics/9208.pdf}}</ref> 

===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.<ref>
{{cite book
 | title = Offset Reduction Techniques in High-Speed Analog-to-Digital Converters: Analysis, Design and Tradeoffs
 | author = Pedro M. Figueiredo, João C. Vital
 | publisher = Springer
 | year = 2009
 | isbn = 978-1-4020-9715-7
 | pages = 54–62
 | url = https://books.google.com/books?id=El9Ki0spMEwC&q=%22output+voltages+of+the+latched+comparator%22+regeneration+reset+phase&pg=PA55
 }}</ref>  
This is in contrast to a continuous comparator, which can only employ weak positive feedback since there is no reset period.