LOVELY PROFESSIONAL UNIVERSITY

JALANDHAR

ECE:- 210

TERM PAPER :- COMPARATOR AND ITS APPLICATION

SUBMITED TO:-

MR.SHAKTI R CHOPRA

SUBMITED BY:-

SHAMSHER SINGH

SEC.NO. D6803

REG.NO. 10805489

ROLL.NO A-20

Acknowledgement

I, Shamsher singh would like to thank lovely professional university for introducing a concept

of term papers at this very stage which helped me a lot in knowing me my skills abilities and

gathering extra knowledge about the subject. I would like to thank my subject teacher Mr.

Shakti raj Department of Electronics sciences, for his help and co-ordination in completing

this term paper.

I would also like to thank my friends and classmates without whose cooperation this project would never have been completed.

TABLE OF CONTENTS

INTRODUCTION

KEY SPECIFICATIONS

FUNCTIONS OF COMPARATORS

COMPARING COMPARATOR AND OP AMPS

SUPPLY VOLTAGE

BASIC FEATURES OF COMPARATOR

COMPARATOR OUTPUT

APPLICATION OF COMPARATOR

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.

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 ± VS) this action can be written:

where sgn(x) is the sign function. Generally, the positive and negative supplies VS 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 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 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).

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 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. 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. 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.[5] When hysteresis is added then a comparator cannot resolve signals within the hysteresis band.

Output type

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.

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. 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

Continuous Vs. 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 instantly 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. Often Clocked latches can employ strong positive feedback when a clock is high and reset when the clock is low. This is in contrast to a continuous comparator which can only employ weak positive feedback since there is no reset period.

The Function of a Comparator

A comparator accepts two analog signals and produces a binary signal at the output, a function of which input voltage is higher. The output signal remains constant as the differential input voltage changes. When described that way, the comparator resembles a 1-bit ADC.

Comparing Comparators and Op Amps

An op amp running without negative feedback can serve as a comparator, because its high voltage gain enables it to resolve very small differences in input voltage. Op amps used this way are generally slower than comparators and lack other special features, such as hysteresis and internal references.

Comparators cannot generally be used as op amps. They are trimmed to provide excellent switching times at the expense of the frequency-response correction that makes op amps so

versatile. The internal hysteresis employed in many comparators, which prevents oscillation at the output, also prevents their use as op amps.

Supply Voltage

Comparators operate with the same supply voltages used by op amps. Many older comparators require bipolar (e.g., ±15V) or unipolar supply voltages as high as 36V. These supply voltages are still used in industrial applications.

For most new applications, however, the comparator operates within the range of low unipolar voltages typically found in battery-operated devices. Modern applications for comparators require low current consumption, small packages, and (in some cases) a shutdown function. The MAX919, MAX9119, and MAX9019 comparators, for example, work with voltages from 1.6V or 1.8V to 5.5V, draw a maximum of 1.2µA/1.5µA over the entire temperature range, and are available in a SOT23 and SC70 packages. The MAX965 and MAX9100 families of comparators operate with supply voltages as low as 1.6V and 1.0V, respectively. See Table 1.

Table 1. MAX9015-MAX9020 Selection Guide

Part Comparator(s) Int. Reference (V) Output Supply Current (µA)

MAX9015A 1 1.236, ±1% Push-pull 1

MAX9016A 1 1.236, ±1% Open drain 1

MAX9017A 2 1.236, ±1% Push-pull 1.2

MAX9017B 2 1.24, ±1.75% Push-pull 1.2

MAX9018A 2 1.236, ±1% Open drain 1.2

MAX9018B 2 1.24, ±1.75% Open drain 1.2

MAX9019 2 - Push-pull 0.85

MAX90120 2 - Open drain 0.85

Comparators in Tiny Packages

Nano-powered comparators in space-saving chip-scale packages (UCSP) with a low 1µA supply current, such as the MAX9025-MAX9098 families, are ideal for ultra-low-power system applications. Available in small 5-pin SC70 packages, the MAX9117-MAX9120 single-comparator families feature an ultra-low 600nA supply current with two outputs from which to select, push-pull or open-drain. See Table 2. These comparators are ideal for all 2-cell battery-monitoring/management applications.

Table 2. Tiny Space-Saving Comparators

Package Part Comparator(s) Int. Reference Output Supply Current (µA)

6-UCSP MAX9025 1 Push-pull 1.0

6-UCSP MAX9026 1 Open drain 1.0

6-UCSP MAX9027 1 Push-pull 0.6

6-UCSP MAX9028 1 Open drain 0.6

5-SC70 MAX9117 1 Push-pull 0.6

5-SC70 MAX9118 1 Open drain 0.6

5-SC70 MAX9119 1 Push-pull 0.35

5-SC70 MAX9120 1 Open drain 0.35

Basic Comparator Features

A comparator normally changes its output state when the voltage between its inputs crosses through approximately zero volts. Small voltage fluctuations, always present on the inputs, produce very small voltage differences. When the voltage difference is near zero volts, it can cause undesirable changes in the comparator's output state . To prevent this output oscillation, a small hysteresis of a few millivolts is integrated into many modern comparators. In place of one switching point, hysteresis introduces two: one for rising voltages, and one for falling voltages (Figure 1). The difference between the higher-level trip value (VTRIP+) and the lower-level trip value (VTRIP-) equals the hysteresis voltage (VHYST). For comparators with hysteresis, the offset voltage (VOS) is simply the mean value of VTRIP+ and VTRIP-.

Figure 1. Switch thresholds, hysteresis, and offset voltage.

For comparators without hysteresis, the voltage difference between the inputs needed to switch the comparator is the offset voltage, rather than the zero voltage required by an ideal comparator. However, the offset voltage (and, consequently, the switching voltage) changes with temperature and supply voltage. One measurement of that dependence is

the power-supply rejection ratio (PSRR), which shows the relationship between a change in the nominal supply voltage and the resulting change in offset voltage.

The inputs of an ideal comparator exhibit infinitely high input resistance, and thus no current flows into its inputs. For actual comparators, however, the currents that flow into their inputs also flow through the internal resistance of any voltage source that is attached to them, thus generating an error voltage. Bias current (IBIAS) is defined as the median value of the two comparator-input currents. For the MAX917 and MAX9117 comparator families, for example, the maximum IBIAS current is 2nA over the entire temperature range, and less than 1nA at room temperatures, TA = +25°C. See Table 3.

Table 3. Low IBIAS

Part IBIAS

MAX9025—MAX90281nA (max) @ TA = +25°C2nA (max) @ TA = TMIN to TMAX

MAX9117—MAX91201nA (max) @ TA = +25°C2nA (max) @ TA = TMIN to TMAX

MAX9171nA (max) @ TA = +25°C2nA (max) @ TA = TMIN to TMAX

As lower supply voltages become common, Maxim expanded the input-voltage range of comparators beyond the supply voltages. Some Maxim comparators employ the parallel switching of two npn/pnp input stages, which has allowed input voltages as high as 250mV beyond each supply rail. Such devices are called Beyond-the-Rail comparators. The range of input common-mode voltages available can be found in the comparator's data sheet.

Comparator Outputs

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.

One criterion for selecting a comparator is the time its output takes to alter its state after a signal has been applied at its input. This propagation time must account for propagation delay through the component and rise/fall times in the output driver as well. A very fast comparator like the MAX961, and MAX9010-MAX9013, for example, has a typical propagation delay of only 4.5ns or 5ns, and a rise time of 2.3ns and 3ns, respectively. (Remember that the propagation delay measurement includes a portion of the rise time). One should note the different influences that affect propagation time (Figure 2). These factors include temperature, load capacitance, and voltage drive in excess of the switching threshold (input

overdrive). Propagation time is called tPD- for the inverting input, and tPD+ for the noninverting input. The difference between tPD+ and tPD- is called skew. Supply voltage also has a strong effect on propagation time.

Figure 2. The effect of external influences on propagation time.

For a given application, select either a comparator with high speed or one that saves power. Maxim offers a range of performance for this purpose: from the MAX919 (800nA, 30µs) to the MAX9075 (6µA, 540ns); from the MAX998 (600µA, 20ns) to the MAX961 (11mA, 4.5ns); and from the MAX9107 (350µA, 25ns) to the MAX9010 (900µA, 5ns). The recent MAX9010 (in a SC70 package) represents a useful compromise in these parameters, with a 5ns propagation time and 900µA supply current.

For ultra-high-speed ECL and PECL outputs with 500ps propagation delay, refer to the MAX9600/MAX9601/MAX9602 part families.

Comments about Particular Comparators

The most frequent application for comparators is the comparison between a voltage and a stable reference. Maxim offers various comparators in which a reference voltage is integrated on 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. The MAX9117 device family, for example, requires only 1.3µA maximum (including reference) over the entire temperature range. The precision of an integrated reference typically ranges from 1% to 4%. For high accuracy, however, references in the MAX9040 family of comparators offer 0.4% initial accuracy and a maximum 30ppm/°C temperature drift.

The MAX9017/MAX9018, MAX923, and MAX933 dual comparators and the open-drain-output MAX973 and MAX983 dual comparators are ideally suited for window-comparator applications. Because the integrated reference within all four of these devices can connect to the comparator's inverting or noninverting input, overvoltage and undervoltage thresholds can be implemented with just three external resistors. These components also provide a hysteresis

pin. By adding two additional external resistors, this pin allows the addition of a hysteresis threshold, as shown in Figure 1. Some comparators such as the MAX912/913 offer complementary outputs - i.e., two outputs that transition in the opposite direction of each other for a change of relative input polarity.

Fast propagation delay (1ms typically at 5mV overdrive) makes the MAX9201/MAX9203 ideal for fast ADCs and sampling circuits like receivers, V/F converters, and many other data-discriminating applications.

Other high-speed, low-power comparators like the MAX9107/MAX9108/MAX9109 are low-cost upgrades to the industry-standard comparators, MAX907/MAX908/MAX909. The dual comparator, MAX9107, is offered in a space-saving 8-pin SOT23 package. The single comparator, MAX9109, is available in a tiny 6-pin SC70, while the quad comparator, MAX9108, is offered in a 14-pin TSSOP. See Table 4 and Figure 3.

Table 4. Ultra-Fast Comparators

Speed (ns) Part Comparator(s) Supply Current (A) Package

4.5 MAX999 1 5m 5-SOT23

4.5 MAX962 2 5m 8-µMAX

5 MAX9010 1 0.9m 6-SC70

5 MAX9011 1 0.9m 6-SOT23

5 MAX9012 2 0.9m 8-µMAX

5 MAX9013 1 0.9m 8-µMAX

7 MAX9201 4 4.7m 16-TSSOP

7 MAX9202 2 2.5m 14-TSSOP

7 MAX9203 1 1.3m 8-SOT23

8 MAX900 4 2.5m 20-SO

8 MAX901 4 2.5m 16-SO

8 MAX902 2 2.5m 14-SO

8 MAX903 1 2.5m 8-SO

10 MAX912 2 6m 16-SO

10 MAX913 1 6m 8-µMAX

25 MAX9107 2 350µ 8-SOT23

25 MAX9108 4 350µ 14-TSSOP

25 MAX9109 1 350µ 6-SC70

40 MAX9140 1 150µ 5-SC70

40 MAX9141 1 165µ 8-SOT23

40 MAX9142 2 150µ 8-SOT23

40 MAX9144 4 150µ 14-TSSOP

40 MAX907 2 700µ 8-SO

40 MAX908 4 700µ 14-SO

Figure 3. Illustration of the best speed/power choices for a comparator in an SC70 package.

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 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 +/-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.

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.

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.

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.

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.

Some other applications of comparator

This section introduces three applications that require comparators.

The first example application is a level shifter from 3V logic to 5V logic. As shown in Figure 4, this circuit requires only a single comparator with an open-drain output as in the MAX986. The circuit provides great flexibility in choosing the voltages to be translated. It also allows the translation of bipolar ±5V logic to unipolar 3V logic by using the MAX972. In that application, take care that no voltage exceeds the maximum voltage allowed on any pin and that the current into the output is limited by a sufficiently large-valued pull-up resistor (refer to the MAX986's Absolute Maximum Ratings in its data sheet).

Figure 4. Level translation from 3V to 5V logic.

The circuit of Figure 5 solves another frequently encountered problem. Configured as shown, a single unipolar comparator converts a bipolar input signal (a sine wave in this case) to a unipolar digital output signal. The required offset voltage is calculated as:

Figure 5. Unipolar comparator with bipolar input signal.

As shown above in Figure 5, two equal-valued resistors (labeled R4) establish the comparator's trip threshold at half the supply voltage. In the circuit of Figure 6, four

comparator outputs form a thermometer gauge indicating one of four ranges for the input-current level. The shunt resistor converts the input current to a voltage, and resistors R1 and R2 set the op-amp gain as required for the desired level of reference voltage. Resistors R4 to R7 denote thresholds for the desired digital outputs.

Resolving a current measurement into one of four ranges.

A similar version of this article appeared in the July 1, 2001 issue of ECN magazine.

The Schmitt Trigger

The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point.

The Schmitt Trigger

Schmitt trigger action is a double threshold comparator process. The current equation at A gives:

where

This dependence upon the output voltage gives the dual threshold. The two output states give the thresholds shown at right.

ConclusionIn some applications where a comparator is used to square off high-speed sinusoidal signals in order to generate a clock signal, it is important to know the output jitter specification of the comparator. This application note has shown how to extrapolate the output jitter measurement of the MAX999 even in the presence of a non ideal source generator. The limitation of this measurement and its sources of error have been discussed. Finally, the output jitter has been correlated with the input-referred voltage noise.

Reference

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