Comparators don't get no respect, or at least they may be underrated and under utilized. The reason may simply be because its more famous sibling, the op amp, usually gets the spotlight.
“In large measure, the versatility of the op amp allows it to dominate the analog design world,” said Jim Williams, Staff Scientist for Linear Technology. “Comparators are frequently perceived as devices that crudely express analog signals in digital form ” a 1-bit A/D converter. This is true but is constrictive because they do more than just compare signals.”
A typical list of applications for comparators could include clock generation and regeneration, signal conditioning, threshold detection, pulse processing, and even automated test. This little underrated mighty mite is also used for many other applications such as voltage monitoring, voltage level translation, and voltage to frequency conversion. You can find it being used in sample/track and hold circuits, zero crossing detectors, pulse width modulators, line receivers, oscillators, Schmitt triggers, peak detectors, and even delay lines. Typical system applications for comparators included portable and battery-powered systems, scanners, set top boxes, and high-speed differential line receivers. So, as you can see it's extremely useful and versatile.
Currently, comparators are finding their way into many new applications. For example, high-speed data communications with sub 10 nano second (ns) propagation delay requirements need comparators and so do ultra-low power devices such as hand-held medical equipment that draw sub 10 micro amp current.
Analog Devices (ADI) has high-speed comparators that offer performance up to 150 pico second (ps) propagation delay, greater than 8-GHz input equivalent bandwidth, 500 femto second (fs) jitter and less than15 ps of overdrive and slew rate dispersion. The AC performance makes ADI's new line of high-speed comparators ideal for a wide variety of demanding applications ranging from high-speed triggers to clock generation and recovery. These devices feature true differential inputs making them ideal for line receiver applications. Differential latches are included and many feature programmable hysteresis. Analog Devices says that it offers exceptionally low random and deterministic jitter, world class dispersion and overdrive performance, fast propagation delay to meet the needs of the new applications. How are these performances met? The company's SiGe bipolar process and sophisticated design techniques are the reasons given.
The low power comparators are ideal for power sensitive applications such as battery powered equipment. This type comparator offers features such as power supply currents down to 5 micro amps, and internal references and outputs with programmable delays.
To meet some of the low-power application needs Linear Technology Corp. (LTC) offers the micro power, low voltage LT6700. It has low power down to 6.5 micro amps, and is available in a small outline transistor (SOT) -23 package type. It allows input and output common-mode voltages to exceed the supply voltage (good for harsh automotive environments), 400 mV trimmed on-chip references, and multiple input polarity combinations.
National Semiconductor says that faster response time or propagation delay, low power, precision or low offset voltage and shut down are also needed for new applications. The company's LMV761 and 762 are precision comparators intended for applications requiring low noise and low input offset voltage. The LV761 single has a shutdown pin that can be used to disable the device and reduce the supply current. It is available in a space saving SOT23-6 or small outline IC (SOIC) -8 packages. The LMV762 dual is available in SOIC-8 or micro small outline package (MSOP) -8 packages. Both comparators feature a CMOS input and push-pull output stage. The push-pull output stage eliminates the need for an external pull-up resistor. The input offset voltage has a typical value of 200 micro volts at room temperature and a 1-mV limit for over temperature.
Comparators also are used in digital systems to recover clock timing signals and for high-speed square waves transmitted over a distance as small as tens of centimeters because they can become distorted due to stray capacitance and inductance. Poor layout or improper termination can also cause reflections on the transmission line, further distorting the signal waveform. A high speed comparator can be used to recover the distorted waveform while maintaining a minimum of delay.
AMI Semiconductor introduced the CMP001 to meet the needs of today's new designs. It has an output response time that ranges from 0.5 micro seconds to 1.5 micro seconds with a 20 mV overdrive. Its bias startup time from power on is a mere 1 micro second.
Texas Instruments offers the TLV3011 for low-power needs, and has an open-drain output with an uncommitted on-chip voltage reference. The TLV3011 features 5 micro amps quiescent current, the input common-mode range is 200mV beyond the supply rails, and single-supply operation from 1.8V to 5.5V. The integrated 1.242V series voltage reference is stable with up to 10nF capacitive load, and can provide up to 0.5mA output current. The TLV3011 is currently available in the SOT23-6 package with single chip (SC) -70 packages available Q1 2004.
The ADCMP565 from ADI is an ultra-fast voltage comparator fabricated using the company's proprietary XFCB process. The device features 300 ps propagation delay with less than 50 ps overdrive dispersion. Overdrive dispersion, a particularly important characteristic of high-speed comparators, is a measure of the difference in propagation delay under differing overdrive conditions. A fast, high-precision differential input stage permits consistent propagation delay with a wide variety of signals in the common-mode range from -2.0 V to +3.0 V. Outputs are complementary digital signals fully compatible with ECL 10 K and 10 KH logic families. The outputs provide sufficient drive current to directly drive transmission lines terminated in 50 ohms to -2 V. A latch input is included that permits tracking, track-and-hold, or sample-and-hold modes of operation. The ADCMP565 is available in a 20-lead plastic leaded chip carrier (PLCC) package.
The ADCMP565 comparator is a very high speed device and consequently, high-speed design techniques must be employed to achieve the best performance. ADI says that the most critical aspect for using the ADCMP565 in a design is the use of a low impedance ground plane. A ground plane, as part of a multilayer board, is recommended for proper high-speed performance. Using a continuous conductive plane over the surface of the circuit board can create this, allowing breaks in the plane only for necessary signal paths. The ground plane provides a low inductance ground, eliminating any potential differences at singular ground points, which can be caused by ground bounce. A proper ground plane also minimizes the effects of stray capacitance on the circuit board.
It is also important to provide bypass capacitors for the power supply in a high speed application. A 1 micro Farad electrolytic bypass capacitor should be placed within 0.5 inches of each power supply pin to ground. These capacitors will reduce any potential voltage ripples from the power supply. In addition, a 10 nF ceramic capacitor should be placed as close as possible from the power supply pins on the comparator to ground. These capacitors act as a charge reservoir for the device during high frequency switching.
Match Specs to Applications
Now you know what applications comparators are used for and what new comparators are available for these designs. But, what comparator specs should designers look for and what ranges are acceptable for different applications?
National Semi says that for low power the minimum operation voltage should be 1.8V, 2.5V or 2.7V. For propagation delay it depends on the application but you should look out for the input over-drive. Typically the delay should fall in a range from about 7 ns to the microsecond range. As for precision, the offset voltage should be in the uV to mV range.
Linear Technology's Erik Soule, Signal Conditioning Market Manager, says you need to decide on the comparator toggle rate or propagation delay, the acceptable power consumption, and the appropriate power supply range before you dig into the details. You also may want to determine if there is an application specific comparator available that meets your needs. For example, the LTC1921 is optimized for “48V telecom supply monitoring, with preset under/over voltage thresholds to meet relevant standards.
The LTC1998 is designed for supply monitoring of Lithium-ion battery-powered devices. The new LT6700 is a dual comparator with on-chip reference, and versions of the LT6700 are available with three different input polarity combinations to minimize the number of external components. Additionally, check if hysteresis is pre-set (internal) or adjustable. You can always add more by adding positive feedback. Finally, you should determine if the outputs are latched or standard, if they are open collector/drain or push/pull types, and if you need rail-to-rail operation.
ADI's Carlton Lane, Product Line Manager for Comparators, says you should look for the lowest RMS jitter spec, the minimum dispersion and a variation of the propagation delay with the varying input conditions, such as overdrive and slew rate to get superior AC timing accuracy at high speeds. Additionally, Mr. Lane says “don't forget the capability for programmable hysteresis.”
What about designer gotchas? What should designers be mindful of when designing with a comparator? National Semi says one of the most important points is to be sure you eliminate comparator chatter by adding hysteresis. While ADI says the inputs need to be isolated from outputs as effectively as possible.
“Any parasitic coupling from the output back to the input can cause instability when overdrive conditions are low,” said ADI's Carlton Lane. Analog Devices offers programmable hysteresis because it is impossible to completely isolate input from outputs. “Programmable hysteresis can be used very effectively to reduce or prevent instability,” said Lane. “Also, high speed comparators must have clean 50 ohm environments with proper termination to avoid waveform distortion prior to the inputs and to fully utilize the AC timing accuracy of these comparators,” concluded Mr. Lane.
“You should ensure that the pin out configuration is optimized for high-speed layouts,” said LTC's Erik Soule. “If speed is everything, then pick the supply voltages for which the device will have the lowest propagation delay/toggle rate. You also need to answer other important questions about your design including; is external input protection required or is the device designed to handle worst case input swing? You also need to know if special power supply sequencing is required if you have separate input and output supply device. Next, you need to know what type of supply bypassing is required to keep the circuit stable. Finally, you should ensure the comparator specs are guaranteed over the desired operating range.”
These observations may seem cumbersome but they will certainly help you get your design right the first time and maybe you will find that you use them in a new application.
Company Contact List
Analog Devices, Inc.
Tel: 800-ANALOGD (262-5643)
Linear Technology Corp.
National Semiconductor Corp.
Texas Instruments Inc.
Tel: 800-477-8924, ext. 4500