The previous article in this series (Part 16, link at end) introduced the voltage comparator. One issue with comparator applications is caused when a slow-moving signal is applied to the input. As the applied signal crosses the switch point, even a small level of voltage noise riding on the signal can cause the comparator to switch. The comparator may switch several times as the noisy applied signal crosses the switch-point voltage, Figure 1.

Figure 1: Comparator chatter

When the comparator is applied to a circuit such as a high-speed counter, this causes many false counts for one transition. For comparators with a push-pull output stage, one solution is hysteresis, Figure 2.

Figure 2: Hysteresis network

Analysis of this circuit is simplified by the fact that the output has only two states: V out low (Vol) or V out hi (Voh). In this circuit, Voh is equal to the positive supply voltage and Vol is ground or zero. The transfer function is shown in Figure 3. Notice that the switch point depends on the state of the output just before the change of state.

F igure 3: Transfer function of a comparator with hysteresis

For these two cases. the current through the feedback circuit can be calculated by:

and

The comparator switching voltage for each case is:

and

Combining terms yields:

and

The total hysteresis voltage is the difference between the two levels. With Vol equal GND this reduces to:

This is the result for the inverting-comparator configuration. Notice that the comparator switch points (Vsl and Vsh) are dependent on the power-supply voltage as well as the reference voltage. When Vref is half of the supply voltage, the switch points will be symmetric about Vref.

If Vref is adjusted toward Voh, the voltage from Vref to Vsh will be less than the voltage from Vref to Vol. This causes the mid-point between the switch voltages to differ from Vref. When Vref is below Vref, the opposite occurs.

By interchanging the signal and reference, a non-inverting comparator configuration can be built. This, however, compromises the high input impedance presented to the signal by the inverting input, a factor which may be critical to a design. The comparator switch points are given by:

and

This analysis is for comparators with a push-pull output stage. For those comparators with an open-collector or open-drain output stage, the pull-up resistor and the load resistor form a voltage divider which sets the Voh as a portion of the supply voltage, Figure 4.

igure 4: Open-collector comparator output considerations

R(pull-up) must be small compared to R1+R2, so it does not limit the current in that part of the circuit.

Note that there are devices available on the market which have the hysteresis feature included on the chip.

About the author



William P. (Bill) Klein is a Senior Applications Engineer with the High Performance Analog group at Texas Instruments. Bill joined TI through its acquisition of Burr-Brown in August 2000. His experience as an analog circuit designer covers over 40 years in fields ranging from mineral exploration to medical nuclear imaging. One current role Bill has is hosting the Analog e-LAB Web Cast, presenting real world solutions to real world problems in analog circuit design. In addition to a BSEE from Arizona State University and registration as a Professional Engineer in the State of Arizona, he has authored numerous magazine articles, application notes and conference papers.

Previous installments of this series:

• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208403856, click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter—dynamic parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=208401183, click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter—static parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=207800114, click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", www.planetanalog.com/features/showArticle.jhtml;?articleID=207001505, click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504289, click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
• "SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here