Circuit uses two reference voltages to improve hysteresis accuracy
Increase the hystereis of your comparator by switching between two reference voltages.
Marián Stofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Martin Rowe and Fran Granville -- EDN, January 7, 2010
In advanced IC comparators, programmable hysteresis eliminates 0V-centered differential-input-voltage noise (Reference 1) and improves the comparator’s response if its differential-input voltage is low or varies slowly over time. For example, the ADCMP609 comparator from Analog Devices lets you program its hysteresis from 0 to 160 mV with a single resistor that connects between the HYS (hysteresis) pin and ground. That voltage range may be too narrow for some applications, however. The circuit in Figure 1 lets you widen the hysteresis by using two reference voltages applied to the noninverting input. The circuit uses IC3, an Analog Devices ADR390B, to generate a high reference voltage, VREFH, of 2.048V. Resistor divider R1/R2 produces a low reference voltage, VREFL, of 0.2048V, or a difference of 1.8432V. Thus, the hysteresis equals the high reference voltage minus the low reference voltage. IC2, an Analog Devices ADG772 dual-SPDT (single-pole/double-throw) switch, routes the voltages to the comparator’s noninverting input (Reference 2).
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Whenever the input voltage at the comparator’s inverting input exceeds the high reference voltage, the output goes to a logic low. That action forces S1B to connect the low reference voltage to the comparator’s noninverting input. The circuit remains in that state until the input voltage drops below the low reference voltage. At that point, the switch connects the high reference voltage back to the comparator. For fast-ramping waveforms at the input, the hysteresis increases because of signal-propagation delays in both IC1 and IC2. The 35-nsec propagation delay in the ADCMP609 occurs at approximately 10 mV of input overdrive, and this overdrive roughly doubles in effect as an addition to the hysteresis voltage, increasing it by approximately 1%.
Because ADCMP60x comparators are rail-to-rail I/O devices, the low reference voltage could be 0V. In this case, however, the value of 0.2048V lets the comparator cooperate with other rail-to-rail I/O ICs from the same supply voltage. The outputs of these ICs can swing between 0V and the power-supply voltage with a margin of millivolts to tens of millivolts, depending on the load. An ADR390B provides the reference voltages (Reference 3). The 2.2-nF decoupling capacitors suppress variations of these voltages during voltage transitions at the Q output. These values are sufficient because the parasitic-charge transfer from the switch’s control input, IN1, to the respective channel’s source electrode is typically 0.5 pC. The short-term variation of the decoupled reference voltages is less than 250 µV.
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Firstly, I would like to point, who Rube Goldberg was. It might be useful for those, who considered this name to belong to famous series of Schmitt, Eccles-Jordan, Darlington and others.
Reuben Lucius Goldberg (July 4, 1883 – December 7, 1970) was a Jewish American cartoonist, sculptor, author, engineer, and inventor. Goldberg is best known for a series of popular cartoons he created depicting complex devices that perform simple tasks in indirect, convoluted ways – now known as Rube Goldberg machines (from Wikipedia).
Now to the circuit described in the DI.
For those, who understand a comparator as any off-the-shelf functional unit; as cheap as possible, comprising lowest count of elements; for those a classical solution with a resistive positive feedback will do. If, however, a high accuracy of both thresholds is required, then these thresholds can not be derived from output of the IC directly, as the saturated-output voltage-level is a subject of changes with temperature, load and even duty-cycle of operation. Also the dynamic performance of the circuit proposed is much better. In the comparator with a simple positive resistive feedback, the parasitic input capacitance of the IC is charged through the feedback resistor, thus increasing delay and rise- and fall-time of output as well.
Marian Stofka - 2010-1-6 07:40:46 PDT -
Interesting circuit, but why go to all the trouble of an active reference voltage, resistive dividers, and the switch to replace the three resistors used to create a bias and Schmitt trigger function on the positive input of the comparator. Three resistors must be significantly less costly and complex than the Rube Goldberg circuit shown. It only takes two resistors for a non-inverting, but low input impedance solution. Maybe I'm just a little old but this has been around for decades in a simplified form.
wikipedia.org/wiki/Schmitt_trigger
Chris - 2010-8-1 13:37:00 PST -
Are Design Idea's Not avaialble in pdf form any more?
Ben Wolodko - 2010-7-1 13:42:00 PST
