Schmitt Trigger

Another circuit that is often used to remove contact bounce noise is the Schmitt trigger, which is shown in Fig. 21.3. This has nicely adjustable hysteresis, without requiring two kinds of transistors. (Note that electronics designers often refer to transistors as Ql, Q2, etc.) Initially, Q2 is on and Q1 is off When Ql is turned on by raising the pot setting (simulating an incoming 

Once Ql is on, it tends to short-circuit the voltage on the base of Q2, so the 5K variable pot now has to be turned way down to the point where the resistance of Ql goes up, which can then drive current into the base of Q2. (If a 56K resistor is substituted instead of the lOOK, then Q2 would not go on until an even lower setting of the variable pot is reached, giving more hysteresis.) 

The capacitor does not limit the time that Q2 is on, because current can flow around it ( bypass it) through the 2.2K. The purpose of the capacitor is, instead, to speed up the switching action. 

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One of the more useful functions of the DC Sweep is to plot transfer curves. A transfer curve usually plots an input versus an output. A DC transfer curve plots an input versus an output, assuming all capacitors are open circuits and all inductors are short circuits. In a DC Sweep, all capacitors are replaced by open circuits and all inductors are replaced by short circuits. Thus the DC Sweep is ideal for DC transfer curves. The Transient Analysis can also be used for DC transfer curves, but you must run the analysis with low-frequency waveforms to eliminate the effects of capacitance and inductance. Usually a DC Sweep works better for a transfer curve. The one place where a transient analysis works better is plotting a hysteresis curve for a Schmitt Trigger. For a Schmitt Trigger, the input must go from positive to negative, and then from negative to positive to trace out the entire hysteresis loop. This is not possible with a DC Sweep. 

In this section we will use an operational amplifier to create a Schmitt Trigger. A non-ideal operational amplifier must be used because the ideal op-amp model has trouble converging when it is used as a Schmitt Trigger. Wire the circuit ... 

This op-amp circuit is a Schmitt Trigger with trigger points at approximately 7.5 V. A sinusoidal voltage source will be used to swing the input from +14 V to -14 V and from -14 V to +14 V a few times. The frequency of the source is 1 Hz. This low frequency is chosen to eliminate the effects of the op-amp slew rate on the Schmitt Trigger performance. If you... 

We can see that the Schmitt Trigger changes state at approximately VIN = 7.2 V. 

The next thing we would like to do is plot the hysteresis curve for this Schmitt Trigger. The hysteresis curve is a plot of V0 versus V N. We must delete the trace Vfl/IN/. Click the LEFT mouse button on the text V(VlNf. It should turn red, indicating that it has been selected. To delete the trace, press the DELETE key. You should have the Probe screen shown... 

EXEHCI5E 6-17 Run a non-inverting Schmitt Trigger with a 14 volt sine wave input. 

Like the Wein-bridge oscillator, the Schmitt trigger only needs a power supply of 5 to 15V to begin its oscillation. This entails that the maximum lead that the clock pulse can drive is in the 1 mA range. The oscillation is controlled by the RC time constant and the hysteretic native to the Schmitt trigger. 

The most important characteristics of a good model are the positive and negative hysteresis points. Unfortunately, manufactures of the Schmitt trigger give a loose specification as to what these points are. Table 8.5 shows data taken from Radiation Hardened Product Databook (Harris Semiconductor 1993) on the CD4093BMS NAND Schmitt trigger. 

The IsSpice model shows that the ramp voltage peaks at 3.7 V and has a minimum of 1.75 V. The maximum is below the extreme high specification of 4 V, and the minimum is above the extreme minimum specification of 1.4 V. This seems to line up pretty close to the data sheet, but the model s hysteresis voltage is 1.95 V, and the specified maximum is only 1.6 V. The IsSpice version of the Schmitt trigger would work for some applications where the hysteresis voltage is not so critical, but for this application, the large hysteresis value caused a much lower frequency then expected. The IsSpice model s frequency is 5.29 kHz. 

TABLE 8.5 CD4093BMS NAND Schmitt Trigger Specifications... 

In PSpice both a 7414 and a CD4093 were used. The 7414 is the digital model for the Schmitt trigger inverter. To use the digital device for an analog measurement, E sources (voltage-controlled voltage sources) were used as buffers. The schematic used for the PSpice model is shown in Fig. 8.59. The results are displayed in Fig. 8.60. 

Figure 8.60 PSpice results of Schmitt trigger oscillator using a 7414.

V limits. PSpice s and Micro-Cap s models of the Schmitt trigger are clearly valid for the parameters evaluated in this comparison. 

Figure 8.62 Circuit board data from Schmitt trigger oscillator.

Voltage Comparators and Schmitt Triggers, The voltage comparator is an analog device with two inputs and an output. It is generally used to compare one voltage... 

A device somewhat similar to a comparator is the Schmitt trigger Figure... 

Figure 23.27. Schmitt trigger. From S. P. Perone and D. O. Jones, Digital Computers in Scientific Instrumentation, New York McGraw-Hill, 1973, by permission of the publisher. Copyright 1973 by McGraw-Hill, Inc.

The difference between the upper threshold and lower threshold is commonly called the hysteresis or backlash level of the Schmitt trigger it provides noise immunity for the device. 

Electrical contacts in relays and switches often bounce several times when making contact ("closing"), instead of just smoothly closing once. This can cause the electricity to start and then stop quickly, which can enter digital data the wrong number of times. A "debouncer" circuit (such as a Schmitt trigger) can prevent the multiplicity of startings. 

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