18. The Comparator, Positive Feedback and Schmitt Trigger

18.1. Objective

The objective of this activity is to investigate the voltage comparator, the use of positive feedback and the operation of the Schmitt Trigger configuration. The use of conventional operational amplifiers as a substitute for voltage comparators will described in basic op amp circuits section.

18.2. Notes

In this tutorials we use the terminology taken from the user manual when referring to the connections to the Red Pitaya STEMlab board hardware. Extension connector pins used for 5V voltage supply are show in the documentation here. Oscilloscope & Signal generator application is used for generating and observing signals on the circuit.

18.3. Background

18.3.1. The Voltage Comparator

A Differential Voltage Comparator such as the AD8561 from the analog parts kit has a pinout similar in many ways to that of a conventional opamp but with many important differences (figure 1). There are the usual \(V_+\) and \(V_-\) power supply pins but a comparator will also have a ground (GND) pin as well. The differential \(+IN\) and \(-IN\) pins are essentially the same as a conventional op-amp. There will also be an output pin as in an opamp but there is often a second “inverting” ( or complementary ) output. Also, while the voltage at the output of an opamp can generally swing close to the \(+\) and \(-\) supply rails, the output of a comparator will swing only between ground(gnd) and the \(+\) supply. This makes the output more like a digital signal and compatible with standard logic gates such as TTL or CMOS. The voltage comparator can be thought of as a single bit analog-to-digital converter (ADC). The AD8561 also includes a LATCH input which will latch or freeze the output and prevent it from changing even if the inputs change.

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Figure 1: AD8561 datasheet and pin assignments

18.4. Materials

  • Red Pitaya STEMlab
  • OPAMP: 1x AD8561 voltage comparator
  • Voltage regulator: 1x LM317
  • Resistor: 2x 4.7 \(k \Omega\)
  • Resistor: 1x 20 \(k \Omega\)
  • Resistor: 1x 47 \(k \Omega\)
  • Resistor: 1x 100 \(k \Omega\)
  • Capacitor: 1x 0.1 \(\mu F\)

18.5. Directions

Construct the comparator test circuit as shown in figure 2 on your solder-less breadboard. The two 4.7 kΩ pull-up resistors are optional and are used to increase the peak positive output swing to closer to the +5 V supply.

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Figure 2: AD8561 comparator circuit

Note

Voltage Comparators are extremely sensitive to the noise and glitches on the power supply rail. Noisy power supply rail will cause glitches on the output signal. This glitches will be present at switching threshold voltages. In other words, comparator will have some trouble deciding to switch on V+ or to V- when comparing two input signals affected by the power supply noise. Because of that here we use a voltage regulator to stabilize our power supply rail and prevent noisy output from the comparator. You can try directly using 5V power supply rail and observe the results and compare them with the results obtained using voltage regulator. Note: It is not necessary to drop down voltage from 5V to 2.5V but we chose that just form simplicity.

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Figure 3: AD8561 comparator circuit on the breadboard

LM317 Voltage regulator is described in previous section.

18.6. Procedure

  1. Set Oscilloscope probes attenuation; IN1 to x1 and IN2 to x10
  2. Start the Oscilloscope & Signal Generator application.
  3. To apply input voltage \(V_{in}\) in the OUT1 settings menu set Amplitude value to 0.5V and DC offset to 0.5V. From the waveform menu select TRIANGLE, deselect SHOW button and select enable.
  4. In the OUT2 settings menu set Amplitude value to 0.5V, from the waveform menu select DC and select enable.
  5. In the IN2 settings menu set probe attenuation to x10
  6. On the left bottom of the screen be sure that IN1 V/div is set to 500mV/div (You can set V/div by selecting the desired channel and using vertical +/- controls)
  7. On the left bottom of the screen be sure that IN2 V/div is set to 1V/div (You can set V/div by selecting the desired channel and using vertical +/- controls)
  8. On the left bottom of the screen be sure that OUT2 V/div is set to 500mV/div (You can set V/div by selecting the desired channel and using vertical +/- controls)
  9. Set t/div value to 200us/div (You can set t/div using horizontal +/- controls)
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Figure 4: AD8561 comparator circuit measurements

You should see a square wave that is high ( near +2.5 V ) when the input signal level is a greater than 0.5 V (OUT2 DC value) and low ( near 0 V ) when the input signal is less than 0.5 V. Note the levels of the input triangle wave where the output changes from low to high and from high to low.

Now connect Channel IN1 (set probe attenuation x10 and IN1 settings menu set probe attenuation to x10) to the inverting output ( pin 8 ). You should again see a square wave but with opposite phase to pin 7(IN2). Also change DC level of OUT2 (set amplitude to 0.7V) - this will change switching level of the voltage comparator causing different times duration of HIGH and LOW states of the comparator output. You should again see two square waves with opposite phases but now with opposite HIGH and LOW time durations.

  1. Set Oscilloscope probes attenuation; IN1 to x10 and IN2 to x10
  2. In the OUT2 settings menu set Amplitude value to 0.7V, from the waveform menu select DC and select enable.
  3. In the IN2 and IN2 settings menu set probe attenuation to x10 and offset level -1700mV

Note

From description above you can maybe see how to make an PWM (pulse width modulation) signal using constant frequency triangle signal and changeable DC \(V_{ref}\) value.

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Figure 5: AD8561 both output measurements at different \(V_{ref}=0.7V\)

Zoom into the falling edge of the output (IN2) square wave by adjusting the Horizontal position and time per division settings such that the falling edge is centered on the time axis and the time per div is small enough to see the transition time of the edge (5 uS/div). You should see that the output does not go from the high output level all the way to the low output level all at once but stops part way and spends some time at an intermediate level before continuing the rest of the way to the low output level. You should should also see this delay when transitioning from low to high (IN1). This delay is caused by noise as the input signal slowly passes through the input threshold level ( 0.7 Volts in this case ) and can cause problems. This is the reason why it is good to have low noise power supply and low noise input signals on voltage comparator. Try to repeat switching noise measurement at more noisy power supply (5V pin directly form STEMlab board)

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Figure 6: Switching noise measurements.

Note

Usually our intuition is to correlate high possibility of noise issues with high frequency signals. In case of voltage comparator this is not always true. If we increase OUT1 (\(V_{in}\)) frequency to 100kHz the switching noise will be much lower. Way is that? You may think like this: Voltage comparator has very sensitive inputs and it is constantly comparing values of \(V_{in}\) and \(V_{ref}\). Now let’s set \(V_{in}\) to be noiseless signal and \(V_{ref} = DC +(-) A_{noise}\). When triangle wave \(V_{in}\) signal is slowly approaching \(V_{ref}\) the voltage comparator will start switching and if the \(V_{ref}\) amplitude swingings around DC value by \(A_{noise}\) the comparator output will change states according to the \(V_{in} - (V_{ref} = DC +(-) A_{noise})\) ratio. So, as long \(V_{in}\) amplitude stays in the range of \(V_{ref} = DC +(-) A_{noise}\) value the comparator output will effectively switch on \(A_{noise}\) and not on the input signals. Once \(V_{in}\) goes below \(V_{ref} = DC - A_{noise}\) or above \(V_{ref} = DC + A_{noise}\) the comparator output will switch high or low but now on input signal values not on noise values. You can see that low frequency triangle wave \(V_{in}\) amplitude will spend more time near \(V_{ref} = DC +(-) A_{noise}\) causing voltage comparator to produce noisy output while high frequency triangle wave \(V_{in}\) amplitude will quickly pass by \(V_{ref} = DC +(-) A_{noise}\) range preventing voltage comparator to produce any noise switching.

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Figure 7: Switching event at high input signal frequency (100kHz)

18.7. Using positive feedback to add hysteresis: the Schmitt trigger

Along side low noise power supply a common solution to the problem just outlined is to add noise immunity to the comparator circuit by incorporating hysteresis into the transition threshold voltage \(V_{th}\), as shown in figure 8. By “hysteresis” we mean that the threshold voltage is a function of the system’s current operating state, which is defined for this circuit by its output voltage: positive or negative saturation. Because \(V_{th}\), the voltage at pin 2, is determined by the voltage divider constructed from resistors R1 and R2, it changes in response to a change in the output voltage: once the output has gone high in response to an input which has passed below the threshold voltage, the threshold voltage is changed to a higher value \(V_{th+}\) ( \(V_{ref}\) + a fraction of the output high voltage ); conversely, an input voltage climbing through \(V_{th+}\) will change the output to its low state and cause the threshold voltage to be set to a lower value \(V_{th-}\) ( \(V_{ref}\) - a fraction of the output low voltage).

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Figure 8: Schmitt trigger

This difference between \(V_{th+}\) and \(V_{th-}\) means that once a transition is triggered by a change in \(V_{in}\), noise excursions smaller than this difference on the input will not cause \(V_{in}\) to cross the hysteresis gap \(V_{hist} = V_{th+} - V_{th-}\) and cause an undesired reversal of the output state. If the hysteresis gap is made large enough, then the system can be made completely impervious to the noise on the input signal, eliminating the spurious output levels suffered by the basic comparator circuit (figure 1).

18.7.1. Calculating the threshold

Let’s call the maximum and minimum output voltages \(V_{high}\) and \(V_{low}\). The threshold voltage when the output is at \(V_{high}\) and at \(V_{low}\) is:

The resulting hysteresis gap for the circuit of figure 8 is given by:

\[ \begin{align}\begin{aligned}V_{th_{high}} = \frac{R_1}{R_1+R_2} (V_{high}+V_{ref})+V_{ref} \quad (1)\\.\\V_{th_{low}} = \frac{R_1}{R_1+R_2} (V_{low}-V_{ref})+V_{ref} \quad (2)\end{aligned}\end{align} \]

The resulting hysteresis gap for the circuit of figure 8 is given by:

\[V_{hist} = V_{th_{high}}-V_{th_{low}} = \frac{R_1}{R_1+R_2} (V_{high}-V_{low}) \quad (3)\]

For the AD8561 with a +2.5 V power supply and pull-up resistor, \(V_{high} - V_{low} \approx 2.3 V\). Because the other end of the voltage divider (bottom of R1) is connected to \(V_{ref} = 0.5 V\), the threshold voltages \(V_{th_{high}}\) and \(V_{th_{low}}\) will be centered around 0.5V (\(V_{ref}\)) assuming that \(V_{high}\) and \(V_{low}\) are more or less centered around 0.5 V). Connecting the bottom of R1 to a different voltage reference source rather than to mid supply will not affect the hysteresis gap, but it will center that gap around a threshold proportional to the new reference voltage. In fact the negative input pin of the comparator could be connected to the fixed reference voltage and the end of R1 considered as the input. This in effect reverses or inverts the sense of the two outputs. Above stated can be represented in Schmitt Hysteresis plot shown on figure 9.

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Figure 9: Schmitt Hysteresis

Note

Hysteresis gap equation places a potential restriction on the ratio R1/R2 for a Schmitt trigger: unless R1 < R2, the hysteresis gap will be larger than one half of the peak to peak output voltage swing range of the comparator and depending on the reference voltage value, one or the other of the Schmitt trigger thresholds might be beyond the range of the output voltage. Assuming the input signal voltage range is also limited to the output swing range ( in other word the power supply rails ) then the circuit’s output could lock-up and no longer respond to any changes in the input rendering the circuit useless.

18.8. Procedure

Add the two positive feedback resistors to your circuit as shown in figure 8. Use values for R2 = 100 KΩ and R1 equal to 10 KΩ. Using IN2, again observe the output square wave but note the level of the input triangle wave when the output changes level from low to high and high to low. Explain your results. Try a value for R2 less than R1. Does the circuit still work?

  1. Set Oscilloscope probes attenuation; IN1 to x1 and IN2 to x10
  2. Start the Oscilloscope & Signal Generator application.
  3. To apply input voltage \(V_{in}\) in the OUT1 settings menu set Amplitude value to 0.5V and DC offset to 0.5V. From the waveform menu select TRIANGLE, deselect SHOW button and select enable.
  4. In the OUT2 settings menu set Amplitude value to 0.5V, from the waveform menu select DC, deselect SHOW and select enable.
  5. On the left bottom of the screen be sure that IN1 V/div is set to 200mV/div (You can set V/div by selecting the desired channel and using vertical +/- controls)
  6. On the left bottom of the screen be sure that IN2 V/div is set to 500mV/div (You can set V/div by selecting the desired channel and using vertical +/- controls)
  7. In the IN1 settings menu set probe attenuation to x1 and offset level to -500mV
  8. In the IN2 settings menu set probe attenuation to x10 and offset level to -1000mV
  9. In the TRIGGER settings menu select source IN2, select positive edge and set trigger level to 2V
  10. Set t/div value to 200us/div (You can set t/div using horizontal +/- controls)
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Figure 10: Schmitt Hysteresis and output signal

Compare results from figure 10 and figure 4. Look at the levels of IN1 when IN2 goes high and low.

To see if the delay caused by the input noise has changed, again zoom into the falling and rising edges of the output square wave by adjusting the Horizontal position and time per division setting. Does the output pause at the same intermediate level as it transitions or does it no longer have this delay?

  1. In the TRIGGER settings menu select source IN2, select positive edge, NORMAL and set trigger level to 2V
  2. Set t/div value to 5us/div (You can set t/div using horizontal +/- controls)
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Figure 11: Switching noise with hysteresis

As you can see from figure 11 when using hysteresis switching noise is not present at all. Compare figure 6 and figure 11.