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Oct 16 2020

LM311 Voltage Comparator: 4 Things Need to Pay Attention

I Introduction

The LM311 devices are single high-speed voltage comparators. The devices are designed to operate from a wide range of powersupply voltages, including ±15-V supplies for operational amplifiers and 5-V supplies for logic systems. The output levels are compatible with most TTL and MOS circuits. These comparators are capable of driving lamps or relays and switching voltages up to 50 V at 50 mA. All inputs and outputs can be isolated from system ground. The outputs can drive loads referenced to ground, VCC+ or VCC−. Offset balancing and strobe capabilities are available, and the outputs can be wire-OR connected. If the strobe is low, the output is in the off state, regardless of the differential input.

Catalog

I Introduction

II Precautions of LM311

2.1 Choose Components Reasonably

2.2 Increase Amplitude of Input Signal

2.3 Add Filtering Appliances to Output of Comparator

2.4 Adopt Lagging Technology

FAQ

II Precautions of LM311

LM311 is a commonly used linear comparator, which is widely used in comparison and shaping circuits, and as is shown in Figure 1. 

Figure 1. LM311 Circuit Diagram

Figure 1. LM311 Circuit Diagram

However, LM311 often has unexpected problems in the application, that is, the output pulse signal is not as ideal as theoretical analysis. Instead, high-frequency oscillation occurs near the front and back edges of the output pulse, as shown in Figures 2 and 3. 

High Frequency Oscillation before Output Pulse

Figure 2. High Frequency Oscillation before Output Pulse

 

Figure 3. High Frequency Oscillation after Output Pulse

When the input signal Vi amplitude of the LM311 is smaller and the frequency is lower, the high-frequency oscillation is more serious. This kind of waveform containing high-frequency oscillation cannot be used directly. It will cause misoperation to subsequent circuits, such as frequency measurement. Therefore, this situation must be paid attention to, and try to avoid or eliminate high-frequency oscillation. The following will give a brief analysis of the causes of oscillations, and at the same time put forward several methods to effectively avoid eliminating oscillations on the basis of experiments.

Figure 4. LM311

Figure 4. LM311

When a high-speed comparator is used for high-speed input signals and low source impedance input signals, the normal output response should be fast and stable. However, when the input signal is a slowly varying signal or a high-impedance signal source (1.0KΩ-10KΩ), the comparator may oscillate suddenly at the comparison threshold point, which is caused by the high gain and wideband of the comparator, and the presence of interference is also one of the direct causes of this oscillation.

In application, to avoid this kind of oscillation and instability, careful consideration should be made in advance and overall arrangements should be made. The following will propose several effective methods to avoid and overcome oscillations:

2.1 Choose Components Reasonably

Reasonably arranging the occurrence of structural oscillations has a lot to do with structural arrangements. The output signal should be far away from the input terminal pin, and should also be far away from the two balanced terminal pins, because the feedback signal sensing or touching any pin may almost cause oscillation. If the comparator uses a resistor at the input, its position and resistance are worth considering. The resistance should be placed near the tube base, and the general resistance value should be less than 10K (or even less), please refer to the corresponding manual when using.

Positive and negative power supply should add 0.1μ filter capacitor to filter out the interference of the power supply, and put the capacitor near the pin. The two balanced ends should be properly handled. When not in use, they can be shorted together. For specific use, you can also refer to the relevant manual.

2.2 Increase Amplitude of Input Signal

The magnitude of the input signal amplitude is directly related to the oscillation. Experiments show that the smaller the signal amplitude, the lower the frequency, the greater the possibility of oscillation. The following will make a simple analysis of the above conclusions. If there is a zero-crossing comparator, the input signal is Vi=V0sinω0t. The slope of the signal at t=0 is:

The amount of voltage change in △t time is: △Vi=K·△t=V0sinω0t, which shows that △Vi is proportional to V0,ω0, that is, the greater the amplitude of the input signal, the higher the frequency of the signal. Then in the △t time, the longer the amplitude change of V is, when dvi/dt is large enough, the input signal will quickly cross the comparison threshold, so as to achieve the purpose of eliminating oscillation. Because the input voltage range of the comparator is generally relatively wide (for example: the voltage input range of the LM311 is ±30V), this method is the most simple and feasible.

The experiment proves that as long as the amplitude of the input signal is greater than 0.7V, this design can work reliably in the range of 10Hz ~ 60KHz, continue to increase the voltage amplitude, the working range can be extended to the low frequency end.

2.3 Add Filtering Appliances to Output of Comparator

Pulling a resistor at the output of the comparator and connecting a capacitor with an appropriate capacity has a significant effect on filtering and reducing oscillation. The capacity of the capacitor should be determined on the basis of the experiment. The capacity of the capacitor should not be too large, otherwise the leading edge of the output pulse will be deteriorated. It was found in the experiment that this negative effect is particularly serious at higher frequencies, and even make the pulse amplitude smaller, so that the counter of the subsequent stage can not work, the situation is shown in Figure 5.

Figure 5. Pulse Amplitude at Higher Frequencies

Figure 5. Pulse Amplitude at Higher Frequencies

Therefore, this method has certain limitations in the application, and the reasonable choice of capacitance is the key to applying this method. Of course, the deteriorated front can be restored by the 74LS14 with a shaping effect. The negative effect of this method is to shift the original pulse front backward. In this design, capacitance C=0.01μ is taken. Within the range required by the system, the value of the pull-up resistor that the circuit can work reliably cannot be too large. In this design, R=510Ω.

2.4 Adopt Lagging Technology

In the comparison circuit, when the input signal reaches the comparison level, the comparator should be reversed immediately, but if the measured signal is superimposed with a certain amount of interference, the comparator may oscillate near the comparison level, as shown in the following figure (Figure 6-7).

Figure 6. Output of a Common Zero-crossing Comparator

Figure 6. Output of a Common Zero-crossing Comparator

Figure 7. Output with Lag Technology

Figure 7. Output with Lag Technology

The effective method to overcome the oscillation of the comparator is to use the lag technology, that is, add a small amount of positive feedback to its non-inverting end. The comparison level of the lag comparator is no longer a single level, but has two power levels near the original comparison level. In general, for the circuit in figure 8, the upper comparison level is represented by V+H, and the lower comparison level is represented by V+L.

Figure 8. Circuit with Two Levels

Figure 8. Circuit with Two Levels

The hysteresis voltage can be adjusted by R1 and R2. As long as △V is selected properly, the oscillation phenomenon of the comparison circuit can be eliminated. Therefore, the anti-interference ability is greatly improved, but the presence of the lag level △V will make the detection sensitivity worse. Therefore, △V should not be too large, usually R1≤R2. For the LM311 comparator, adding 3mv of hysteresis will eliminate the oscillation in the circuit.

Therefore, we must consider carefully and treat separately when using LM311. Only in this way can we be handy when using it.


FAQ

  • How to use LM311?

LM311 is a single-channel comparator. When using it, connect the reference voltage and the compared signal voltage to its non-inverting and inverting input terminals (pin 2 and pin 3), and its output is the result of the comparison. If you want the foward output result, pin 7 is connected to the positive power supply and pin 1 is the output. If the result is to be output in reverse, pin 1 is grounded and pin 7 is the open collector output.

  • lm311 and lm393 are both voltage comparators, so what is the difference between them?

LM311 is single voltage comparaotor, LM393 is dual voltage copatpr. LM311 has a load current of up to 50MA and a voltage of 40V. It can drive relays with a minimum power supply voltage of 5V.

The LM393 load current is 16MA, and the minimum voltage is 2V for a single power supply.

  • Whats the difference between LM311 and LM111?

Their functions are the same, and the 1XX series can be used in harsher environments.

The 3XX series can only be used in a commercial environment, typically the applicable temperature range of the device.

The price of 1xx is much more expensive than 3xx.

  • What does the 5 and 6-pin balance strobes of LM311 mean?

The function of balancing the mirror current of the reverse circuit is realized by connecting a potentiometer in the middle. In addition to the balance function, the 6 pin also has a strobe function, and the 6 pin can be grounded through the transistor drive circuit for strobe output.

  • What is the difference between lm311 voltage comparator dual power supply and single power supply?

The comparators are all open-collector outputs, without load resistance, they cannot output voltage signals.

Dual power supplies can detect signals lower than 0, and single power supplies can only detect signals higher than 0.

  • Can the lm311 comparator be powered by a positive and negative five-volt dual power supply?

Of course, LM311 can be powered by ±5V dual power supply. Its requirement for working power supply is that the voltage difference between the positive and negative power supply (or single power supply voltage) is at least 3.5V and the maximum is 30V, as long as it is within this range.

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