Window Comparator Definition and Circuit Examples

The window comparator is a commonly used analog circuit that has two thresholds: an upper limit and a lower limit. The input signal is considered a valid signal only when it is between these two thresholds. If the input signal is greater than the upper limit or smaller than the lower limit, the window comparator will sound an alarm or trigger the corresponding logic circuit.

Working Principle

In the circuit, the input signal is compared with two thresholds and a high-level or low-level signal is output, thereby limiting, filtering, and determining the signal. Here's the working principle:

Typically a window comparator consists of multiple operational amplifiers and a voltage reference source. The operational amplifier is used to compare the magnitude relationship between the input signal and the reference voltage. When the input signal is between the reference voltages, the output signal is high level; otherwise, the output signal is low level. At the same time, by adjusting the reference voltage, the upper and lower limits of the window can be set.

The circuit is shown in the figure, the window reference voltage u1>u2

Only when the input voltage ui is greater than u2 and less than u1, both A1 and A2 output high level, so uo outputs high level.

1) When ui is greater than u1, it is naturally greater than u2 (because u1>u2). At this time, although A1 outputs high level, A2 outputs low level. The output voltage uo is clamped by the diode at the output end of A2, and the output is low level.

2) If ui is smaller than u2, it is naturally smaller than u1. At this time, although A1 outputs high level, A2 outputs low level. The output voltage uo is clamped by the diode at the output end of A2, and the output is low level.

Rules

1. Input signal comparison

When the input signal enters the window comparator, it is compared with the upper and lower thresholds. If the input signal exceeds the window range, the comparator will limit it within the window and output a corresponding high or low signal.

2. Output signal control

Depending on the comparison between the input signal and the threshold, the window comparator will output a corresponding high-level or low-level signal. Normally, a high-level signal indicates that the input signal is greater than the upper threshold or less than the lower threshold, and a low-level signal indicates that the input signal is within the window range.

3. Comparator adjustment

In order to adapt to different input signal ranges and requirements through the comparator, the window comparator usually has an adjustable threshold function. Users can set the appropriate window range by adjusting the threshold according to actual needs, and achieve precise control and judgment of the input signal.

Features

1. Double-threshold comparison: Unlike single-threshold comparators (such as zero-crossing comparators), window comparators can perform comparisons by setting different thresholds, thereby providing more flexibility and adaptability. This makes the window comparator better to cope with complex signal processing needs.

2. Anti-interference ability: Due to its internal structure and the use of multiple operational amplifiers, the window comparator has strong anti-interference ability and is suitable for noisy environments or situations where fuzzy processing is required.

3. Multi-state output: The window comparator can produce multiple output states by setting different thresholds. These states can be used to indicate whether the input signal is within a specific level range. This design allows it to maintain output stability despite discontinuous signal changes.

4. Wide range of applications: Window comparators are not only used for traditional comparison functions, but can also be widely used in fields such as mechanical assembly, material thickness measurement, component position identification, color identification, quantity counting, and evaluation of welding connection strength. In addition, it is often used as the time command unit in microcontrollers and has important applications in digital logic.

5. Nonlinear response: Compared with a single-threshold comparator, a window comparator can provide a smoother nonlinear response, which is particularly useful in some cases (such as temperature detection) because it can avoid output changes frequently caused by signal fluctuations.

Applications

The main function of the window comparator is to detect whether the input signal is within a predetermined threshold range. If it is within this range, it will output a high level or logic "1", otherwise it will output a low level or a logic "0". Its specific functions are as follows:

Detect input signals: The window comparator can monitor the input signal in real time and determine whether the signal is within the specified threshold range. This function is required in many automatic control systems and sensors, such as measurement of temperature, pressure, light intensity, etc.

Limiting effect: The window comparator can provide the effect of limiting the amplitude of the input signal. When the input signal exceeds the preset upper or lower limit, the output signal will change. This function can be widely used in analog signal processing, audio processing, and video signal processing.

Signal trigger: The window comparator can also be used as a signal trigger. When the input signal enters or leaves the preset threshold range, the output signal will trigger the relevant event or process. This capability can be used in areas such as electronic design, robotic control, and automated processes.

Window comparators are widely used in industrial control, instrument measurement, medical instrumentation, audio processing, video signal processing and other fields, and can help engineers and technicians effectively implement functions such as signal detection, limiting effects and signal triggering.

Window Comparator Example: LM393

LM393 is a high-gain, wide-band voltage comparator chip that contains two independent comparators that can be used to compare the magnitude relationship between two input voltages. The output of the comparator can control external circuits, such as switches, relays, LEDs, etc., in response to the comparison results of the input voltages. In addition, the LM393 chip is designed as a low-power component, suitable for use in battery-powered applications that require long-term operation. It operates over a wide voltage range and is usually powered from a single power supply, with typical operating voltages ranging from 2V to 36V. In addition, LM393 has good temperature stability and is suitable for applications under different ambient temperatures.

Features:

1. Due to its advantages of high speed, low power consumption and reliability, the LM393 chip is widely used in amplifiers, oscillators, counters, remote controls, audio/video processing, liquid level detection, infrared detection, photoelectric sensors, temperature/humidity monitoring, etc. in the field.

2. LM393, like most comparators, is prone to oscillation if there is coupling from the output end to the input end due to parasitic capacitance. This only occurs in the gap between the output voltage transition when the comparator changes state, and adding bypass filtering to the power supply cannot solve this problem. The design of the standard PBC board is helpful to reduce the input-output parasitic capacitive coupling. Reducing the input resistance to less than 10K will reduce the feedback signal, and adding even a small amount of positive feedback (hysteresis 1.0~10mV) can cause fast switching, making oscillations due to parasitic capacitance impossible, unless hysteresis comparator is used. Because plugging directly into the IC board and adding resistors to the pins will cause the input-output to oscillate over a very short conversion period, and if the input signal is a pulse waveform with reasonably fast rise and fall times, hysteresis comparator is not necessary.

3. All unused pins of the comparator must be connected to ground.

4. The LM393 bias network establishes that its quiescent current has nothing to do with the power supply voltage range of 2.0~30V.

5. Usually the power supply does not need to add a bypass capacitor.

6. The differential input voltage can be greater than Vcc without damaging the device. And the protection part must be able to prevent the input voltage from exceeding -0.3V to the negative terminal.

7. The output part of LM393 is an NPN output transistor with open collector and grounded emitter, which can provide or O-Ring function with multi-collector output. The output can be operated as a simple open circuit to ground SPS (when no load resistor is used), the sink current in the output section is limited by the available drive and the beta value of the device. When the limit current (16mA) is reached, the output transistor will exit and the output voltage will rise quickly. And the output saturation voltage is limited by the γSAT of the output transistor of approximately 60ohm.

8. When the load current is small, the low offset voltage of the output transistor (about 1.0mV) allows the output to be clamped at zero level.

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