A comparative analysis about comparator
Published time: 2018-07-02
Abstract:-Comparator is one of the most important analog circuits required in many analog integrated circuits. It is used for the comparison between two different or same electrical signals. The design of Comparator becomes an important issue when technology is scaled down. Due to the non-linear behavior of threshold voltage (VT) when technology is scaled down, performance of Comparator is affected. Many versions of comparator are proposed to achieve desirable output in sub-micron and deep sub-micron technologies. The selection of particular topology is dependent upon the requirements and application. In this paper, we have shown the implementation of different topologies in 0.5 μm technology using the Mentor Graphics Tool. We have done the pre-layout simulation of two different topologies. We have performed DC, AC and transient analysis. We have also calculated output impedance. We have prepared a comparative analysis about them.
Comparators seem to be pretty simple.They compare two signals voltages and set the output high or low accordingly. However, if both input signal voltages are really close, even a little noise on the input signal can cause the output to oscillate between high and low logic levels. Adding hysteresis is the easiest way to solve this problem.
Jotrin Electronics think that Hysteresis is depends on its previous state when the system output. When hysteresis is added to a comparator, the upper switching threshold is set higher and the lower switching threshold is set lower. This is basic principle how an AC thermostat works, in case you can’t noticed. Let’s take a moment to imagine that if thermostats didn’t have hysteresis: the AC might cycle on and off every few seconds at the smallest temperature fluctuations, which would be noisy, inefficient, and put stress on the AC unit. Adding hysteresis to the AC thermostat，that will allows the system to operate more effectively.
Jotrin Electronics think that some comparators have built-in hysteresis, usually around a few millivolts. This may be adequate for certain applications, but other situations may require the addition of external hysteresis. Adding external hysteresis allows the rising and falling thresholds to be specific to the system requirements.
Hysteresis is implemented in comparator circuits through positive feedback. This is one of those few instances where positive feedback is a positive thing! Instead of having one threshold point, hysteresis creates different rising and falling thresholds. This keeps the output consistently at a low or high state instead of oscillating, when the input signal is hovering around the reference voltage. A comparator with hysteresis added through positive feedback is also called a Schmitt trigger.
The example below shows the TL331 device configured as an inverting Schmitt trigger. The TL331 is a single channel, low power, open collector comparator without internal hysteresis. The resistor divider created by the R1 and R2 resistors sets the reference voltage on the non-inverting pin, establishing the threshold voltage at which the comparator output switches. Since this is an open collector comparator, a pull-up resistor is connected to the output. The feedback resistor adds hysteresis through positive feedback. Typically, a relatively large feedback resistor value is used, at least 100 kΩ.
Figure 1. Comparator configured as inverting Schmitt trigger
For this inverting configuration, when the input signal is low relative to the threshold, the output pin is high, pulling the threshold voltage higher through the feedback resistor. This way, small voltage fluctuations on the input signal don’t trigger the comparator output to switch until the input voltage reaches the higher, adjusted, rising threshold. Once the input signal reaches the rising threshold, the output is pulled low. This pulls the threshold voltage lower through the feedback resistor, so that the output will stay low until the input voltage drops to the below the lower adjusted threshold voltage.
The non-inverting configuration works in a similar way as far its use of positive feedback. However, in this case, the threshold voltage set by the resistor divider doesn’t change as it does with the inverting configuration. Instead, feedback adjusts the input signal at the non-inverting node.
Figure 2. Comparator configured as non-inverting Schmitt trigger
In this configuration, Jotrin Electronics think that when the input signal is low, the output pulls low, causing the voltage at the non-inverting node to drop lower. Once the input signal is high enough to pull the non-inverting node higher than the reference voltage, the output is pulled high, which then pulls the non-inverting node even higher.
When our test lab test those comparators, our test method also adding hysteresis，it just need to only one or two external resistors, and the resistor values can be adjusted to the threshold values best suited for the specific application. When testing comparator, if there is any chance that the voltages on the input pins ，it will approach each other for any significant amount of time, adding hysteresis is a simple testing way of reducing issues caused by noise on the input signal.
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