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Application of voltage comparator in detection system

1 Introduction

A voltage comparator is a feature that compares two or more analog levels and gives the result. It compares an input analog voltage signal with a set reference voltage. In the vicinity of the same amplitude, the output voltage will jump to a corresponding high level or low level. It is widely used in analog and digital signal conversion fields. application.

Multisim is a relatively good circuit simulation software. It provides virtual instruments and analysis methods not only to see the running status of the circuit and the performance indicators of the measuring circuit in time, but also to design and test simultaneously, and to complete various types of circuit design. And experimentation. Based on Multisim10.1, this paper simulates and designs three comparator circuits with LM339 from American NS.

2 Comparator features and classification

2.1 Characteristics of the comparator

Since the comparator has only two different output states, low level or supply voltage, the comparator output stage with full supply swing characteristics is the emitter follower, which makes the output signal and the power supply swing only Very small pressure difference. This voltage difference is dependent on the voltage between the collector and the emitter in the transistor's internal saturation state. The output voltage of a CMOS rail-to-rail comparator depends on the MOSFET under saturation. Compared to a bipolar transistor structure, the voltage is closer to the supply voltage under light load conditions.

2.2 Classification of comparators

According to the number of voltage comparators contained in a device, it can be divided into single, double and quad voltage comparators; according to the function, it can be divided into general-purpose, high-speed type (propagation delay less than 50ns), micro-power comparator (static Current less than 20mA), low voltage type (supply voltage less than 5V) and high precision voltage comparator; according to output mode, can be divided into collector (or drain) (Open-Drain) open output and push-pull (Push -Pull) Output structure in two cases. The output of most comparators is an open-collector structure. For example, LM339, MAX918, etc. require pull-up resistors.

3 voltage comparator selection

3.1 Power supply voltage range, common mode range

When the comparator is used, there are two cases of dual power supply and single power supply. The power supply mode should be selected according to the functional requirements. In order for the comparator to work properly, first the supply voltage should be within the allowable voltage range of the comparator used, and secondly the input signal at both ends must not exceed the common mode range specified by the comparator.

3.2 hysteresis voltage and offset voltage

Since the input of the comparator is often superimposed with a small ripple voltage, the differential mode voltage generated by these fluctuations causes a continuous change in the comparator output. To avoid output oscillations, new comparators typically have hysteresis voltages of a few mV. The presence of a hysteresis voltage causes the comparator switching point to be two: one for detecting the rising voltage and one for detecting the falling voltage (Figure 1). The difference between the high voltage threshold (VTRIP+) and the low voltage threshold (VTRIP-) is equal to the hysteresis voltage (VHYST), and the comparator's offset voltage (VOS) is the average of VTRIP+ and VTRIP-. The offset voltage (ie, the switching voltage) generally varies with temperature and supply voltage.