A dual-limit comparator with arbitrarily selected center value and amplitude

Function of the circuit

Usually, the dual-limit comparator must set the upper and lower limits, and make comparisons within the upper and lower limits. This circuit sets the center value first, and then completes the comparison by comparing the amplitude from the center value. The amplitude of the center value can be freely selected by the external voltage, just like the integrated circuit three-terminal regulation output voltage detector. First, the center value is selected as +5V, and then the positive and negative percentages thereof are selected as the reference dual-limit voltage VW (5*X%).

How the Circuit Works

A1 is an OP amplifier, A3 is a buffer amplifier, A2 is an inverter, and when EI>V0, A4 is equivalent to an ideal diode circuit that outputs a negative voltage. A5 is a zero-crossing comparator, and VR1 is used to adjust its hysteresis voltage to make it work stably.

The solid line in Figure A represents the output of OP amplifier A4. The voltage drift is caused by the existence of -C0 in the circuit. If EI-VO=0 is used as the origin, only when EI>-V0, diode D1 is turned on and outputs a negative voltage. The dotted line in the figure represents the output when the input is less than -V0. The output is superimposed with the output of A4 at the input of the comparator to obtain the absolute value output. The amplitude of the double limit is determined by multiplying the comparator voltage by the deviation value of VW. When EI=0~-V0-VW area, the output is "H" level, and the output is "L" level within the range of -V0 positive and negative VW. The comparator only performs zero-crossing detection, and the point shown by the dotted line in the figure is the apparent zero point, so it can be used as a double limit comparator.

Component selection

This circuit uses a dual OP amplifier, and a quadruple circuit TL084 can also be used. The offset adjustment circuit is omitted in the circuit, so the input voltage must be greater than 1V. If the input voltage is relatively low, a low-bias OP amplifier should be selected.

The proportional relationship between resistors R5~R11 is very important, but the E series nominal resistors do not have resistance values ​​of 1, 1/2, and 1/4, so you can select the resistance value according to the circuit diagram or use R=30K, R/2=15K, R/4=7.5K.

Adjustment

When the input voltage of the comparator LM311 is close to the reference voltage, it is easy to oscillate, causing the output to jump. In order to make the comparison more stable, a hysteresis voltage must be added. The positive feedback amount is determined by the voltage divider circuit composed of R12 and VR1.