COMPARATOR CIRCUIT SET-UP WITH A LM3XX SERIES

Composed By: Dan Tse, Ravi Grewal, Ryan Malay, Mike Phan & Dave W.

MOST VOLTAGE COMPARATORS WILL COME WITH AN 8-PIN CONFIGURATION SHOWN IN FIGURE 1.

FIGURE 1.

THE ABOVE CIRCUIT SETUP SHOWN IS USED TO OBTAIN DIGITAL OUTPUTS FROM ANALOG SIGNALS, SUCH AS FROM A VIDEO CAMERA. THE COMPARATOR CAN BE CONFIGURED TO TRIGGER WHEN THE INPUT VOLTAGE AT PIN 2 IS GREATER THAN THE THRESHOLD SET AT PIN 3 FOR ACTIVE HIGH LOGIC.

Vpin2-Vpin3 > 0 => Vpin6 = 5 V

Vpin2-Vpin3<=0 => Vpin6 = 0 V

TYING A RESISTOR TO A VOLTAGE SUPPLY SETS THE OUTPUT VOLTAGE AT PIN 6. DEPENDING UPON THE FPGA, EITHER 5 OR 3.3 VOLTS WILL WORK. A RESISTOR SHOULD BE CHOSEN TO MINIMIZE THE CURRENT SUNK BY THE FPGA. THE REFERENCE THRESHOLD VOLTAGE CAN BE SET USING EITHER RESISTORS CONNECTED TO THE POWER SUPPLY OR A POTENTIOMETER FOR MORE PRECISE CONTROL.

FIGURE 2.

HOW TO READ FIGURE 2.

THE CIRCUIT IS STRAIGHT FORWARD.

  • PINS 1, 4 & 8 ARE GROUND, NEGATIVE AND POSITIVE VOLTAGE SUPPLY RESPECTIVELY. THE COMPARATOR CAN OPERATE IN THE RANGE OF +/-5V TO +/-15V.
  • PINS 2 & 3 ARE THE INPUT SIGNAL (IE. FROM VIDEO CAMERA) AND THE VOLTAGE THRESHOLD RESPECTIVELY. THE COMPARATOR WIL TAKE YOUR INPUT SIGNAL AND CHECK TO SEE IF THE VOLTAGE IS EQUAL TO OR GREATER THAN YOUR THRESHOLD SIGNAL. IF IT IS, IT WILL OUTPUT A LOGIC ‘1’, ELSE IT WILL OUTPUT LOGIC ‘0’.
  • LASTLY, PIN 7 IS THE COMPARATOR OUTPUT. IT IS RECOMMENDED THAT A PULL-UP RESISTOR IS USED TO CONNECT THE OUTPUT TO VCC. THIS WILL ENSURE THAT A CLEAN LOGIC ‘1’ SIGNAL IS OUTPUTTED AND AVOIDS FLOATING VOLTAGES.

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