EDN, May 26, 2011

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Logic gates form high-impedance voltmeter Raju Baddi, Tata Institute of Fundamental Research, Maharashtra, India ↘ You can use the circuit described in this Design Idea to estimate voltages across 10- to 100-MΩ resistances. It also works for reverse-biased diodes. The common CMOS gates in Figure 1 have an input threshold voltage in which the output swings from logic zero to logic one, and vice versa. The thresh- old voltage depends on the supply voltage (Figure 2). Because of each CMOS gate's high input impedance, input currents are approximately 0.01 nA. If you apply 5V to 100 MΩ, you get 50 nA. Thus, you can connect the gate input at a point at which it draws a negligible amount of current. You can vary the CMOS gate's supply voltage to attain the desired threshold voltage for the gate input. If you apply the unknown voltage to one of the gate's inputs and then connect the other input to the posi- tive-voltage supply, you can vary the supply voltage, VS , until you reach a point at which the threshold voltage at the input becomes equal to the unknown voltage. MOVING COIL METER − + OR 10k R2 IC1A R1 CD4011 DVM NOTE: R1 AND R2 ARE IN TENS OR HUNDREDS OF MEGOHMS. Figure 1 Use CMOS gates and a variable power supply to find an unknown voltage. IC1B 220 LED 1 TO 15V VARIABLE SUPPLY _ + Simply Magnetic. Broadest range of high performance magnetic linear and rotary encoder ICs EDN 110512DI5141 FIGURE 1 DIANE 8- to 16-bit resolution Insensitive against external magnetic stray fi elds AEC-Q100 automotive qualifi ed www.austriamicrosystems.com/MagneticEncoders MAY 26, 2011 | EDN 57 Booth #621

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