EDN, May 26, 2011

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designideas EDITED BY MARTIN ROWE AND FRAN GRANVILLE READERS SOLVE DESIGN PROBLEMS Waveform generator minimizes amplitude dependency Marián Štofka, Slovak University of Technology, Bratislava, Slovakia ↘ Engineers have long used func- IC4; and comparator IC5 ; dual analog SPDT (single-pole/ double-throw) switch IC3 , IC2 tion-generator circuits employ- ing analog integrators and high-hyster- esis comparators. The outputs of these circuits often depend on variations in temperature, power-supply voltage, load, and parts. However, you can pump new life into the classic triangular/ rectangular-waveform generator using the circuit in Figure 1. This circuit uses a precision reference-voltage source and the diode network comprising IC1 and IC6 , ; integrator . The result is a ramp- and square-wave generator that holds its output stable in the face of those variations. Comparator IC5 uses a switching technique to achieve its sta- ble high hysteresis. References 1 and 2 5V HSMS-282P IC6 VREF+ ADR431 VIN IC1 GND VOUT TRIM VREF NC 2.5V +IN0.4× +IN0.8× −IN0.8× −IN0.4× 100 nF VCOM 200k –VS –VS 200k VS AD8475 IC2 VREF– ADG736 IC3 S2B IN2 VCOMP COM VS 1.25k R+IN0.4 1.25k R–IN0.4 1.25k + 1.25k − 1k 1k −OUT +OUT S1A S2A S1B D1 D2 IN1 VDD R1 5k 0.1% 3 1 − 4+ 5 IC4 10 nF C+ R+ 5k 0.1% 8 7 2 4 3 NC 100 nF − + IC5 5 2 6 1 25 pF C1 OUTPUTS VINT ADA4899-1 ADCMP601 charge injection. The output of IC5 provide more details, including how the Schottky-barrier diodes of IC6 suppress comparator, is initially low, forcing the ADG736 analog switch, IC3 , an ADCMP601 , to connect resistor. As long as the comparator's output is low, the integrator's output, VINT VREF+ a positive reference voltage, VREF+ IC5's inverting input. Simultaneously, the S2 switch connects a negative refer- ence voltage, VREF−, to the integrator's RI , to , increases linearly until it reaches , the comparator's positive-thresh- old level. At that point, the output of the comparator changes to high, which turns on the B channels of both mul- tiplexers as their A channels turn off. When the switch positions change, the integrator integrates positive refer- ence-voltage VREF+, and VINT decreases linearly until it reaches the negative threshold VREF− IC2 creates VREF+ and VREF− sets. The cycle then repeats. A bipolar reference source comprising IC1 , which the comparator and . This part DIs Inside 54 Produce current from positive or negative high-voltage supplies 55 Arrange LEDs as seven-segment displays 57 Logic gates form high-impedance voltmeter 58 Measure resistance and temperature with a sound card ▶To see all of EDN's Design Ideas, visit www.edn.com/ designideas. Figure 1 Creating hysteresis by switching precision reference voltages within the waveform generator ensures high insensitivity of the amplitude of triangular waveforms on supply-voltage variations. 52 EDN | MAY 26, 2011

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