201303.pdf 36567KB Mar 22 2013 09:11:22 PM

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designideas ANALOG SIGNALS DIGITAL SIGNALS 10 0 –10 H DAC OUTPUT H L H L OC 4 PWM REF L 0 10 20 30 40 50 60 70 80 90 100 TIME (μSEC) microcontroller with the internal 16-bit timer. C 3 and R 9 differentiate the resulting PWM signal; IC 3B then inverts it, forming a 1-μsec sample signal for the sample-and-hold IC 2 . Pin 5 of IC 2 forms S/H SIGNAL FOR 344 COUNTS IC 1B - PIN 7 RAMP OC 1 PWM DAC SIGNAL Figure 2 The differentiated OC 1 falling edge generates the S/H sample pulse at the ramp −5V point. the DAC output and is adjusted to −10, 0, and +10V for OC 1 PWM counts of 88, 600, and 1112, respectively, corresponding to a 10-bit count of 1024. The count offset of 88 helps to avoid the initial nonlinear region of the ramp so that the PWM DAC shows good linearity with a LSB of 20 mV and an accuracy of ±40 mV. Additional PWM DACs could also be implemented using capture PWM outputs OC 2 and OC 3 . Figure 2 shows the waveforms to be expected for a DAC output corresponding to 256 on a 10-bit scale of 1024. OC 4 forms the PWM reference based on which a 20-kHz bipolar ramp signal is output at Pin 7 of IC 1B . This ramp is sampled and held at a count of 256+88=344, corresponding to a DAC output of −5V. EDN RefeR ences 1 Kester, Walt (editor), The Data Conversion Handbook, section 3-1, pg 3-28, Newnes, 2005, http://bit.ly/ KjU8fU. 2 Raman, Ajoy, “Universal Analog Hardware Testbench,” http://bit.ly/ QCYXmb. A circuit for mains synchronization has two separate outputs for each half-period EDNDI5324 Fig 2.eps DIANE Dušan Ponikvar, University of Ljubljana, Ljubljana, Slovenia ↘ Often a measurement of weak signals has to be performed in the presence of strong interference from the ac power mains. If the interfering signal cannot be filtered out, then you can still obtain a clean result by making two consecutive measurements separated in time by an odd number of half-periods of the mains and calculating the average of the two measurements. The interfering signals have opposite polarities in consecutive measurements, and averaging cancels them out. If you average several consecutive pairs of measurements, the results will improve still further. Instead of counting the half-periods of the mains, you may find that a circuit having two outputs for synchronization with odd or even half-periods of the mains can come in handy. The circuit shown in Figure 1 provides two separate and optically isolated outputs, ISO 1 and ISO 2 , for synchronization with the desired half-period of the mains. Figure 2 shows the results of simulation (using the free version of MAINS R 1A 120k R 2A 120k R 1B 120k C 1 R 2B 10 nF 120k D 1 D 2 D 3 D 4 R 3 47k R 4 47k D 5 C 3 1 μF 10V C 2 1 μF 10V Q 5 2N3904 OC 1 4N33 OC 2 4N33 R 6 10k 5V R 7 10k ISO 1 ISO 2 Figure 1 Mains zero crossings are marked by the optically isolated outputs. 52 EDN | March 2013 [ www.edn.com ]
ANALOG INTEGRATION ISN’T FOR EVERYONE © 2013 Maxim Integrated Products, Inc. All rights reserved. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc., in the United States and other jurisdictions throughout the world.
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