Fluke JW EDN p2 free download

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designfeature By Jim Williams, Linear Technology Corp PART 2 OF THIS SERIES ON THE DESIGN OF A 20-BIT DAC DIS- CUSSES THE ALL-IMPORTANT TECHNIQUES FOR TESTING THE PERFORMANCE--LINEARITY, SETTLING TIME, AND NOISE--AT SUCH MINUSCULE DATA LEVELS. SEE PART 3 IN THE NEXT ISSUE. Measurement techniques help hit the 1-ppm mark true 20-bit DAC that fits on a circuit board A and costs approximately $100 to build is a de- sign milestone (see EDN, April 12, 2000, pg 95 or www.ednmag.com/ednmag/reg/2001/04122001/ R=100k SEVEN-DECADE SWITCHED WIPER POSITION PERMITS SETTING TO 0.1-PPM LINEARITY 08ms743.htm). Claiming to achieve this level of per- formance is one thing, but proving it with precise (a) measurements is another. The measurement tech- 10k 2k 400 80 niques are at times more exacting than the actual cir- INPUT cuit's design. Part 2 of this series presents approaches and circuits for measuring linearity, settling time, and noise. MEASURE LINEARITY TO 1 PPM Of these three measurements, determining the DAC's linearity requires the greatest effort. Verifying 1-ppm linearity of the DAC and the integral ADC requires special considerations, and, interestingly, some help from the 19th century. Test- Figure 1 80 ing necessitates some form of voltage OUTPUT source that produces equal-amplitude output steps 80 for incremental digital inputs. Additionally, for measurement confidence, it is desirable that the source be substantially more linear than the 1-ppm requirement. This demand is stringent and painful- ly close to the state of the art. The most linear "digital-to-analog" converter is also one of the oldest. Lord Kelvin's KVD (Kelvin- Varley divider) is, in its most developed form, lin- ear to 0.1 ppm. This manually switched device fea- COMMON tures 10 million individual dial settings arranged in (b) seven decades. You can think of the device as a three- 10k terminal potentiometer with fixed "end