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designfeature By Jim Williams, Linear Technology Corp
FOR THE PRICE OF A SAN FRANCISCO DINNER, YOU CAN PUT STANDARDS-
LAB PERFORMANCE ON A CIRCUIT BOARD. PART 1 OF THIS THREE-PART
ARTICLE CHRONICLES A 1-PPM DAC AND ITS VARIOUS DESIGN OPTIONS,
PART 2 IN THE NEXT ISSUE PRESENTS VERIFICATION TECHNIQUES, AND
PART 3 DISCUSSES THE HANDLING OF PARASITIC EFFECTS.
20-bit DAC demonstrates
the art of digitizing 1 ppm
Part 1: exploring design options
f you design and use precision instruments, a of DACs are still in use. (Part 2 discusses this type
I 1-ppm measurement is a nearly impossible of DAC in more detail.)
dream. This level of performance used to be at- Thus, a practical, 20-bit (1-ppm) DAC that is easy
tainable only from large, slow, and expensive in- to construct and does not require frequent calibra-
struments that require extreme care in handling and tion is a useful development. The scheme in Figure
use. However, now a DAC design that uses a simple, 1 is based on the performance of a true 1-ppm ADC
powerful feedback loop can capture the magic 1- with scale and zero drifts less than 0.02 ppm/