Jim Williams, Linear Technology
Application Note 120, Appendix A
People have been converting digital-to-analog quantities for a long time. Probably among the earliest uses was the summing of calibrated weights (Figure A1, left) in weighing applications. Early electrical digital-to-analog conversion inevitably involved switches and resistors of different values, usually arranged in decades. The application was often the calibrated balancing of a bridge or reading, via null detection, some unknown voltage. The most accurate resistor-based DAC of this type is Lord Kelvin’s Kelvin-Varley divider (Figure, large box). Based on switched resistor ratios, it can achieve ratio accuracies of 0.1 ppm (23+ bits) and is still widely employed in standards laboratories.
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| Figure A1. | Historically Significant Digital-to-Analog Converters Include: Weight Set (Center Left), 23+ Bit Kelvin-Varley Divider (Large Box), Hybrid, Board and Modular Types, and the LTC2757 IC (Foreground). Where Will It All End? |
High speed digital-to-analog conversion resorts to electronically switching the resistor network. Early electronic DACs were built at the board level using discrete precision resistors and Germanium transistors (Figure, center foreground, is a 12-bit DAC from a Minuteman missile D-17B inertial navigation system, circa 1962).
The first electronically switched DACs available as standard product were probably those produced by Pastoriza Electronics in the mid 1960s. Other manufacturers followed and discrete-and monolithically-based modular DACs (Figure, right and left) became popular by the 1970s. The units were often potted (Figure, left) for ruggedness, performance or to (hopefully) preserve proprietary knowledge. Hybrid technology produced smaller package size (Figure, left foreground). The development of Si-Chrome resistors permitted precision monolithic DACs such as the LTC2757 (Figure, immediate foreground).
In keeping with all things monolithic, the cost-performance trade-off of modern high resolution IC DACs is a bargain. Think of it! An 18-bit DAC in an IC package. What Lord Kelvin would have given for a credit card and LTC’s phone number.
