Fluke JW EDN p3 free download

File name JW_EDN_p3.pdf

designfeature By Jim Williams, Linear Technology Corp UNDESIRABLE AND UNINTENDED THERMOCOUPLES ARE THE PRIMARY SOURCES OF ERROR IN LOW-DRIFT CIRCUITS. ATTENTION TO LAYOUT AND OTHER CONSTRUCTION DETAILS IS THE ONLY WAY TO TACKLE THE PROBLEM. Minimizing thermocouples maintains 20-bit DAC precision ubtle parasitics can have pronounced and S seemingly inexplicable effects on the perform- ance of low-level circuits, and a 1-ppm DAC is certainly in this category. Part 1 of this three-part se- COPPER JUNCTION BISMUTH ries discussed the circuit design of a 20-bit Figure 1 DAC with 0.1 ppm/ C of drift, and part 2 dis- cussed the measurement techniques (references 1 N and 2). This third and final part discusses how you JUNCTION deal with cables, connections, solder, component W E choice, terror, and circuit arcana. Perhaps the most prevalent detractors to micro- S volt-level circuitry are unintended thermocouples. COMPASS (Reference 2 also includes considerable discussion on dealing with thermocouples.) In 1822, Thomas Joining pieces of bismuth and copper led Thomas Seebeck Seebeck, an Estonian physician, accidentally joined to his accidental discovery of what he called "thermomag- semicircular pieces of bismuth and copper while netism" and what we now call the Seebeck effect. studying thermal effects on galvanic arrangements (Figure 1). A nearby compass indicated a magnetic disturbance. Seebeck experimented repeatedly with 3 different metal combinations at various Figure 2 2.8 temperatures, noting relative magnetic-field 2.6 strengths. Curiously, he did not believe that electric 2.4 current was flowing and preferred to describe the ef- 2.2 fect as "thermomagnetism." He published his results 2 in a paper (Reference 1). Subsequent investigation 1.8 showed the "Seebeck effect" to be fundamentally 1.6 electrical in nature, repeatable, and quite u