A reference current source needs high accuracy, low temperature drift, and high output impedance. Available IC current sources come with some of these features. However, at current levels greater than 1 mA, their output impedance decreases to less than 10 MΩ. Figure 1 shows a composite 10-mA current-source configuration that has a compliance voltage of 5 to 42V, a set-current error of less than 1%, a temperature drift of less than 45 ppm/℃, and an output impedance of greater than 100 MO. One application of this accurate 10-mA current source with high impedance is as a 4- to 20-mA current-loop calibration reference that has a maximum loop voltage of 40V and that operates over the -40 to + 85℃ industrial-temperature range.
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