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 42 V, a set-current error of less than 1%, a temperature drift of less than 45 ppm/°C, and an output impedance of greater than 100 MΩ. 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 40 V and that operates over the –40 to +85 °C industrial-temperature range.
|Figure 1.||This composite configuration of a voltage reference and a
voltage regulator has high output impedance.
IC1's VREF output and R1 set IOUT. IOUT equals VREF/R1 plus IC1's bias current, which is typically 50 µA. This bias current is a small error of less than 0.05% at IOUT=10 mA and changes by only 10 µA over –40 to +85 °C. Changes in VREF and R1 over temperature more directly contribute to IOUT's accuracy and temperature coefficient. Inexpensive resistors with 0.1% tolerances and 25 ppm/°C drift over temperature are common. The LM4130 has a VREF grade of 0.05% and 20 ppm/°C over –40 to +85 °C. Thus, the worst-case current-setpoint error is within 0.15% and 45 ppm/°C, which results in an upper limit on the current error over temperature of 0.45%, or 45 µA.
The circuit's high performance would degrade if its output impedance were not very high. The circuit's output impedance is an undesirable parasitic in parallel with IOUT. The product of IC1's line regulation times IC2's line regulation determines the current source's output impedance. IC2's 1.2 V output sets IC1's input-minus-output to a near constant. IC1's VREF over R1 sets IOUT, which is twice removed from VIN. VREF of IC1 has an overtemperature line regulation of 500 ppm/V, and IC2's output has an overtemperature line regulation of 350 ppm/V. Output impedance is greater than 300 MΩ, which is good, if you calculate it using only the line-regulation effects. Although line regulation is the dominant source of output impedance, other thermal errors beyond line regulation degrade the potential of keeping very high output impedance over temperature. Bench measurements made on the composite showed output impedance greater than 300 MΩ at 25 °C and 100 MΩ over –40 to +85 °C.
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