Common Ground for Older VPW Circuit:  (Electronic circuit added 4/05)

Transmitter accurately Transfers voltage input:  05/15/03 EDN Design Ideas  /  (added 1/06)  When you connect remote sensors to a central process controller, a simple, robust, and commonly used interface is the 4- to 20-mA loop. The advantages of this current loop include the simplicity of just two twisted wires that share both power and signal, the loop's high noise immunity in harsh environments, and the de facto loop standard within the process-control industry....

Voltage reference improves JFET:  10/26/00 EDN-Design Ideas / (added 05/03) /The common JFET current-source in Figure 1a has average output impedance and depends heavily on the JFET's VP and IDSS variations. These manufacturing variations limit the initial accuracy of the current setpoint that the circuit can obtain with a fixed R1. Most users of JFET current-sources sample VP and IDSS for each lot of JFETs and then select R1 by first solving the quadratic equation ID=IDSS(1–VGS/VP)2 for VGS and then solving the current-setpoint equation ISOURCE=VGS/R1. After completing these lot-specific calculations, the accuracy error for ISOURCE can be less than ±15%. In addition to initial accuracy error at ambient, the VGS overtemperature performance limits the current temperature drift to 0.3%/°C. The temperature dependence of VGS is a function of both the mobility variation of IDSS with IDSS negative-temperature coefficient and the built-in potential of VP's positive coefficient. VGS has one ID operating point at which it has a zero temperature coefficient, but this one current setpoint is probably not the one that you wanted. With resistors commonly available at 0.1%, 25 ppm/°C grade, R1 will be a small contributor to any errors in the current setpoint. Compliance voltage across the current source is VGS to BVDSS. The output impedance, RO, for this current source is approximately 1 MΩ. You can improve RO to approximately 10 MΩ by adding another JFET in a cascode configuration.....