Small circuit forms programmable 4- to 20-mA transmitter

  
One of the key challenges in the design of 4- to 20-mA current transmitters is the voltage-to-current conversion stage. Conventional transmitters use multiple op amps and transistors to perform the conversion function. These approaches have been around for a long time, but they are usually inflexible, have poor power efficiency, and have limited current compliance. An improved Howland current pump, on the other hand, can be cost-effective, because it addresses the cited problems. In addition, it closely models an ideal current source with the potential for nearly infinite output impedance. Figure 1 shows the improved Howland-current-pump topology, implemented with a high-resolution DAC, a precision reference, and a high-current op amp.
Small circuit forms programmable 4- to 20-mA transmitter - schematic

You can make R3′ as small as necessary to achieve the desired current and improve the load range. As an alternative, you can make the other resistors large to keep the quiescent current low for high power efficiency. The improved Howland current pump is flexible. It offers both current-sink and-source capability. The input voltage at VX is polarity-insensitive; you can apply it to either R1 or R1′. You can connect the load to the supply rail as a high-side load, or you can refer it to a low-side supply or ground (Figure 1). Further, one of the primary advantages of this topology is that the current pump provides potentially infinite output impedance, like that of an ideal current source. However, you must pay strict attention to resistor matching. Given the requirements of small form factor and high precision, the design in Figure 1 uses IC2, the a 16-bit current-output AD5544 DAC, with an external op amp instead of a voltage-output DAC. You face some important trade-offs in deciding whether to use a current-output or a voltage-output DAC. Current-output devices typically cost less than voltage-output DACs. The design must convert the current to a voltage to run the current pump, and the external op amp determines the accuracy of this conversion. Thus, you have control of the amount of accuracy as your application requires. Voltage-output DACs generally cost more than current-output devices because the...




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