Small circuit forms programmable 4- to 20-mA transmitter | | |
http://www.edn.com/article/CA289969.html
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.
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