W. Stephen Woodward
The accurate transmission of analog signals over long distances in noisy industrial environments is a difficult design problem. One of the oldest solutions – isolated current loops – is still one of the best. This Idea For Design presents a simple optically isolated linear current loop receiver that uses generic parts.
The receiver operates from a single non-isolated power rail (12 V) to generate a convenient analog 5-V voltage-mode output that’s ready for further analog signal conditioning, digital conversion, or whatever the application requires. In combination with an earlier IFD , it can complete a robust, analog multi-drop data link.
The circuit employs a simple technique to inexpensively implement accurate analog isolation: operation of a multiple-channel LED/transistor opto-isolator (NEC PS2501-4) in a linear mode (see Figure 1).
|Figure 1.||Using inexpensive generic parts, this circuit creates an optically isolated
linear current loop receiver.
The incoming 4- to 20-mA current-mode signal is subdivided in a 1:4 ratio between the active coupler pair U2b and passive pairs U2a and U2d. This division provides a more manageable 0.8- to 4-mA signal to the downstream receiver circuitry. The 1:4 current ratio is approximately established by the 1000- and 249- series resistors and improved by the 31-k linearity compensation resistance in parallel with LED b.
The current density in LEDs a and d is twice that of LED b, which results in the forward voltage drop of a and d being about 35 mV higher than that of b. The current shunted around LED b by the parallel resistor cancels the resulting current offset and the possible nonlinearity of about 1%.
The U3 feedback loop ensures an accurate mirror of the LED b current on the opposite side of the optical isolation barrier in LED c. The LED c current drive is served to force equality of the U2b and U2c phototransistors. The only way this can happen is for the LED c current to track that of LED b.
Any minor mismatch between the transfer gains of the U2b and U2c optical pairs is calibrated out in test with the CAL trimpot, establishing an accurate overall voltage/current conversion function of 0.25 V/mA. Stability of the calibration against component aging and temperature variation is ensured by the similar operating points of the coupled pairs.
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