For long-distance analog control wiring, which signal type is preferable?

• A. Current
• B. Voltage
• C. Digital
• D. Pulse-width Modulation

Answer: (A)

For long runs of analog control, sending a current signal is preferred because the loop current stays constant despite cable resistance and electrical noise. The receiver measures the current, so voltage drops along the wire don’t distort the sensed value, giving better accuracy and reliability over distance. This is why two-wire loop-powered transmitters (often using a 4-20 mA range) are common in industrial analog control—the transmitter can power itself from the loop and still convey the process variable consistently.

Voltage signaling, by contrast, suffers from voltage drops over long cables, which degrades the signal. Digital and PWM are not ideal for straightforward analog transmission over long distances: digital signals require regeneration to preserve information, and PWM would need filtering at the receiving end to recover an analog value, adding complexity and potential latency.