Current vs. Voltage Mode Signals

BIRKET Engineering News, Sept/Aug 1994

In a control system, analog variables such as speed, pressure, temperature, and position are often communicated by a varying voltage or current. In the industrial automation world, the 4-20ma current loop is the standard. We note that those who specify entertainment control systems favor 0-10 volt DC signals without, it seems, due consideration of the merits of the 4-20ma standard. There are several good reasons why current mode signals should be considered.

Although errors can be introduced into any signal, voltage mode signals are susceptible to more than their share of problems. These include the impedance of the voltage source and other supply fluctuations, wire and connection resistance, the integrity of wire insulation, electrostatic and electromagnetic noise, and ground potential differences. Care must be taken not to load a voltage signal as new devices are added. The only real advantage of voltage mode signals is that they interface directly with D/A and A/D converters and analog multiplexing devices.

Current mode signals have comparative strengths in most of the areas where voltage mode signals have weaknesses. They are immune to loop resistance found in long wire runs and faulty connections. Additional devices generally can be added to the loop without concern for the signal, supply permitting. Current mode signals are relatively immune to noise, the only exception being electromagnetically induced noise which can be substantially eliminated through the proper use of shielded twisted pairs. Intrinsically safe system practices (as described in NEC Article 504-2) are inherently compatible with 4-20ma signals; something which is becoming more of a concern with the growing interest in gas effects. Shorts and opens in the circuit are readily revealed for use with automated diagnostics. Finally, 4-20ma current loop transmission is a more widely accepted industry standard. Any perceived incompatibility with D/A, A/D and multiplexing devices is no longer an issue if the designer uses the widely accepted input and output cards from major manufactures such as Allen-Bradley.

We believe that the 4-20ma standard should be given more consideration in applications where voltage mode signals are common place, especially if the voltage signal is used only because “it has always been done that way.”