Basics of Analog & Digital Sensor Signals

For industrial process control, there are a variety of ways that sensors and transmitters communicate measurement information. This article provides a general primer to help understand each signal and where it may be commonly used for measurement and control.

What are Analog Signals?
 

Analog signals are continuous electrical signals that represent the measurement variable. The analog signal could be based on either voltage or current. The signal is scaled based on the range of the measured variable. A possible analogy for an analog signal could be something like a dimmer control for a light bulb. As the dimmer control is moved the intensity of the light changes.

Visual representation of analog signals

In control system terminology analog signals may be abbreviated as follows:

AI – Analog input

AO – Analog output

Common Analog Output Types
 

4-20mA – The most common analog output signal is the 4-20mA current output. This is a universal signal that could be related to temperature, pressure, or analytical measurements like pH or conductivity. In most cases a 24 volt DC supply powers the current output signal.

The transmitter converts the measured variable into the 4-20mA current output where 4mA = 0% and 20mA = 100%. There are multiple wiring schematic for transmitter wiring:

  • 2-wire (current and voltage on the same +/- wires. Normally referred to as “loop powered”)
  • 3-wire (a common 24VDC power supply is used for both the transmitter/sensor and the 4-20mA current signal)
  • 4-wire (separate 24VDC power supplies are used to power the transmitter and the 4-20mA current

0-10 VDC (also 1-5 VDC) - DC voltage outputs are also commonly used as a universal signals for process control. For example, 0-10 VDC is prevalent in building control (HVAC) applications thus may have some cross-over into industrial control. When in doubt, check the specifications of your device to learn about its voltage output range. Also note that mA current signals may be converted to voltage by adding resistors in parallel with the circuit.

Millivolt - sensors such as pH probes or thermocouples may generate a millivolt signal in relation to the measurement. For example, a glass membrane pH sensor will generate a signal of + 414mV to – 414mV corresponding to the 0 to 14 pH scale. Often the millivolt signal is too small to be transmitted long distances so it is converted to 4 – 20mA within the transmitter.
 

Nanoamp – Electrochemical sensors such as polarographic dissolved oxygen products generate a nanoamp current signal in relation to the density of oxygen molecules within the process. As is the case with millivolt outputs, the nanoamp signal is typically converted to 4-20mA within a transmitter so it can be more easily used by existing control systems.