Analog and digital control signals are the two types of inputs and outputs on your various low voltage equipment controllers. An analog control output signal is used when a device needs to function at more than two points in a range. A common output range for an analog signal is 0-10 volts (other analog signals used commonly are 2-10V, 4-20mA and 0-20mA). A digital control output signal allows a device to operate at only two points. Digital outputs are used with devices that only require an On/Off or Open/Closed function.
Your air handler’s outdoor air damper motor actuator is an example of a device receiving an analog output signal from its controller. Typically this damper motor actuator will control to a minimum of three positions. Closed position will be used when the unit is in unoccupied mode to prevent unwanted outdoor air infiltration. Full open position will be used when the unit is in occupied cooling mode and the outdoor air is suitable for free cooling (economizer mode). The third position is set between the fully open and fully closed positions to allow minimum outdoor air during occupied periods. This minimum outdoor air requirement is dictated during design by the utilization and occupancy of the space served by the air handler. A great reference to use for figuring out the minimum outdoor air requirements for a space is ASHRAE 62.1. For our example, we’ll say that the design engineer has determined that your space needs 400CFM minimum outdoor air during scheduled occupied time. The certified testing and balancing contractor has measured that the outside air damper needs to be open to a 30% position to meet that 400CFM requirement. To achieve this 30% open damper position, the air handling unit controller will send an analog output signal of 3 volts (using a 0-10V controller) to the damper motor actuator. To close the same damper during unoccupied hours, a signal of 0 volts will be sent.
Knowing the difference between analog and digital comes in handy when reviewing your controls sequence of operations and controller diagrams. For every device that requires modulation there needs to be an available analog output at the controller. For every device requiring two positions there needs to be an available digital output at the controller. Aside from what is required, we recommend having a couple of spare analog and digital outputs to remain flexible for future equipment configurations.
Commissioning tip: When verifying an analog operating device in the field it is important that the voltage range of the device matches the voltage output range of the controller. As discussed previously this range can be 0-10V, 2-10V, 4-20mA, 0-20mA, etc. For example, if the controller is sending a 0-10V analog signal but the device is set to receive a 2-10V signal, no modulation will occur at the device when the controller is operating between 0-2V. This is an error easily corrected improving both the accuracy and efficiency of your building automated system.