Sinking and Sourcing

Sinking vs Sourcing Digital Outputs

A sinking digital (input/output) provides a grounded connection to the load, whereas a sourcing digital (input/output) provides a voltage source to a grounded load. Typically, a device will specify whether its interface terminals are sinking or sourcing and any necessary power supply/loading considerations.

Figure 1: Sinking digital output is connected to a sourcing digital input. In this circuit, the load is pulled to ground by the provided sinking digital output.

• Sinking Output: the device's output signal does not provide power. It must be connected to a sinking input with a power supply loop or a sourcing input.
• Sourcing Input: the device receiving the signal provides power for the input signal. It must be connected to a sinking output, such as a 2-wire transmitter, which uses the power from the receiving device.

Figure 2: Sourcing digital output is connected to a sinking digital input. In this circuit, the load is pulled up to receive the voltage provided by the sourcing digital output.

• Sourcing Output: The device's output signal powers the output circuit. It must be connected to a receiving device that provides no power and acts as a resistive load, such as a 2-wire transmitter.
• Sinking Input: the device receiving the input signal does not provide power. It must be connected to a sinking output with a power supply loop or a sourcing output or a sinking output.