Sinking and Sourcing with NI DAQ C Series Hardware

Issue Details

• I am setting up digital inputs and outputs on my system, and I want to make sure that I have enough current to drive the correct digital level. How do I know whether to use the sinking or sourcing digital I/O?
• Can my data acquisition (DAQ) device that is a sourcing digital input be utilized with a sinking digital output signal?

Solution

Sinking and Sourcing are terms used to define the control of direct current flow in a load and is an Input/Output type for NI digital modules.

Sinking digital I/O provides a grounded connection to the load and sourcing digital I/O provides a voltage source to the load. Because both a voltage source and a ground reference are needed to create a complete circuit, you must have a sourcing input or output connected to a sinking output or input, respectively.

Note: If you wish to connect a sourcing input to a sourcing output or a sinking input to a sinking output, you will need to add an additional resistor. For further information on connecting two I/O of the same type, please refer to Connecting Two Sinking I/O or Two Sourcing I/O Together .

There are a couple of hardware considerations you need to take:

• You must determine whether your hardware can either be a sink, a source, or both. Checking the datasheet of your NI hardware for schematic diagrams and information will help inform you.
• Consider the current limit specifications for NI I/O modules.
  • Digital Output (DO) modules typically have a maximum continuous output current defined; these modules don't typically have a minimum operating current defined.
    • Using the NI-9472  as example, this is 0.75 A maximum continuous output current per channel.
  • Digital Input (DI) modules differ, as their internal receiver circuitry draws a small amount of current when performing the measurement.
    • Using the NI-9426 sourcing input module as example, which is specified as follows:
      • OFF State: Up to 150 μA from the DI pin.
      • ON State: At least 330 μA from the DI pin.

Please refer to the Input/output type section in the Digital Subsystem Specifications section of the Specifications Explained: C Series Modules document for more information on sinking and sourcing.

Now, for example, consider a simple circuit that consists of one digital input connected to a digital output. The circuit needs a voltage source, a ground, and a load.

• A sourcing digital I/O provides the voltage needed for the circuit.
• A sinking digital I/O provides the ground needed in the circuit.
• The digital input provides the load required for the circuit to work.

Additional Information

While Sinking and Sourcing is primarily used for digital modules, the same concept can be applied to analog modules.

For example, the NI-9266 Datasheet contains the figure below. The figure below shows that the current flows from the Vsup pin connected to the external power supply to the AO pin. Additionally, from the datasheet we can see that the current range of the NI-9266 is from 0 mA to 20 mA. With these two pieces of information, we know that the NI-9266 can only source current, it cannot sink current from its Analog Out pins.