For single-ended devices, voltage levels are usually specified in terms of the voltage placed on the output terminal when driving a high level signal or when driving a low level signal, and by the voltage required on the input terminal for the signal to be recognized as a high or low level signal.

For the digital waveform generators and analyzers, the single-ended voltage levels are defined as follows:

  • Generation Voltage High—When configured for active drive generation, this is the voltage produced at the channel electronics when the pattern generation engine generates a binary one. When configured for open collector generation, Generation Voltage High Level is equivalent to setting the data channel to a high-impedance state.
  • Generation Voltage Low—The voltage produced at the channel electronics when the pattern generation engine generates a binary zero.
  • Acquisition Voltage High—The pattern acquisition engine senses a binary 1at or above this voltage level.
  • Acquisition Voltage Low—The pattern acquisition engine senses a binary 0at or above this voltage level.
Note On the NI 655x devices these levels are configurable.

When connecting an digital waveform generator and analyzer to a Device Under Test (DUT), you must ensure that the interface voltage levels are compatible. The relationship between the single-ended voltage levels and the DUT voltage levels are shown in the following figure:

To accurately communicate with a DUT, you must configure the module such that the following conditions are met:

  • Generation Voltage High Level ³ DUT VIH
  • Generation Voltage Low Level £ DUT VIL
  • Acquisition Voltage High Level £ DUT VOH
  • Acquisition Voltage Low Level ³ DUT VOL
  • Acquisition Voltage High Level > Acquisition Voltage Low Level

The extra margin between the voltage level that is driven by the source and the voltage level that is required at the destination is called the Noise Immunity Margin (NIM). The NIM indicates the amount of noise tolerable on the connecting cable with a data bit being received in correctly. The total NIM is computed by the following formula:

N I M = m i n G e n e r a t i o n   V o l t a g e   H i g h   -   D U T   V I H   , G e n e r a t i o n   V o l t a g e   L o w   -   D U T   V I L   , D U T   V O H - A q u i s i t i o n   V o l t a g e   H i g h ,   D U T   V O L - A q u i s i t i o n   V o l t a g e   L o w  

The NI SHC68-C68-D4 shielded cable for single-ended high-speed digital signals provides excellent protection against external noise sources. If your system operates in a particularly noisy environment, some data bits might be incorrect. In such an environment, increase the NIM if possible.