Synchronous sampling is a key feature of a data acquisition module for acquiring voltage and current signals. Some power measurements require the following types of synchronous sampling:
  • Active power
  • Reactive power
  • Power factor
Synchronous sampling is automatic in any acquisition when you perform voltage and current measurements with an NI 9225 or NI 9227. Traditionally, you implement synchronous sampling by using simultaneous sample-and-hold circuitry. The following different types of timing can exist between I/O channels:
  • Perfect, shared timing and triggering in order to achieve zero inter-channel sampling delay.
  • Deterministic timing and triggering to achieve a deterministic nonzero inter-channel sampling delay.
  • Shared, non-deterministic triggering and/or independent timing in which the separate clocks are not phase-locked.
  • Independent subsystems with free-running timing and separate triggers.
For non-deterministic delays, you can use the common features in the data to measure sampling delay between channels. For deterministic delays, you can compensate the data to measure event timing and system response. You can compensate for deterministic delay by adjusting the timing in the folowing ways:
  • Post-trigger delay
  • Signal conditioning through an all-pass filter
  • Scaling the measured phase responses directly
Depending on the test signal, you can express this delay in a number of different units: time (s), samples, and phase (deg). The following equation describes the conversions between these common delay units:
Time = Samples Samples rate = Phase 360 × Frequency

where

  • The time is in seconds.
  • The sample rate is in hertz.
  • The phase is in degrees.
  • The frequency is in hertz.

The following figure demonstrates the relationships among these units.

Waveform graph showing amplitude versus time with two overlaid waveforms.

In the previous figure, the delay time is 0.01 seconds. This delay time equals 10 samples/1000 (samples/s) and 90 deg/(360 deg * 25 Hz).