For long-term DAQ measurements running free-running (without external time reference or synchronization), the accuracy of the internal timebase must be considered. Over time, the measurement time axis may drift relative to ideal (absolute) time.

DAQ systems derive the sample clock from an internal oscillator. Its accuracy is typically specified in ppm (parts per million) of the sample rate.

Example: 50 ppm corresponds to a relative error of 50 × 10⁻⁶ = 0.005 %

A first-order estimation of time drift is:

Drift ≈ elapsed_time × timebase_accuracy

• Timebase accuracy: 50 ppm of the sample rate
• Measurement duration: 3 months ≈ 90 days ≈ 7,776,000 s

Calculation:

Drift ≈ 7,776,000 s × 50 × 10⁻⁶
      ≈ 390 s
      ≈ 6 min 30 s

Expected result: After ~3 months of free-running acquisition, the time axis may drift by approximately 6.5 minutes relative to ideal time.

• Timestamps remain monotonic and internally consistent
• Absolute timing accuracy degrades over long durations
• Often acceptable for trend analysis
• Relevant when correlating data with external systems or clocks

• External time reference (e.g. GPS, PTP, IRIG-B)
• Periodic resynchronization
• Use of hardware with higher timebase accuracy (e.g. cRIO)

A successful write operation does not prove that an output signal is physically present. This applies regardless of whether communication uses DAQmx, VISA, a serial protocol, or a vendor DLL.

• Commanded value: The last value successfully passed to the driver or communication interface.
• Readback: A value returned by the driver or device. It may represent a cached command, buffer, register, device setpoint, status, or internally measured value.
• Measured actual value: A value obtained through an independent measurement path with defined accuracy.

• Level 0 – Commanded value: Store and display the value after the write operation returns successfully.
• Level 1 – Device/API readback: Query the device or driver. The exact meaning is device-specific and must be documented.
• Level 2 – Electrical feedback: Independently measure the voltage, current, or logical level. This verifies the signal only at the measurement point.
• Level 3 – Functional feedback: Verify the actual response of the connected load or process.

• Use Level 0 as the default.
• Use a higher level only when a specific and relevant failure must be detected.
• Do not present a commanded or returned value as an actual value unless it is backed by a documented measurement.
• Consider marking values as unknown after communication loss, device reset, or power cycle.