Causes of Jitter Between the Trigger and AO on cDAQ Modules with Delta-Sigma ADCs

Issue Details

When using the NI‑9260 module in a cDAQ‑918x chassis, the first analog output (AO) sample shows random jitter at certain sampling rates. The jitter appears inconsistent and does not follow an obvious pattern.

Symptoms

• The timestamp of the first AO sample varies from run to run.
• Jitter magnitude depends on the selected AO sampling rate.
• Example jitter values observed:

• • ~600 µs at 1650 S/s
  • ~200 µs at 5120 S/s
  • ~20 µs at 52.1 kS/s

The next picture shows the jittering using many different sample rates 

For n=10 and n=31 the jitter looks random. For n=1, 2, 3, and 5, it appears that either the first sample is immediately sent or there is some range of variance after a delay.

Solution

The jitter and latency observed on the first AO sample are normal for this kind of module. They result from the module’s delta‑sigma ADC design and are not caused by NI-DAQmx, the programming environment, or the chassis. This design inherently introduces up to one sample clock period of jitter.

This behavior is expected due to its delta‑sigma ADC architecture. Modules that use a resistor‑divider ADC with an update pin allow NI-DAQmx to align the first output sample precisely with a trigger. In contrast, those who use a delta‑sigma ADC are driven by a continuous sample‑clock timebase rather than a discrete update mechanism.

The DAC oversamples at 256 * f_s and requires around 130 sample periods to initialize. Because of this long initialization time, NI-DAQmx cannot start the ADC exactly at the trigger without adding a large latency. Instead, NI-DAQmx starts the ADC early, and the trigger only selects which sample clock period begins the output. This design inherently introduces up to one sample clock period of jitter.

If this delay is unacceptable, consider alternative AO modules like the NI-9263, which do not have this delay.

Additional Information

The NI‑9260 also adds a fixed output delay, which is calculated as: 30.767 / f_s + 3.2 µs.

Note that f_s refers to: Data rate range