Timestamping accuracy for the NI 5112

[GcDago]

I've red for the NI 5112 the following specification:
- Timestamping of records to 2 ns accuracy
What does it mean?

I know that the NI 5112 adopts the AD converter AD9054A-135, manufactured by Analog Devices.
The following specifications are reported on the data sheet:
Aperture Delay at +25°C 0.5 ns.
Aperture Uncertainty at +25°C 2.3 ps rms.

I'm driving the NI 5112 with an esternal reference clock (rubidium oscillator) and I believed that the limit in timing accuracy was the limit of the reference clock.
Am I wrong?

Thanks
GcDago

[psisterhen]

The number that you give (2ns) does not refer to the timing accuracy of the sample clock, but rather the accuracy of the trigger timestamp. This timestamp marks, with sub-sample precision, when the reference trigger is received. Although this timestamp can tell you when you sampled relative to a trigger, it doesn't affect the timing of the sample clock.

The sample clock jitter, as you point out, is better than 2ns.