Chapter 5 Analog Output
NI USB-621x User Manual 5-8 ni.com
AO Sample Clock Signal
Use the AO Sample Clock (ao/SampleClock) signal to initiate AO samples. 
Each sample updates the outputs of all of the DACs. You can specify an 
internal or external source for AO Sample Clock. You also can specify 
whether the DAC update begins on the rising edge or falling edge of 
AO Sample Clock.
Using an Internal Source
One of the following internal signals can drive AO Sample Clock:
• AO Sample Clock Timebase (divided down)
•Counter n Internal Output
A programmable internal counter divides down the AO Sample Clock 
Timebase signal.
Using an External Source
Use any input PFI line as the source of AO Sample Clock.
Routing AO Sample Clock to an Output Terminal
You can route AO Sample Clock (as an active low signal) out to any output 
PFI terminal.
Other Timing Requirements
A counter on your device internally generates AO Sample Clock unless you 
select an external source. AO Start Trigger starts the counter and either the 
software or hardware can stop it once a finite generation completes. When 
using an internally generated AO Sample Clock, you also can specify a 
configurable delay from AO Start Trigger to the first AO Sample Clock 
pulse. By default, this delay is two ticks of AO Sample Clock Timebase.