The ADCs employed on DSA devices belong to a class of components called delta-sigma ADCs. One distinguishing feature of delta-sigma converters is that they use an oversample clock to drive the conversion. As the name implies, the physical frequency of the oversample clock signal is greater than the sample rate. When a single DSA device acquires data, the high-frequency oversample clock is locally generated by a Direct Digital Synthesis (DDS) chip on the device. If two or more 447x and 4461/4462 DSA devices are to be synchronized, they must share the oversample clock to ensure tight synchronization of the acquisition. Refer to the User Manual for your device for more details on the oversample clock. These devices do not support the reference clock functionality.
In NI-DAQmx, the oversample clock is referred to as the Sample Clock TimeBase. In PXI, for PXI-447x and PXI-4461/4462 devices the Sample Clock Timebase can only be driven from slot 2 onto the PXI Star trigger line. As such it is required that the device from which all other devices receive the Sample Clock Timebase must be in slot 2. Note:
Only full backplane PXI-4461/4462 and PXI-447x can be synchronized in this manner as hybrid devices do not have access to the PXI Star trigger lines.
In PCI, the Sample Clock Timebase can only be routed to RTSI 8. Note:
The 447x devices require a divided down version of the 4461/4462 oversample clock, since there is only a single RTSI line which a Sample Clock Timebase signal can be sent on this makes it impossible to have both PCI-4461/4462 and PCI-447x as slave devices in a synchronized system.
DSA devices can also export their Sample Clock. This signal goes high every time a conversion takes place. In PXI, this signal can be routed onto any of the PXI trigger lines, but not PXI Star. In PCI, it can be routed to RTSI lines 0-6. DSA devices can not import an external Sample Clock signal as its own Sample Clock, however this exported Sample Clock can be used by other DAQ devices for synchronization purposes.
Understanding these clock signals is important when synchronizing DSA devices with each other or with different devices. Below are some examples of how to perform some basic types of DSA synchronization using NI-DAQmx:
For the first two examples below, please review the DSA User Manual for more details on Synchronization of DSA devices. Synchronizing Analog Input on Multiple NI-447x or NI-4461/2 Devices:
LabVIEW 2012 and later includes a shipping example with NI-DAQmx called Analog Input - Synchronization
(NI Example Finder » Hardware Input and Output » DAQmx » Synchronization). Synchronizing Analog Input and Analog Output on Multiple NI-4462 Devices:
LabVIEW 2011 and earlier includes a shipping example with NI-DAQmx called Multi-Device Synchronization of DSA Analog Input and Output Using a Shared Timebase & Trigger with DAQmx.
Download NI-DAQmx Shipping Examples for LabVIEW 2012 or Compatible
Alternatively, from NI-DAQmx 8.0 onward there is a Multidevice Task feature which simplifies the synchronization process between DSA devices called Channel Expansion. This feature automatically shares the Sample Clock Timebase, Sync Pulse and Start Trigger between DSA devices specified in the NI-DAQmx Task to achieve synchronization. See the related links below for more information on this topic.