I have two or more C Series modules that have a delta-sigma ADC, each in their own cDAQ chassis. How can I synchronize these modules?

Unlike other C Series modules, modules with delta-sigma ADCs (DSAs) require synchronized Sample clock timebases in order to achieve sample clock synchronization.

Since Programmable Function Input (PFI) lines built into CompactDAQ chassis have a maximum bandwidth of 1 MHz, which is lower than the internal timebases of all DSA modules, these PFI lines cannot be used to implement Sample Clock Synchronization between DSA modules on multiple cDAQ chassis.

In order to synchronize DSA modules, there are several potential solutions. In order of simplicity:

1. Connect two or more Time-Sensitive Networking (TSN)-enabled cDAQ-9185 or cDAQ-9189 chassis and utilize their time synchronization features to synchronize the timebases of C Series DSA modules. Data collected from these modules will still need to be adjusted for group delay, but will be automatically synchronized.
2. If you don't count with TSN enabled chassis, use a NI-9469 Chassis Synchronization Module in each cDAQ chassis to export timebases in a master-slave configuration. Refer to How to Synchronize Multiple cDAQ Chassis Using the NI-9469 C-Series Module for details.
3. You can alternatively use NI 9402 C Series Digital Module to share the required signals. Unlike the CompactDAQ chassis built-in PFIs, the NI 9402's have enough bandwidth to share the signals. You can use the DAQmx Connect Terminals VI to export the internal timebase on a Delta-Sigma module in the master CompactDAQ through the PFI on the NI 9402. In the slave chassis, you can set the PFI from a second NI 9402 as the sample clock timebase source. This method can also be used for PXI - CompactDAQ synchronization.

NI recommends using TSN enabled devices for multi-chassis synchronization whenever possible or the NI 9469 of those are not available. Method 3 should only be used if the other methods are not feasible since inappropriate cabling and the added programming complexity can introduce additional possible error sources for the synchronization.

Refer to Time-based Synchronization of Analog Input C Series Modules with NI-DAQ™mx in LabVIEW for more details about performing the synchronization using TSN enabled chassis, and to Signal-based Synchronization of Analog Input C Series Modules with NI-DAQmx in LabVIEW for instructions using the NI 9469 to synchronize the devices. While not explicitly mentioned, the later can be used to identify the properties required to use method 3.

Additional Information

This article: How to Achieve High-Accuracy Measurements With NI-DAQmx-Based TSN Devices explains that the synchronization accuracy of TSN protocol is between 1μs to hundreds of nanoseconds, depending on the system's configuration.

The NI-9469 accuracy is +-3.5 ppm, so the synchronization accuracy (Time Drift) = ( [ Accuracy in PPM ] / 1 Million) * ( # Seconds Per Day ) = (3.5/ 1 Million) * ( 86400) =  +-0.3024 [Seconds/Day]. ​​

DAQmx Connect Terminals enabled routes remain after stopping the task. You need to use the DAQmx Disconnect Terminals VI or power cycle the device to disable the routes.

If you're not sure whether you require this type of synchronization, you can find a complete list of the C Series modules that need the NI-9469 or a TSN-enabled cDAQ-9185/cDAQ-9189 in order to be synchronized across multiple cDAQ chassis by looking at the "C Series Delta Sigma Devices" section of C Series Device Groupings.