Correct Delayed Data When Synchronizing Dynamic Signal Analyzer Devices

Issue Details

I observe a delay, skew, or phase shift between waveforms when synchronizing a Dynamic Signal Analyzer (DSA) device with another acquisition device. For example:

• I synchronize a DSA device and a standard NI-DAQ device using a shared digital start trigger, but the acquired waveforms do not align.
• I acquire the same signal at the same sample rate with a shared start trigger, but the waveform from the DSA device lags behind the waveform from the NI-DAQ device by several samples.
• I synchronize multiple DSA devices or NI C Series DSA modules using channel expansion, but one waveform appears delayed relative to another.

Solution

This behavior is typically caused by the anti-aliasing filter delay present in devices that use Delta-Sigma analog-to-digital converters (ADCs).

Use one of the following methods to compensate for the delay.

Option 1: Use Channel Expansion

If the devices can be included in a single task using channel expansion and support Filter Delay Removal, NI-DAQmx automatically compensates for the filter delay.

Requirements:

• Configure the devices in a single task.
• Use the same sample rate for all devices.
• Use hardware that supports Filter Delay Removal.

Devices such as NI C Series DSA modules, PXI-4461/4462, and PXI-447x modules can be synchronized using channel expansion, but NI-DAQmx does not automatically remove their filter delay.

Option 2: Use Filter Delay Removal or Filter Delay Adjustment

If the devices cannot be configured in a single task, and the DSA device supports Filter Delay Removal, enable the following DAQmx property:

Note: Not all DSA devices support Filter Delay Removal.

If the device supports Filter Delay Adjustment, you can configure the following DAQmx properties:

These settings can be used to fine-tune alignment between synchronized acquisitions.

Option 3: Compensate for the Delay Manually

If Filter Delay Removal is not supported, manually discard the initial samples affected by filter delay.

To compensate:

1. Start both acquisitions.
2. Read and discard an appropriate number of samples from the DSA device.
3. Begin reading data simultaneously from all devices.
    

Note: Determine the number of discarded samples either experimentally or by reviewing the filter delay, input delay, or group delay specifications for the device.

Option 4: Use Analog Triggering

If the acquisition uses analog triggering, the filter delay is typically not observed in the triggered data because the analog trigger circuitry is located after the anti-aliasing filter.

For additional details, refer to the Dynamic Signal Acquisition User Manual.

Additional Information

Many Dynamic Signal Analyzer (DSA) devices use Delta-Sigma ADCs that incorporate anti-aliasing filters. These filters remove frequency components above the Nyquist frequency but introduce a delay commonly referred to as:

• Filter delay
• Group delay
• Input delay

The delay value depends on the device architecture and operating configuration and is typically documented in the device specifications.

Because this delay exists only on devices that use Delta-Sigma ADC technology, waveforms acquired from these devices may appear delayed when compared with data from devices that do not use Delta-Sigma ADCs.