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Archived: Differences Between Analog and Digital Triggered Acquisition for NI DSA Devices
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When I configure my DSA device for analog acquisition with a digital trigger, I receive pretrigger samples that I did not request. When I configure a DSA board for analog triggered acquisition, I do not get any pretrigger samples. Why is this?
The following NI DSA hardware has a different architecture than that of other data acquisition devices.
- NI 4461 and 4462
- NI 4472, 4472B, and 4474
- NI 4492, 4495, 4496, 4497, 4498, and 4499
In this document we will compare the acquisition of standard MIO devices with the above listed NI DSA devices using the NI 4472 as an example. Specific hardware characteristics and specifications will be different, but the documented theme or behavior is accurate for the listed NI DSA devices.
Related Links: Knowledge Base 2UI8PGX4: Why Is My Data Delayed When Using DSA Devices?Knowledge Base 2WF8F22L: How Can I Synchronize NI PXI-44xx DSA Devices across Multiple PXI Chassis?NI Dynamic Signal Acquisition User Manual
- NI 4472 Special Hardware Features
Each analog input of a 4472 board is equipped with an analog and a digital filter.
- The analog filter consists of a two-pole low-pass Butterworth filter with a cutoff frequency of about 400 kHz which removes all of the high-frequency components before the signal reaches the Analog-to-Digital converters (ADC).
- Each channel has its own delta-sigma ADC, which include a digital filter that employs a FIR (Finite Impulse Response) architecture whose cut-off frequency tracks the sampling rate. The digital filter adjusts automatically to follow the Nyquist frequency, setting a cutoff frequency of 0.4863fs, where fs is the sampling rate specified. The inherent delay of the finite impulse response (FIR) filter is 38.7 sample periods.
Note: More information about filters and their inherent delay can be found in the NI Dynamic Signal Acquisition User Manual page(s) 2-7 to 2-10.
So, how does the inherent delay of the FIR filters affect my acquisition?
You will begin receiving data from your board 38.7 sample periods after the acquisition has started. For example, when acquiring a voltage signal, the acquired data will be placed in a 38.7 samples "pipe" of the FIR filter. If you are using a sampling rate of 102.4KS/s, it will take 38.7 * (1/102.4k) = 377.9us for the board to return the first sample after the acquisition has started. For the NI 4472, the data returned by the board will look different depending on the type of trigger being used because the triggers arrive at different moments (see Figure 1 below).
Figure 1: Trigger comparison for NI 4472
- Digital Triggered Acquisition
The ADCs on NI 4472 board are running continuously, so there are always samples in the 38.7 sample "pipe" of the FIR filter. When using a digital trigger, the trigger is received by the board before the ADC (see Figure 1 above), so the first sample that will be returned by the board corresponds to data that occurred 38.7 sample clock periods ago. So roughly, the first 39 samples of your acquisition will correspond to 39 points before the digital trigger was received. Most of the time these first 39 samples will not be valid data, and they need to be discarded. In the case of a standard MIO device, the "pipe" is very short (one sample), so the data being returned by the board corresponds to the moment when the digital trigger was received (see Figure 2).
Figure 2: Digital trigger comparison of NI 4472 to DAQ device
- Analog Triggered Acquisition
When using an analog trigger, the comparison between the level specified and the sampled data is done once the conversion has been done, after the sample has traveled through the pipeline (see Figure 1 above). So when using an analog trigger you will not get the previous 39 samples that you get when using a digital trigger. In this configuration, both a NI 4472 and a standard MIO device would return the same data. The NI 4472 would still take 38.7 sample periods to return the data to the program, but both data would be time-stamped at the same time (see Figure 3).
Figure 3: Analog triggering comparison of NI 4472 and DAQ device