Applications for Various Trigger Modes in Data Acquisition Systems
Chris Ni, ADLINK Technology Inc.
A typical data acquisition system uses various types of sensors to collect signals from the surrounding environment or from each piece of equipment undergoing testing. In general, these signals are randomly generated. Therefore, data acquisition systems must constantly take samples in order to ensure that all essential data is collected, resulting in a large quantity of data. However, a storage limiting problem exists because the capacity of memory or hard drives is fixed, restricting the storage space of such a data acquisition system. Hence, sampling the significant part of a continuous signal is a vital consideration for data acquisition card (DAQ) designers. Accordingly, triggers are an important means by which to single out vital data and eliminate this problem.
 What is a trigger?
During data acquisition, users can set certain signal conditions, such as the logical high or low of a digital signal, or a specific voltage value. Once the signal meets these specified conditions, the DAQ will then be triggered to sample data and send it to the system. This is the fundamental principle of triggering. Trigger functions can be used in numerous applications, such as pulse testing in power transmission systems (which directly sets the pulse voltage as the trigger condition), synchronizing the operation of multiple DAQs (which uses a shared clock source to generate a trigger signal), pinpointing signal sampling of integrated motion controller dynamical systems (which sends a trigger signal after the mechanism is positioned to begin data acquisition) and etc. Properly applying triggers can ensure accurate sampling of valuable data, greatly improving system effectiveness and test precision.
Figure 1. Basic Trigger Diagram
An introduction of trigger technologies is currently available on DAQs, and the description of how to design these trigger methods in your systems illustrate below.
Trigger Signal Types
As described above, the fundamental principle of triggering is to give a trigger signal to "stimulate" the DAQ to take a sample. Trigger signal types can typically be divided in to the following:
1. Digital Trigger
A TTL signal triggers the DAQ through an external input. Users can usually set triggers based on the raising or falling edge of TTL signals. Implementing digital signals is the simplest; generally achieved through the logic gates of a CPLD. Hence, most DAQs, including the NuDAQ series from ADLINK Technologies, offer digital triggering capabilities.
Figure 2. Digital Trigger Diagram
2. Analog Trigger
Another triggering method is by a voltage signal and presetting a certain voltage value. When the voltage signal is above or below the specified value, a trigger is fired. Analog triggers can be used to detect swift changes in a continuous voltage signal. For example, in a power transmission system, users can set the input signal trigger voltage level. Once this level is exceeded, sampling will take place. This can be used to detect pulses in power systems. Analog triggers require a relatively complex circuit design-typically including an extra ADC component and comparator circuit. Hence, usually only high-end DAQs, such as the DAQ-2000 series from ADLINK Technology, or the E and M series from National Instruments, offer analog triggering.
Figure 3. Analog Trigger Diagram
In addition to the triggering methods described above, where an analog trigger is fired when the signal passes above or below the specified voltage level, a new generation of DAQs also support more complex analog triggering conditions. For example, the DAQ-2000 series of DAQs from ADLINK Technology allows users to set two groups of triggering voltage levels (high threshold and low threshold). Based on the relationship between trigger signal and trigger level values, users can set multiple trigger conditions, including below-low, above-high, high-hysteresis, low-hysteresis, inside-region and etc. The following is an example using high-hysteresis to describe these advanced triggering conditions.
Figure 4. High-Hysteresis Trigger
High-hysteresis trigger (Figure 4) occurs when the trigger signal exceeds High_Threshold, the triggering conditions are met and sampling will begin. However, how this differs from traditional voltage level triggers is that there are no triggers fired until the trigger signal falls below Low_Threshold. How can such triggering conditions be used? In the real world, trigger signals may also carry high levels of noise, causing the trigger signal to constantly be above or below the trigger level, resulting in unanticipated triggering behavior. After high-hysteresis trigger conditions are met, the trigger signal must drop below Low_Threshold in order for the next trigger to fire, providing tightly controlled and extremely accurate triggering conditions.
Trigger Signal Source
Regardless whether an analog or a digital trigger signal is used, it must be connected to the trigger signal source input of the DAQ in order to be effective. In general, trigger signal sources include:
1. Dedicated analog trigger input
2. Dedicated analog/digital trigger input
3. Specific analog input channel
4. Specific digital input channel
The pin definition for the DAQ-2010 from ADLINK is used as an example below. Pins 5 and 48 are dedication AI analog trigger inputs. Pin 47 is a dedicated AO digital trigger input. Channels 1 through 4 are analog trigger inputs.
Trigger Modes
After a certain condition is met and the signal enters the DAQ, a logic circuit (FPGA or PLD) on the board will drive an ADC to start the sampling processes. The basic way to think of it is once a trigger is fired, sampling will start immediately. Yet with the advancement of FPGAs and PLDs, we can plan for other trigger modes in these logic components. The following includes a list of trigger modes commonly found on mid- to high-end DAQs.
1. Post-trigger
Post-trigger is the simplest trigger mode. After a user sends a "start sampling" command, the FPGA or PLD will start DMA and wait for the trigger. After the trigger is generated, sampling begins immediately and will continue until the number of sample points (as set by the user) is collected, or a "stop" command is received. For pulse testing, several DAQs in synchronization can use this trigger mode.
2. Delay-trigger
Delay-trigger is used in the situation where there is a certain amount of delay from when the trigger occurs to when we want to sample data. For example, in the 802.11 WLAN protocol, there is a "guard period" (think of it as a time when the signal voltage is 0) between each transmitted frame. Each frame also has a preamble that follows a set format; followed by modulated significant data. To capture the significant data, we can set the preamble as the triggering condition. After the trigger fires, there is a slight delay before sampling starts. Hence, we can ignore the guard period and preamble to obtain significant data. This mode is called delay-trigger. As shown in the figure above, when using a delay-trigger, and when trigger conditions are met, the FPGA/PLD will delay a period of time before driving the A/D circuit to begin sampling. Users can use program setting delay time values to accurately capture necessary data.
3. Pre-trigger
In some applications, vital data might not be available after a trigger, but before the trigger fires. For such cases, a pre-trigger mode can be used. In the pre-trigger mode, data acquisition starts immediately after a user sends the "start" command. Data DMA will continuously steam in to the system buffer. Once trigger conditions are met, data acquisition will stop and data collected before the trigger will be sent back to the user. During destructive testing, researchers usually only care about what happens before structural break-down. For this type of application, we can set the structural break-down as the triggering condition (when structural break-down occurs, there is usually a large vibration or sound signal associated with it) for a pre-trigger mode. This can be used to easily capture signals before the break-down.
4. Middle-trigger
Middle-trigger is an extended pre-trigger. With a middle-trigger, users can acquire data both before and after the trigger. As shown in the figure above, users can set M (pre-trigger) + N (post-trigger) data to observe signal changes before and after a trigger.
Conclusion
Signals sampled by every type of data acquisition system have distinct characteristics. When designing a system, selecting appropriate trigger conditions and modes can help users filter out unnecessary data to retain only the significant portion. As hardware technologies advance, newly-designed DAQs should support all types of triggering functions to meet the numerous types of signaling requirements. For each data acquisition or test and measurement system design, in order to collect data efficiently, many factors must be considered, such as thoroughly analyzing signal characteristics, determining proper triggering conditions/modes and selecting the appropriate DAQ.
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