Data acquisition.

A DAQ system typically includes hardware for measuring, controlling and switching. The measurement hardware consists of the analog, digital and counter inputs of the DAQ. The analog inputs usually record DC voltages, which are then converted into digital data via an analog-to-digital converter (ADC). The measured voltages correspond, for example, to defined temperature values or speeds. DAQs with a digital module can scan the bit patterns of external devices under test (DUT). On the other hand, data loggers with a counter module can count external events such as the number of digital pulses, the pulse duration or the frequency.

The control hardware contains the output component of a DAQ. A digital-to-analog converter (DAC) interprets the commands from the control hardware and outputs them as DC voltages or currents. This analog output can control the position of a valve or the flow rate of a pump.
DAQ systems with a digital module convert commands into a bit pattern and control, for example, lights or other systems.

The switching hardware can be used, for example, to record the measured values from sensors sequentially or synchronously. Signal conditioning hardware converts the signals from the transducer into a measurable form by amplifying, attenuating, linearizing or isolating the signals before they are sent to the measurement hardware.

Yes, modern systems allow the visualization of measurement data in real time. The values are displayed graphically or in tabular form directly during recording. This requires high-performance hardware and optimized software solutions.

Modular DAQ systems, e.g. from NI, are generally PC-based data acquisition systems that offer various possible combinations of hardware and software components for customized test solutions. Modular systems can be adapted to specific requirements and offer flexible application options. The hardware is connected to the PC via USB or Ethernet. This connection can be direct, as a multifunction I/O device, or via a chassis equipped with conditioned I/O measurement modules. For the optimum configuration of a modular data acquisition system for a test application, it is important to precisely define the requirements of the application.

Multifunction I/O data acquisition devices are connected to a PC via USB or PCI/PCIe; they offer combinations of analog and digital inputs/outputs as well as counter/timer functions. The devices are suitable for industrial DAQ applications as well as for automation applications in the laboratory, research and design verification. Associated software products usually provide the corresponding measurement and analysis functions.

The measurement resolution describes how many details are recorded during a measurement, i.e. how many significant measurement points are recorded per unit of time. Accuracy is a measure of how reliable the measurement results are. A higher resolution does not per se mean higher accuracy. A 6.5-digit voltmeter with poor accuracy is no better than a 5.5-digit voltmeter with good accuracy.

The sensitivity of the DAQ system describes the smallest change in the measured signal that can be detected. It depends on both the resolution and the smallest measuring range of the DAQ. The smaller the measuring range, the smaller the signal changes that can be measured.

The measuring speed is the speed at which the analog-to-digital converter of the DAQ acquires the measurement data, i.e. the time span between the acquisitions. As the sampling speed of the ADC increases, the resolution of the measuring device decreases. The data sheet can be used to assess whether the highest required speed of the DAQ meets your requirements for the measurement solution.

In addition to its specified accuracy (data sheet), the measuring accuracy of a data logger depends on various factors that the user can influence and optimize. Common measures include, among others:

• Selecting the connected sensor to match the behavior of the physical parameters.
• Selecting the measurement hardware according to the desired signal processing (particularly with modular DAQ systems).
• Using shielded twisted pair cables.
• Selecting the amplitude range and frequency bandwidth to match the physical parameters.
• Using differential inputs.
• Using a DAQ system with noise suppression.

Various criteria have to be taken into account when selecting the right data logger in order to configure the DAQ system optimally for a specific test application. It is therefore important to define the requirements of the application at an early stage. The answers to the following questions provide a guide to the selection of the device:

• What type of signals are to be measured?
• How much data has to be recorded?
• What maximum/minimum input voltage is to be connected to the data logger?
• How many measuring channels are required?
• Are all the channels to be scanned simultaneously (synchronously) or asynchronously?
• What is the available budget for the equipment?
• What types of switching relays are required (electromechanical, Reed or solid-state relays)?
• How high does the minimum/maximum sampling rate need to be (number of measured values per second)?
• How long is data collection to take (minutes, hours, days, weeks)?
• What measurement accuracy is required?
• Which interfaces are required for data transfer (storage, analysis)?