Data Acquisition

Each board specializes in one type of transducer; thermocouple boards for interfacing tothermocouples, strain gauge boards for strain gauges, etc. These boards are typically usedfor small, specialized data acquisition systems that have a limited number of transducerslocated near the host computer.3.3.2 Direct connect modular – two-wire transmittersTwo-wire transmitters are two-port modular signal conditioning modules that input anunconditioned signal on the input port and output a conditioned signal on the output port.A single module is required for each type of transducer (or actuator). These signalconditioning modules are not intelligent devices and do not perform on-board A/D conversion.Instead, the conditioned analog signal is transmitted over two lines to the dataacquisition board in the host PC, either as a voltage, or converted into a standard currentloop signal (4–20 mA) to the data acquisition board, hence the name two-wire transmitters.The simplified functional block diagram of a typical two-wire transmitter signalconditioning module is shown in Figure 3.13.Figure 3.13Functional block diagram of a two-wire transmitter signal conditioning moduleVoltage outputs (±10 V or 0–10 V), compatible with the single ended inputs of mostdata acquisition boards allow easy interfacing to the latest data acquisition boardtechnology. However, due to voltage drops, which may occur on the signal lines and theeffects of noise that is proportional to the length of the transmission lines, voltage outputsshould only be used for short transmission lines.Current signals have much greater immunity to noise and can be transmitted over hundredsof meters (up to 1000 meters), to a receiver that converts the currents back into avoltage, for A/D conversion at the PC. The receiver is principally a resistor, nominally inthe order of 500 Ω for full-scale deviation of 10 V (500 Ω X 20 mA). A separate pair ofwires is used for the current loop of each individual sensor, resulting in many cable pairsto the PC. A power supply (between 15–40 V), capable of driving as many current loopsas there are modules, is required.