【正文】 for every data point that is required, the DAQ system acquires and averages many voltage readings. For example, a mon approach is to acquire 100 points and average those points for each measurement needed. IsolationImproper grounding of the DAQ system is the most mon cause of measurement problems and damaged DAQ boards. Isolated signal conditioners can prevent most of these problems by passing the signal from its source to the measurement device without a galvanic or physical connection. Isolation breaks ground loops, rejects high monmode voltages, and protects expensive DAQ instrumentation. Common methods for circuit isolation include the usage of optical, magnetic, or capacitive isolators. Magnetic and capacitive isolators modulate the signal to convert it from a voltage to a frequency.The frequency can then be transmitted across a transformer or capacitor without a direct physical connection before being converted back to a voltage value. This voltage is referred to asmonmode voltage. If a singleended measurement system is used, as shown in Fig. , the measured voltage includes the voltage from the desired signal,VS, as well as this monmode voltage from the additional ground currents in the system,VG. If a DAQ board with differential inputs is used, some of the monmode voltage, typically up to 12V can be rejected. However, larger ground potential differences, or ground loops, will damage unprotected DAQ devices. MultiplexingSignal conditioners equipped with signal multiplexers can effectively expand the input/output (I/O) capabilities of the plugin DAQ board. The typical plugin DAQ board has 8–16 analog inputs and 8–24 digital I/O lines. External multiplexers can increase the I/O capacity of a plugin board to hundreds and even thousands of channels.Analog input multiplexers use solidstate or relay switches to sequentially switch, or scan, multiple analog input signals onto a single channel of the DAQ board. Digital Signal ConditioningDigital signals require signal conditioning peripherals to directly connect the signals with the DAQ board. But it is not a good practice to directly connect digital signals used in research and industrial environments to a DAQ board without some type of isolation because of the possibility of large voltage spikes or large monmode voltages. Some signal conditioning modules and boards optically isolate the digital I/O signals to remove these problems. Digital I/O signals can control electromechanical or solidstate relays to switch loads such as solenoids, lights, motors, and so on. Solidstate relays can also be used to sense highvoltage field signals and convert them to digital signals. Pulse OperationA single pulse can be used to sample a variable capacitor, like a microputer read pulse, or a train of pulses can be used. This method can result in simpler electronics but will have higher noise. Signal Conditioning Systems for PCBased DAQ SystemsThe signal conditioning functions discussed in this section is implemented in different types of signal conditioning products. These products cover a very wide range of price and capability. For example, the National Instruments SC207X Series termination boards have a temperature sensor for use with thermocouples and a breadboard area with silkscreened ponent locations for easy addition of current measurement resistors, simple resistance–capacitance (RC) filters, and other signal conditioning circuitry.The 5B Series of signal conditioning I/O modules is for specific types of transducers or signals. Up to 16 I/O modules can be installed in a backplane and directly connect the modules to a plugin DAQ board. For external signal multiplexing, the AMUX64T analog multiplexer board expands the analog input capability of the I/O multifunction board up to 256 channels. The AMUX64T also includes a temperature sensor and silk screened ponent locations. The SCXI product line is a signal conditioning system that bines the expandability of multiplexing with the flexibility of modular signal conditioning. Signal Conditioning with SCXISignal conditioning extension for instrumentation (SCXI) is a signal conditioning and instrumentation front end for plugin DAQ boards in LabVIEW. An SCXI system consists of an SCXI chassis that houses one or more signal conditioning modules that multiplex, amplify, isolate, and condition both analog and digital signals. The SCXI system then passes the conditioned signals to a single plugin DAQ board for acquisition directly into the PC. A variety of SCXI modules are available with different signal conditioning capabilities.For example, the SCXI1120 module is an eightchannel isolation amplifier module. Each of the input channels includes an isolation amplifier with gains of up to 2,000 and a low pass filter configurable for gains of 4Hz or 10kHz. The SCXI1121 module is a fourchannel isolation amplifier module that also has four channels of excitation. The user can configure each excitation channel for voltage or current. The module also includes halfbridge pletion circuitry for straingauge measurements. Terminal blocks for the SCXI modules include temperature sensors for coldjunction pensation with thermocouples.The signal conditioning connector (SCC) series is a modular portablesignal conditioning system. SCC consists of single and dualchannel signal conditioning modules with builtin signal connectors. For example, the SCCTC02 provides amplification, filtering, coldjunction pensation, and a convenient mini plug connector for one thermocouple input. Any bination of SCC modules can be installed onto an SCC carrier or backplane, such as the SC2345. The SC2345 holds up to 18 SCC modules and cables directly to an E Series DAQ board or module.Signal conditioning is an important ponent of a plete PCbased DAQ system. Signal conditioning has many features that are used to connect sensors such as thermocouples, RTD