【正文】 4T 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, RTDs, strain gauges, and currentoutput devices to PCbased DAQ boards. Signal conditioning improves the accuracy, effectiveness and safety of the measurements, for any type of sensor used in the measurement because of capabilities such as amplification, isolation, and filtering. The National Instruments SCXI product line can supply the signal conditioning and instrumentation front end required for the PCbased DAQ systems. AnalogtoDigital ControlConnecting digital circuitry to sensor devices is simple if the sensor devices are inherently digital in nature. Switches, relays, and encoders are easily interfaced with gate circuits due to the on/off nature of their signals. However, when analog devices are involved, interfacing bees much more plex. Some mechanism is needed to electronically translate analog signals into digital (binary) quantities, and visaversa. An analogtodigital converter, or ADC, performs the former task while a digitaltoanalog converter, or DAC, performs the latter. An ADC inputs an analog electrical signal such as voltage or current and outputs a binary number. In block diagram form, it can be represented as shown in Fig. . Understanding Integrating ADCsIntegrating ADCs provide high resolution A/D conversions, with good noise ideal for digitizing low bandwidth signals,and are used in applications such as digital multimeters and panel meters. They often include LCD or LED drivers and can be used stand alone without a microcontroller host. The following section explains how integrating ADCs work. Discussions include single, dual and multislope conversions. Also, an indepth analysis of the integrating architecture will be discussed.Finally a parison against other ADC architectures will aid in the understanding and selection of integrating ADCs. Integrating analogtodigital converters (ADCs) provide high resolution and can provide good line frequency and noise rejection. Having started with the ubiquitous 7106, these converters have been around for quite some time. The integrating architecture provides a novel and straightforward approach to converting a low bandwidth analog signal into its digital representation. These types of converters often include builtin drivers for LCD or LED displays and are found in many portable instrument applications, including digital panel meters and digital multimeters.SingleSlope ADC ArchitectureThe simplest form of an integrating ADC uses singleslope architecture(Figs. , b). Here, an unknown input voltage is integrated and the value is pared against a known reference value. The time it takes for the integrator to trip the parator is proportional to the unknown voltage (VINT/VIN ). In this case, the known reference voltage must be stable and accurate to guarantee the accuracy of the measurement.One drawback to this approach is that the accuracy is also dependenton the tolerances of the integrator’s R and C values. Thus in a productionenvironment, slight differences in each ponent’s value change the conversion result and make measurement repeatability quite difficult to attain. To overe this sensitivity to the ponent values, the dualslope integrating architecture is used.DualSlope ADC Architecture A dualslope ADC (DSADC) integrates an unknown input voltage (VIN)for a fixed amount of time (TINT), then “disintegrates” (TDEINT)usingaknown reference voltage (VREF) for a variable amount of time (Fig. ). The key advantage of this architecture over the singleslope is that the final conversion result is insensitive to errors in the ponent values. That is, any error introduced by a ponent value during the integrate cycle will be canceled out during the deintegrate phase. In equation form: