National Instruments AMUX-64T User Manual

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National Instruments AMUX-64T User Manual Manual pdf 34 page image

Chapter 3 Signal Connections©National Instruments Corporation3-5AMUX-64T User ManualDifferential ConnectionsOn the AMUX-64T, channels 0 through 31 are connected to channels 0through 7 of the MIO board. AMUX-64T channels 32 through 63 areconnected to channels 8 through 15 of the MIO board. If the MIO boardis configured for differential mode, the AMUX-64T input channels areautomatically used in differential mode. The input screw terminals on theAMUX-64T are grouped together such that for differential mode, all inputsignals (SIG+) and the corresponding signal return path (SIG-) input appeardirectly next to each other. For example, signal return path for channel 0 ischannel 32, and the signal return path for channel 31 is channel 63.Using the AMUX-64T for Thermocouple MeasurementsThe AMUX-64T is equipped with a temperature sensor for thermocouplecold-junction compensation. Because thermocouple output voltages aretypically a few millivolts, you must use a high-gain board (any speed) forbest resolution. Thermocouples may be measured in either differential orsingle-ended configurations. Differential connection tends to yield the bestresults, but single-ended connection allows twice as many thermocouplesto be used on each AMUX-64T.The cold-junction compensation is accurate only if the temperature sensorreading is close to the temperature of the screw terminals. Therefore, whenthermocouples are being read, you should keep the AMUX-64T away fromdrafts or other temperature gradients such as those caused by heaters,radiators, fans, very warm equipment, and so on.Selecting the Gain and Input RangesSince thermocouple output voltages are very low, a gain of 500 or 100 isusually necessary for best resolution. You should set the input range on theMIO board to ±5 V to improve resolution. You can use these settings in allbut a few cases, such as a fairly high-output thermocouple type that is beingused at elevated temperatures. Table 3-2 lists the voltage extremes fromseveral popular thermocouple types. Use it as a guide for determining thebest gain and input range settings to use. If you are using the thermocouplesin a known temperature range, consult a book of thermocouple tables todetermine the approximate millivolt output and the best gain and inputrange settings.