Data Translation DT9828 User Manual

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Data Translation DT9828 User Manual Manual pdf 62 page image

Chapter 562Note: If you are continuously measuring from a properly configured thermocouple inputchannel and the thermocouple opens or becomes disconnected, the open thermocouplepull-up circuit causes the input voltage to rise to approximately 1.5 V over a few seconds.In this case, the temperature value rises very quickly, and you will receive theTEMP_OUT_OF_RANGE_HIGH (88888 decimal) value followed by the OPEN_SENSOR(99999 decimal) value.Input ResolutionThe input resolution of the analog input subsystem on the DT9828 module is 24 bits.A/D Sample ClockThe DT9828 module uses an internal A/D sample clock with a time base of 4.9152 MHz topace analog input operations in continuous mode. This clock paces the acquisition of eachchannel in the channel list.The maximum total throughput is 600 Samples/s. The minimum total throughput rate is0.5865 Samples/s.The data rate per channel is computed as follows:Data Rate per Channel = Throughput Rate/Number of ChannelsFor example, to sample all eight channels at the maximum rate, the data rate per channel is600/8, or 75 Samples/s.Using software, specify the channels that you want to sample in the channel list, and then setthe desired sampling frequency. The software sets the sampling frequency to the closestpossible value. You can use software to return the actual frequency that is used.Note: In QuickDAQ, you specify the per channel sampling frequency. In the DT-OpenLayers Class Library and the Data Acq SDK, you specify the aggregate (total) samplingfrequency. See Table 2 for examples of the per channel sampling frequency based on thenumber of channels.According to sampling theory (Nyquist Theorem), specify a frequency that is at least twice asfast as the input’s highest frequency component. For example, to accurately sample a 20 Hzsignal, specify a sampling frequency of at least 40 Hz. Doing so avoids an error conditioncalled aliasing, in which high frequency input components erroneously appear as lowerfrequencies after sampling.