National Instruments SCXI -1122 User Manual

Also see for SCXI-1122: Getting started Quick start guide Quick start guide Instructions for safe use

Contents

National Instruments SCXI -1122 User Manual Manual pdf 39 page image

Theory of Operation Chapter 4SCXI-1122 User Manual 4-4 © National Instruments Corporationbanks which switch synchronously. The eight upper channels (0 through 7) operate as voltagesense channels and one out of eight is connected to the amplifier at any given point in time. Inaddition, the eight lower channels (8 through 15) operate as current output channels which switchin tandem with the sense channels. At any given point in time one and only one channel isconnected to the current output channels. Table 4-1 indicates the sense/current output channelassociations.Table 4-1. Sense/Current Output Channel AssociationsSense Current OutputChannel 0 Channel 8Channel 1 Channel 9Channel 2 Channel 10Channel 3 Channel 11Channel 4 Channel 12Channel 5 Channel 13Channel 6 Channel 14Channel 7 Channel 15The temperature sensor consists of a thermistor located on the SCXI-1322 terminal block. Thisthermistor connects via the temperature channel to the isolation amplifier. The temperaturesensor is for cold junction compensation of thermocouples. When measuring the temperaturesensor output, set your SCXI-1122 for a gain of five and 4 Hz filter. This will increase themeasurement resolution and accuracy as well as reduce noise.Note: With a 4 Hz bandwidth you must wait one second before you take the temperaturemeasurement to permit the system to settle. If you want to use the 4 kHz filter, take alarge number of samples and average them. To achieve 50 or 60 Hz rejection, youshould acquire data over an integral number of power line cycles, then average thisdata.The filtering consists of one of two low pass filters, 4 kHz (-3 dB) or 4 Hz (-10 dB), which youcan select via software control. These filters are cascaded and are located in two stages. This isdone to eliminate noise introduced by the isolation amplifier.The isolation amplifier fulfills two purposes on the SCXI-1122 module. The isolation amplifierconverts a small signal riding on a high common-mode voltage into a single-ended signal withrespect to the SCXI chassis ground. With this conversion, you can extract the input analogsignal from a high common-mode voltage or noise before it is sampled and converted by theDAQ board. The isolation amplifier also amplifies and conditions an input signal, which resultsin an increase in measurement resolution and accuracy. The isolation amplifier drives the analogoutput stage which consists of hardware circuitry which permits several module outputs tomultiplex into one DAQ board channel.The calibration hardware consists of a software-controlled shunt calibration resistor for straingauge calibration, an auto-zero calibration for nulling the amplifier offsets, and of an EEPROMwhich holds calibration constants for software correction of gain and offset of the isolationamplifier and of the current and voltage excitations. Refer to your software user manual(NI-DAQ, LabVIEW, or LabWindows) for further details and to the Excitation Calibrationsection in Chapter 5, Calibration.