National Instruments NI 4350 User Manual

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National Instruments NI 4350 User Manual Manual pdf 30 page image

Chapter 3 NI 4350 Operation©National Instruments Corporation3-9NI 4350 User ManualAuto-ZeroAuto-zero is a method that instruments use to remove any offset errorsin the measurement. Analog channel 1 (CH1) on the PSH32-TC6,CB-27T, TC-2190, and TBX-68T is dedicated for auto-zero. CH1+ isconnected to CH1– on these accessories. You can measure the voltageoffset on this auto-zero channel and subtract it from the voltagemeasurements on other channels. This way, you can compensate for anyresidual offset error the NI 4350 instrument may have. This isespecially useful when your NI 4350 instrument is operating at anambient temperature other than that of calibration (23° C typical).Note: When using the VirtualBench-Logger along with NI 4350 accessories—PSH32-TC6, CB-27T, TC-2190, or TBX-68T—auto-zeroing isimplemented automatically.Programmable Ground-ReferencingIf you determine that your thermocouple is ground-referenced, switchoff ground-referencing on that channel.If you determine that your thermocouple is floating, switch onground-referencing on that channel. Otherwise, the thermocoupleinputs may float out of the input common-mode limits of the NI 4350instrument.When you use the PSH32-TC6, CB-27T, TC-2190, and TBX-68Taccessories, always switch on ground-referencing on CH1. Doing thisground-references the auto-zero channel.On all the NI 4350 instrument accessories used with thermocouples,analog channel CH0 is dedicated to the thermistor cold-junction sensor.The built-in current source return terminal IEX- is tied to –2.5 Vthrough a resistor. This references any resistor excited by the currentsource to ground. Since this current source excites the cold-junctionthermistor, CH0 is automatically ground-referenced. Therefore, whenmeasuring the voltage across this thermistor, always switch offprogrammable ground-referencing on CH0. Otherwise, the leakagecurrent flowing into the thermistor may cause erroneous measurementsin all the channels that use the current source.Note: When using VirtualBench-Logger, the DAQ Channel Wizard, or theNI435X Instrument Driver, the ground-referencing switch on thecold-junction sensor channel and auto-zero channel is set appropriately,automatically.