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 29 page image

Chapter 3 Signal Connections© National Instruments Corporation 3-7 SCXI-1122 User ManualVs RbCcVcm+-Figure 3-5. AC-Coupled Signal Connection with High Common-Mode VoltageFor AC-coupled signals, connect an external resistor from the positive input channel to the signalreference to provide the DC path for the positive input bias current. Typical resistor values rangefrom 100 kΩ to 1 MΩ. This solution, although necessary in this case, lowers the inputimpedance of the input channel amplifier and introduces an additional offset voltage proportionalto the input bias current and to the resistor value used. The typical input bias current of theamplifier consists of ±80 pA and a negligible offset drift current. A 100 kΩ bias resistor resultsin ±8 μV of offset, which is insignificant in most applications. However, if you use largerresistors, significant input offset may result. To determine the maximum offset the biasingresistor will introduce, use the following equation:V ofsbias = Ibias x RbiasThe input signal range of an SCXI-1122 input channel is ±10 V/ G total referenced to its negativeinput, where G total is equal to the gain selected on the SCXI-1122. In addition, the input channelsare overvoltage protected to 250 Vrms with power on or off at a maximum of 2.5 mArms sink orsource.Note: The SCXI-1122 input multiplexer is composed of relays. Relays have a certain lifeexpectancy, as listed in Appendix A, Specifications. To avoid mechanical wear on therelays, and when you are acquiring a large number of points per channel andaveraging, you should acquire the n samples on a given channel before proceeding tothe next channel. For example, rather than performing 100 scans and taking a singlesample from each channel during each scan, as shown in Figure 3-6a, acquire100 points on each channel then switch to the next channel and acquire a new set ofsamples, as shown in Figure 3-6b.