Model 2790 SourceMeter® Switch System User’s Manual 4-9Figure 4-6Installed Zippertubing® shieldGuardingNOTE The typical high ohms measurement applications for the 7751/7753 V-sourcedoes not require guarding. Guarding only needs to be used for the most demand-ing, noisy situations, or when measuring >1GΩ (which is the maximum resis-tance specified for the 7751/7753).A second inner shield can be used for high ohms measurements that use the 7751/7753V-source and I/V converter. This guarded shield may reduce noise and cable leakage thatcould result in measurement errors.The guard terminals are available at J105 and J107. Guard is connected to a shield thatsurrounds the Source Lo and Sense Lo test leads that connect to the housing of the inflator.In Figure 5-3 (Section 5), use the guard shield to surround the two wires that are con-nected to Bank 3 (J103) of the 7751 module.This guard shielded wire pair should be positioned inside the main cable bundle whoseshield is connected to safety earth ground. The guard shield should be insulated with anouter jacket. A second Zippertubing shield can be used as the guard shield.WARNING The guard terminals are connected to the floating ground of the7751/7752/7753 module. Do not connect V-source Hi to earth ground. IfV-source Hi is connected to earth ground, guard could float up to 500V.WARNING Do not connect V-source Hi to earth ground. The 7751/7752/7753 mod-ule uses a floating ground. This floating ground is not connected tosafety earth ground. Not only will it create a shock hazard by floatingup to 500V, it will also effectively connect a 1nF capacitor across theV-source, providing energy storage that could ignite an airbag underfault conditions.Ground wrappedcable bundleTestStationSafety EarthGroundKeithleySwitchingModuleKeithley 7751, 7752,7753, or 7702GUARDSOURCE SENSE SRCHISRCHISRCLOSRCLOSENHISENHISENLOSENLOAABANK 1BANK 2BBHI HILO LOHISRCHISRCLOSRCLOSRCHISENHISENLOSENLOSENAALOI / VGUARDISOURCEVSOURCEINTERLOCKOUTHIGNDGND HI INLOLO BANK 4BANK 3I / VBB
