Voltages with Large Common-Mode Components
Switching signals with large common-mode components (LO switched from +250 V to -250 V) may, over extended periods of time (tens of Hz for several weeks), begin to degrade the relay performance. The switching life of the relays can be extended in many cases by inserting some small common-mode resistance in series with the LO terminals. Even several hundred ohms (200 to 500 Ω) helps significantly.
Refer to the following figure, where VCM1 or VCM2 are >50 V:
In these situations, break-before-make switching is essential to obtaining any kind of reliable relay life. If S1 and S2 are turned on together, even momentarily, VCM1 + VSIG1, is shorted to VCM2 + VSIG2, which could cause high currents to permanently damage the contacts of S1 or S2. The transient signal generated when switching signals with large common mode voltages closely resemble the effect of applying signals that exceed the V-Hz limit. This could affect the internal transfer of data in the device and would generate an error prompting you to reset the board and reconfigure your measurement. Refer to Handling High DC Voltages for more information.
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Tip If the NI 4070/4071/4072 experiences the conditions where the transients cause the violation of the V-Hz limit of the instrument while the instrument is making measurements, NI-DMM driver returns error NIDMM_ERROR_SERIAL_PORT_ERROR (defined as 0xBFFA401F, or -1074118625). If your system can potentially face this situation, your application should look for this error code after a call to:
Your application can handle this error by calling the following VIs or functions:
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