Cable Effects - Keithley DMM7510 Reference Manual

Section 4: Measure considerations Model DMM7510 7½ Digit Graphical Sampling Multimeter Reference Manual

Cable effects

A voltage this small can easily be generated during handling and connecting of the DUT. Therefore,
you may need to protect the DUT during handling. One method for protecting the DUT is to short the
DUT during handling, and then remove the short before testing.
Figure 140: DMM shorted during handling
As long as the dry circuit limits the voltage between HI and LO, this should be a good solution.
However, when the short is in place, there is test current flowing through the cable that connects the
HI and LO of the DMM to the DUT. When the short is removed, the energy stored in this cable can
force a voltage across the DUT that far exceeds the 20 mV to 30 mV limitation.
If the HI and LO connection is modeled as a transmission line, when the short is opened, the
maximum voltage across the DUT is V = I R . If I is 10 mA and R is 50 ohms, V is 0.5 V,
dut test c test c dut
which is more than 30 times the limit. The voltage on the DUT oscillates as the signal bounces back
and forth in the cable until losses absorb the energy, as shown in the simulation plot below. The
second plot shows the voltage between HI and LO during the same simulation.
Figure 141: HI and LO modeled as a transmission line
4-12 DMM7510-901-01 Rev. B / May 2015