Keithley 6514, Instruction Manual

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3-10 Volts and Ohms Measurements
Input capacitance (settling time)
The settling time of the circuit is particularly important when making volts measurements of
a source that has high internal resistance (Figure 3-5A), or when making high-resistance ohms
measurements (Figure 3-5B).
In both cases, the shunt capacitance (C) has to fully charge before an accurate voltage mea-
surement can be made by V
M
of Model 6514. The time period for charging the capacitor is deter-
mined by the RC time constant (one time constant,
τ
= RC), and the familiar exponential curve
of Figure 3-6 results. Therefore, it becomes necessary to wait four or five time constants to
achieve an accurate reading. For example, if R = 100G( and the input cable has a nominal capac-
itance of 10pF, the RC time constant would be 1 second. If 1% accuracy is required, a single
measurement would require at least five seconds.
There are two basic ways to minimize this problem: (1) keep capacitance in the system to an
absolute minimum by keeping connecting cables as short as possible, and (2) use guarding.
There is, however, a limit to how short the cable can be. Using guard can reduce these effects
by up to a factor of 1000 (see “Guarding Input Cable”).