Keithley 6485, User Manual

Page: 49 / 115

Model 6485 and 6487 User’s Manual Connections 2-9
Figure 2-9
Shielding for Model 6487 measurements (unguarded)
HI
LO
DUT
INPUT
B. Safety Shield
Safety
Earth
Ground
Chassis
Ground
Metal Noise Shield
Metal Safety
Shield
HI
LO
DUT
INPUT
Metal Noise Shield
A. Noise Shield
Input voltage overload (OVRVOLT message)
During normal operation, there should not be a significant voltage between the input HI
and LO terminals of the Model 6485/6487. However, occasionally, as in the case of a
DUT failure, a customer voltage source can become shorted directly to the Model 6485/
6487. Under that condition, protection circuits within the 6485/6487 will limit the current
flow for higher current ranges (20
μ
A to 20mA). Additionally, when operating on the 2mA
and 20mA ranges or when the 6485/6487 auto ranges up to these ranges as a response to
the applied voltage, if the input voltage exceeds 60V, the Model 6485/6487 will change
from a current limit to a 1M
Ω−
3M
Ω
input impedance to prevent excess power dissipa-
tion. The OVRVOLT message will be displayed to indicate the change in the protection
circuit. The same information is available with remote operation.
To return the instrument to normal operation, the over-voltage condition must be
removed. Once the input voltage is reduced to under 60V, the protection circuit will return
to its current limit operation until the current is reduced to a valid (on-scale) level.
Extended operation near, but under 60V, will produce heat inside the instrument and may
require time to cool before returning to accurate readings.
Using a test fixture
Whenever possible, use a shielded low-leakage test fixture to make precision measurements
and for safety when high voltages (>30V) are used.