Fluke 522A/6, Руководство оператора

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5522A
Operators Manual
4-10
sourced output, a softkey LOs appears, which allows you to tie or open an internal
connection between the NORMAL LO terminal and AUX LO terminal. When tied and
Z
is on, then both LO terminals are tied to chassis ground.
External Guard
The guard is an electrical shield, isolated from the chassis, that protects the analog
circuitry. The guard provides a low-impedance path for common-mode noise and ground
loop currents. The internal guard is connected to NORMAL LO through approximately
30
Ω
. There is normally an internal connection between the guard and the NORMAL LO
terminal. By pressing the
B
key, you break this internal connection, which allows you
to connect a lead from the GUARD terminal to earth ground on another instrument in an
interconnected system. Use this external guard connection whenever you are testing a
UUT that has a grounded LO terminal. Remember to always maintain only one earth
ground tie point in a system.
Four-Wire versus Two-Wire Connections
Four-wire and two-wire connections refer to methods of connecting the Calibrator to the
UUT to cancel out test lead resistance to assure the highest precision of the calibration
output. Figures 4-2 through 4-4 illustrate the connection configurations for resistance;
Figures 4-5 and 4-6 illustrate connection configurations for capacitance. The external
sensing capability of the four- and two-wire compensated connections provides increased
precision for resistance values below 110 k
Ω
and capacitance values 110 nF and above.
Part of the setting up the calibrator output for resistance and capacitance includes
selections for four-wire compensation (COMP 4-wire), two-wire compensation (COMP
2-wire) and two-wire no compensation (COMP off). (See “Setting Resistance Output”
and “Setting Capacitance Output” later in this chapter.) Note that compensated
connections for capacitance are to compensate for lead and internal resistances, not for
lead and internal capacitances.
Four-Wire Connection
The four-wire connection is typical for calibrating laboratory measurement equipment.
Increased precision is provided for resistance values below 110 k
Ω
. For other values, the
lead resistances do not degrade the calibration and the Calibrator changes the
compensation to off (COMP off).
Two-Wire Compensation
The two-wire connection is typical for calibrating precision handheld Digital Multimeters
(DMMs) with a two-wire input. Increased precision is provided for resistance values
below 110 k
Ω
and capacitance values 110 nF and above. For other values, the Calibrator
changes the compensation to off (COMP off).
Compensation Off
Compensation off is a typical connection for calibrating handheld analog meters or
DMMs with a two-wire input. This connection is used for all values of resistance and
capacitance and is usually selected when the analog meter or DMM level of accuracy
does not require the additional precision. This is the default condition whenever an ohms
or capacitance output is made, following an output that was not ohms or capacitance.
Cable Connections Instructions
Table 4-1 indicates a figure reference for each type of connection between a UUT and the
Calibrator, referencing Figures 4-2 through 4-10.
When calibrating Resistance Temperature Detectors (RTDs) using the three-terminal
connection shown in Figure 4-9, be sure the test leads have identical resistances to cancel
any errors due to lead resistance. This can be accomplished, for example, by using three
identical test lead lengths and identical connector styles.