When To Use External Sensing; When To Use The External Voltage Guard; Four-Wire Vs. Two-Wire Resistance Connections - Fluke 5730A Operator's Manual

5730A
Operators Manual

When to Use External Sensing

External sensing is normally necessary only when calibrating a device that draws enough
current to produce a significant voltage drop in the cables. An example of such a case is
with the use of the Calibrator as an external dc voltage reference for an ac/dc transfer
standard. In this example, the Calibrator is sourcing 1 V dc into a Fluke 540B AC/DC
Transfer Standard. The 180 Ω input impedance results in a current flow of approximately
5 mA. The Calibrator 90-day uncertainty at 1 V is specified to be ±(6 ppm +1.2 μV) or
±7.2 µV. Cumulative lead and contact resistances of as little as 2 mΩ would cause a
voltage drop greater than the total uncertainty of the Calibrator. External sensing
eliminates this error.
The normal power-up state of the Calibrator is external sensing off, with an internal
connection between the SENSE and OUTPUT automatically made. This is the state when
the External Sense selection says OFF.

When to Use the External Voltage Guard

The voltage guard protects the analog circuitry by placing an electrical shield between the
primary and secondary of the ac line power transformer. An optical cable transmits
control information from the 5730A Calibrator microprocessor to analog circuits. The
voltage guard provides a low-impedance path for common-mode noise and ground loop
currents.
The voltage guard is usually internally connected to the OUTPUT LO terminal. This is
the normal power-up state of the Calibrator, and the connection is automatically made
when the external guard is not selected. This is the state when the External Guard
selection says OFF.
If calibrating a UUT with a grounded low or common input terminal, an external
connection to the V GUARD is necessary. The Calibrator voltage guard must be
grounded at the UUT.
To prevent ground loops, there must be only one ground connection in the
system, so all ground connections should be made at the UUT. To maintain
one ground point, make sure the grounding strap between the GUARD and
GROUND binding posts is disconnected. See Table 3-1, items 17 and 18.

Four-Wire Vs. Two-Wire Resistance Connections

Figure 4-4 shows four different ways to connect to a UUT for resistance calibration.
Figure 4-4A shows a UUT with four-wire sensing. For such meters, always take
advantage of the four-wire sensing capability and use external sensing to get the highest
accuracy. Four-wire sensing is available for all resistance values except 100 MΩ.
For calibrating a meter with only a two-wire resistance mode such as a typical handheld
DMM, refer to Figures 4-4B through 4-4D. For resistances of 19 kΩ or lower in two-wire
mode, compensation circuitry inside the 5730A Calibrator is available to remove errors
introduced by resistance in the path between the front panel terminals and the precision
resistor. Depending on how the meter is connected, two-wire compensation referenced at
the UUT terminals (Figure 4-4C) or at the ends of the UUT test leads (Figure 4-4D) can
be used. See "Resistance Output" for information on turning on and off two-wire
compensation circuitry.
4-6
Note