When To Use The External Voltage Guard; When To Use The Current Guard - Fluke 5700A Series II Operator's Manual

4-20. 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 calibrator's 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 (x pressed so that the indicator is off).
If you are calibrating a UUT with a grounded low or common input terminal, you need an
external connection to the V GUARD. The calibrator's voltage guard must be grounded at
the UUT. (To avoid ground loops you must have only one ground connection in the
system, so all ground connections should be made at the UUT. To maintain one ground
point, make sure there is no grounding strap connected to the calibrator.)

4-21. When to Use the Current Guard

The rear panel CURRENT GUARD is required when the calibrator is supplying low-
level ac current through a long cable, such as in a system. For that reason, a current guard
terminal is supplied on the rear panel where it is convenient for system applications.
The current guard is active only when the calibrator is in the ac current function. The
guard is voltage driven and is in phase with the calibrator's output current to the level of
compliance voltage at the OUTPUT HI terminal. Compliance voltage is the sum of the
UUT burden voltage and the additional burdens of contact resistances and cable losses.
When connected to the inner shield of a triaxial cable, the current guard blocks a
potentially significant leakage path by surrounding the output current with an in-phase
voltage at the same potential. Current leakage still occurs, but only between the guard
voltage on the inner cable shield and the grounded outer cable shield.
An example where current guarding is desirable is when you are sourcing 100 uA at
1 kHz through a 3-meter coaxial cable into a UUT with a burden voltage of 1V. At
100 pF per meter of length, the cable places an additional 300 pF load across the
calibrator's output. The amount of current leakage through the cable capacitance is:
I = V(2πfC)
Where,
V = 1V
f = 1 kHz = 1 x 10
C = 300 pF = 300 x 10
So the total leakage is:
I = (1)(2)(π)(10
This will add in quadrature to the current through a resistive load causing an error of:
+ .
2 2
−100 = 0.018 µA
100 1 9
At 100 µA, the calibrator's 90-day uncertainty is specified to be ±140 ppm + 20 nA, or
±0.034 µA. The current leakage here is over half the calibrator's uncertainty. The error
caused by this leakage can be minimized by using the current guard.
3
Hz
-12
F
3 -12
)(300)(10 ) = 1.9 µA
Front Panel Operation
Connecting the Calibrator to a UUT
4
4-15