Guard Theory; Guard Connection - Fluke 5790B/3 Operator's Manual

5790A
Operators Manual

4-11. Guard Theory

The GUARD is an electrical shield around the sensitive analog circuitry, insulated from
chassis ground and the rest of the 5790A. The GUARD provides a low-impedance path
for common-mode noise and ground currents. The guard reduces the chance of ground
currents in the signal leads caused by plugging interconnected instruments into ac outlets
at different ground potentials.

4-12. Guard Connection

The GUARD is internally connected to the INPUT 1 connector shell or the INPUT 2 LO
binding post in this configuration. The GUARD binding post is disconnected from the
internal guard. This is the normal power-up state of the 5790A, and is selected by
pressing [EX GRD] so that the keycap indicator is off.
For the 5790A it is recommended that most measurements be made in the external guard
configuration. To prevent errors due to common mode signals, especially at higher
frequencies where the guard becomes less effective, the following steps are
recommended. Refer to Figure 4-1 for the recommended test lead connections to test a
measuring instrument, and Figure 4-2 for the recommended test lead connections to test a
source. These illustrations show recommended ways to connect to a source, and to
connect to a source and other measuring device. The hints listed below are identified on
the Figure by number.
1. Use short signal connections to reduce the impedance in the signal return path and to
reduce the current due to capacitive loading.
2. Use relatively low capacitance coaxial cable for connections to reduce the return
currents due to capacitive loading.
3. If attenuation is used, attenuate near the source.
4. Select 5790A [EX GRD]. Connect the GUARD of the 5790A to the GUARD of the
source and UUT. This will help prevent ground signals from being capactivety
coupled to the Lo at the higher frequencies.
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