5520A
Operators Manual
4-10
4-14. When to Use EARTH and EXGRD
Figure 4-1 shows the internal connections made by the Z and B keys.
Chassis ground
Internal guard shield
NORMAL LO
signal ground
NORMAL LO
binding
post
Safety ground
through
ac line cord
Lit = open
Not lit = open
EARTH
EXGRD
GUARD
binding
post
nn003f.eps
Figure 4-1. EARTH and EXGRD Internal Connections
4-15. Earth
The 5520A Calibrator front panel NORMAL LO terminal is normally isolated from
earth (chassis) ground. When it is desired to make a connection between the NORMAL
LO terminal and earth ground, press the Z key, lighting the key annunciator.
To avoid ground loops and noise you must have only one earth ground-to-LO terminal
connection in the system. Usually you make all signal ground connections at the UUT
and verify the Z annunciator is off. Generally, Z is on only for ac and dc volts
where the UUT is isolated from earth ground. There must, however, be a safety ground
for the 5520A. See “Connecting to Line Power” in Chapter 2. When enabled by the
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.
4-16. 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. 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
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