HP 740B Operating And Service Manual page 22

Model 740B
VOLTAGE SOURCE CIRCUIT
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Figure 3-10. Guarding Against AC Ground Currents
Section III
indicate direction of current flow in the conventional
manner: positive to negative.)
3 -52. Guarding is a procedure for eliminating these
problems. Induced ac ground currents can be elim-
ina ted by connecting the guard shield to the ground
reference point
(~
) in the voltage source circuit
(Figure 3 -10). The induced ground current now flows
through the GUARD lead and Z2, bypassing the -INPUT
lead.
3 -53. The error current caused by floating the - IN-
PUT terminal above ground (Figure 3-9) can be elim-
inated by connecting the guard shield to a potential
equal to, but isolated from, the - INPUT potential
(Figure 3-11). In the example shown, this potential
is derived from an added string of resistors, R5 - R8.
VOL TAGE SOURCE
CIRCUIT
The added resistors may be somewhat less accurate
than Rl - R4 and do not even have to be the same
value providing they closely duplicate the voltages
available at the taps on the RI-R4 divider. By con-
necting the GUARD lead to the corresponding tap on the
R5 -R8 divider, the 740B guard shield is referenced
to the same potential as the - INPUT terminal. Cur-
rent through Z1 is thus eliminated and the loading cur-
rent now flows through the GUARD lead, Z2 and the
ground return, eliminating the loading effect on the
Rl - R4 divider.
3-54. If the voltage source circuit is not referenced
to earth ground there is no need for guarding. In
this case the GUARD and - INPUT terminals should
be connected together at the output terminal box using
the shorting strap attached to the GUARD terminal.
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Figure 3-11. Guarding Against DC Ground Currents
3-11