6166 DC Voltage Current Source Operation Manual
5.1.3 How to Use Guard Terminal
5-4
5.1.3 How to Use Guard Terminal
Noise is a big problem in high-speed, high-accuracy and high-resolution measurement or test.
Especially in measurement lines, noise generated between the signal source, load or measuring instrument
and the ground is called common mode noise voltage (or CMV) that largely consists of AC power fre-
quency and its harmonics.
The amount of CMV greatly depends on the cable connection method and the internal structure of the volt-
age or current source. Thus, the 6166 adopts a guard structure that can secure the CMV rejection ratio the
most.
This guard is used when noise is superimposed on output signals, affecting the measurement or test. As
shown in Figure 5-3 and Figure 5-4, the switch that connects the GUARD terminal and the -OUTPUT ter-
minal is opened. (The EXTERNAL GUARD key goes ON.) Then, to prevent induction, a shielded cable
is used as connection cable and one side of the cable shield is connected to the GUARD terminal of the
6166 and the other side is connected to the low impedance side from the ground of the load together with
the output end.
Figure 5-3 shows how to use the guard terminal in 2-wire connection and Figure 5-4 shows how to use the
guard terminal in 4-wire connection.
NOTE: When measurement or test is performed using two or more instruments, connecting them to separate
ground points may cause common mode noise. Connecting them together to the ground wire of the
power supply line can reduce such common mode noise.
Figure 5-3 How to Use Guard Terminal (2-Wire Connection)
Load
(Red)
(Red)
(Black) (Black)
Shielded cable
(Blue)
Case
Chassis
*1 This relay opens when
EXTERNAL GUARD lights up.
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