Service and Maintenance 8
E364xA User’s and Service Guide 219
Common Mode Current Noise
The common mode current is the AC current component which exists between the
output or output lines and chassis ground. Common mode noise can be a problem
for very sensitive circuitry that is referenced to earth ground. When a circuit is
referenced to earth ground, a low level line-related AC current will flow from the
output terminals to earth ground. Any impedance to earth ground will create a
voltage drop equal to the output current flow multiplied by the impedance.
1 Turn off the power supply and connect a 100 kΩ resistor (R
S
) and a 2200 pF
capacitor in parallel between the (–) terminal and chassis ground at the rear
output terminals.
2 Connect a digital voltmeter (DVM) across R
S
.
3 Turn on the power supply. Select the low voltage range (8 V/3 A)
[1]
, enable the
output, and set the display to the limit mode. When the display is in the limit
mode, program the output to the full-scale rated value (8.0 V and 3 A)
[1]
.
4 Record the voltage across R
S
and convert it to current by dividing by the
resistance (DVM reading/100 kΩ). Note that the current is less than 1.5 µA.
[1] For the E3640A model. For other models, refer to Table 8-5.
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