Chapter 3 Calibration Procedures
Constant Voltage (CV) Verifications
56
Common Mode Current Noise
The common mode current is that ac current component which exists between any or
all outputs 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 for +6V output or between the
(COM) terminal and chassis ground for ±25V outputs.
2 Connect a digital voltmeter across R
S
.
3 Turn on the power supply and select the output to be tested using the meter and adjust
selection key on the front panel. Enable the outputs and set the display to the limit
mode. When the display is in the limit mode, program the current of the selected
output to the maximum programmable value and the voltage to the full-scale value
(see Table 3-2).
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 for each of the three
outputs.
5 Repeat steps (1) through (4) for the remaining outputs.
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