Rockwell Automation System Design for the Control of Electrical Noise - Manual (Page 97)

Publication GMC-RM001A-EN-P — July 2001
Measuring Noise Reduction Effectiveness 11-7
Ground Loops
A line-powered oscilloscope may introduce noise via the ground loop
formed by the separate line supply and the connection of the probe to
system ground. Methods to reduce this type of noise are listed below.
• Connect a braided strap between the scope chassis and the panel.
Most scopes have a ground terminal provided for this purpose.
• Pass the scope lead through a ferrite sleeve several times.
• Use a battery powered scope and place it inside the control
cabinet close to the panel.
• Use a scope with differential inputs. Refer to the section
Differential Measurements for more information.
• Extend the scope probe lead. Refer to Figure 11.6 for an
illustration.
Differential Measurements
Differential measurements eliminate ground loops and allow the
scope to be grounded to its own supply ground. Two methods to
reduce this type of noise are given below.
Differential Voltage Probes
Differential voltage probes, as shown in Figure 11.5, use only one
scope input. Since they cancel common-mode voltage between the
measured circuit and the scope common, ground loop problems are
greatly reduced.
The main limitation is that of limited common-mode rejection. To
avoid saturating the amplifier when measuring noise at line voltage
the attenuation must be set to 1/100 or 1/200. This way the noise
signal of interest is attenuated by the same amount. Increasing the
scope sensitivity to compensate amplifies any internal probe or scope
noise. Tips for using differential scope probes are listed below.
• Connect both probe tips to the same point in the circuit under test.
No signal should be seen if the differential function is working
correctly. Refer to the section Checking Your Method for
Effectiveness for details. For best results, use the method as
described.