Reference Section
•
87
Chapter 5
Grounding and Hum
•
Methods for minimizing line-frequency noise.
•
See also Noise, Power Supply.
A perennial bane of electrophysiology is line-frequency pickup, often referred to as
hum. Hum can occur not only at the mains frequency but also at multiples of it.
In a well-shielded enclosure the MultiClamp 700A has insignificant hum levels (less
than 0.01 pA
p-p
). To take advantage of these low levels great care must be taken when
incorporating the MultiClamp 700A into a complete recording system. The following
precautions should be taken.
•
Ground the preparation bath only by directly connecting it to the gold ground
connector on the back of the headstage.
•
Place the MultiClamp 700A in the rack in a position where it will not absorb
radiation from adjacent equipment. A grounded, thick sheet of steel placed
between the MultiClamp 700A and the radiating equipment can effectively reduce
induced hum.
•
Initially make only one connection to the MultiClamp 700A, from the SCALED
OUTPUT BNC to the oscilloscope. After verifying that the hum levels are low,
start increasing the complexity of the connections one lead at a time. Leads should
not be draped near transformers located inside other equipment. In desperate
circumstances, the continuity of the shield on an offending coaxial cable can be
broken.
•
Try grounding auxiliary equipment from a ground distribution bus. This bus
should be connected to the MultiClamp 700A via the SIGNAL GROUND (4 mm)
socket on the rear panel. The Signal Ground in the MultiClamp 700A is isolated
from the chassis and power ground.
