Troubleshooting
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13
Chapter 3
Grounding and Minimizing Noise
To avoid ground-loops, we recommend that you plug-in the Digidata 1322A to the
same power-strip as the PC. Also, be aware that each Analog Input BNC on the
Digidata 1322A is a single-ended input (all BNC shells are connected to signal
ground).
When noise in the system occurs, the first step is diagnosis. Take ALL instruments
out of their racks, and connect them together with only ONE BNC connection.
Observe if the hum (50 – 60 Hz noise) is eliminated. Also observe if the hum is
produced by pickup from the headstage by shielding the headstage and watching
the magnitude of the hum.
If the hum is eliminated at this step, then connect the second BNC cable. If hum is
now observed, this is probably a ground loop that is picking up an alternating
magnetic field. Next try to eliminate the source of the alternating magnetic field: a
cheap transformer, an electric motor, such as found in a nearby fan or refrigerator.
Try to rearrange the two BNC cables to determine if their positioning tells you
anything about the source of the alternating field. High frequency components (20
to 50 kHz) may also appear if there is a ground loop. These originate from the
switching power supply of the computer and can be picked up in the analog signal
inputs of the Digidata 1322A.
If removing the source of the alternating field is not possible, eliminate the ground
loop by making one of the connections between the two instruments without a
shield. Make this either with a naked unshielded wire, or with a BNC cable that has
its shielding cut at one end. Make a break in the shielding away from the interface,
near the connection on the instrument suspected of creating the ground loop.
For users of Axon Instruments amplifiers, more information regarding noise
reduction procedures can be found in the manuals for the Axopatch 200B,
Axopatch-1D, and AxoClamp-2B.
