Microsoft LPF-00004 Manual do Operador Página 38

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Page 20 GENERAL INFORMATION
CYBERAMP 380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.
Grounding and Hum
Grounding
The analog ground inside the CyberAmp 380 is tied to the computer ground via a 10 resistor. The
analog ground is defined as that ground connected to the shields of the BNC connectors and to pin 10
of the multipin input connector. The computer ground comes from the computer and is carried by the
RS-232 serial communications cable.
Hum
Line-frequency pickup, often referred to as hum or line-frequency noise, is a common problem in low-
level recordings. Hum can occur not only at the line frequency but also at multiples of it.
The CyberAmp 380 has inherently low hum levels. To take advantage of these low hum levels great
care must be taken when incorporating the CyberAmp 380 into a complete recording system. The
following procedures should be followed.
1) If practical, provide ground to the recording site from the ground jack on the SmartProbe.
2) Position the CyberAmp 380 so it will not absorb radiation from adjacent equipment. The AC
line transformers in some instruments radiate a magnetic field at the line frequency. A thick
sheet of steel placed between the CyberAmp 380 and the radiating equipment can reduce this
induced hum.
3) Initially, use the CMD300 program, the QUICK300 program, or the data acquisition software
to ground all inputs to the CyberAmp 380. After verifying that the hum levels are low, make
connections one probe at a time and note any increase in the hum level.
4) The cables of the probes and BNC inputs should not run near the AC line transformers of
other equipment.
5) If multiple probes are in use, run the probe cables in a single bundle from the CyberAmp to
the recording sites, if possible.
6) Try to ground auxiliary equipment from a single ground distribution bus.
7) If hum is introduced when you connect the BNC output from a laboratory instrument to the
BNC input of the CyberAmp, proceed as follows:
i) First try to eliminate the hum by carefully re-cabling the laboratory instrument,
following the principles outlined in items 1-5 above.
ii) If hum is still present, consider replacing the CyberAmp end of the BNC coaxial
cable with a 15-pin D-connector. Take the coaxial cable inner conductor to the
positive input pin of the CyberAmp (pin 1) and the coaxial cable shield to the
negative input of the CyberAmp (pin 2). Set the negative input coupling of the
CyberAmp to DC instead of to GND. With this differential input the CyberAmp will
reject the common-mode hum from the other instrument.
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