Connecting Signals
4-11
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Minimize noise from crosstalk and induced-voltage pickup in the flat
cables and screw-terminal accessories by using shielded cable.
Connect the shield to AGND and the inner conductors to Channel LO
and HI. Channel LO and AGND should have a DC return (or
connection) at some point; this return should be as close to the signal
source as possible. Induced noise from RF and magnetic fields can
easily exceed tens of microvolts, even on one- or two-foot cables;
shielded cable eliminates this problem.
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Avoid bi-metallic junctions in the input circuitry. For example, the
kovar leads, used on reed relays, typically have a thermal emf to
copper of 40µV/˚C. Thermals can introduce strange random
variations caused by air currents, and so on.
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Consider filtering. This approach can use hardware (resistors,
capacitors, and so on) but is often accomplished more easily with
software. Instead of reading the channel once, read it 10 or more
times in quick succession and average the readings. If the noise is
random and gaussian, it will be reduced by the square-root of the
number of readings.
Additional Precautions
Do NOT mix data acquisition inputs with the AC line, or you risk damag-
ing the computer. Data acquisition systems give users access to inputs of
the computer. An inadvertent short between data and power lines can
cause extensive and costly damage to your computer. The manufacturer
can accept no liability for this type of accident. To prevent this problem,
use the following precautions:
●
Avoid direct connections to the AC line.
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Make sure all connections are tight and sound so signal wires will not
come loose and short to high voltages.
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Use isolation amplifiers and transformers where necessary.
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