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4-12
Cabling and Wiring
Precautions for Using a DAS-1801ST Board at High Gains
Operating a DAS-1801ST board at input gains of 50 or 250 can lead to
problems if your application is unable to cope with noise. At a gain of
250, each bit of A/D output corresponds to 10
µ
V of analog input. Thus,
with the high speed and bandwidth of this board, analog noise and
performance degradation come easily unless you take precautions to
avoid them. The following collection of ideas and suggestions is aimed at
avoiding these problems.
●
Operate a DAS-1801ST board in 8-channel differential mode. Using
the board in 16-channel, single-ended mode at high gains introduces
enough ground-loop noise to produce large fluctuations in readings.
●
Minimize noise from crosstalk and induced voltage pickup in the flat
cables and screw terminal accessories by using shielded cable.
Connect the shield to LL GND and the inner conductors to Channel
LO and HI. Channel LO and LL GND 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.
●
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 40
µ
V/˚C. Thermals can introduce strange random
variations caused by air currents, and so on.
●
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 your data acquisition inputs with the AC line, or you risk
damaging 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.
Artisan Scientific - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisan-scientific.com
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com