Remote Processing RPC-220 - Manual (Page 35)

ANALOG INPUT SECTION 11
Page 12-2
EXTERNAL TRIGGER
External trigger (CPU signal STADC) at J1-16 causes an
A-D c onversio n on a positive or rising pulse. Tie this
line to groun d if you do not use this signal.
This line can be tied to the optional RTC output at J3-4.
Program mable, per iodic acquisitions from 2 to about
32, 000 times/ second ar e possible. Execution time is
saved because conversions are started outside of a
p r o gr a m .
See application program AIN220-3. C for progr amming
information.
NOISE
An input channel can appear to be noisy (change
reading s at rando m) if unuse d inputs are allowed to floa t.
To m inimize noise (and incr ease accur acy), connect all
unused inputs to ground.
A high im pedance inp ut is, by definition, sensitive to
voltage pickup. Noise is minimized by running wires
away from A C power lines. A low impedance voltage
source helps to reduce noise pick up. Shielded cable can
help reduce noise from high impedance sources. Make
sure the shield is not used for power ground. U sing the
shield for power ground defeats its purpose. Wire pairs
can also be twisted. 5-6 twists/foot provides a good
amount of noise cancellation.
Noise is defined in this section as any random change
from a known input. The amount of noise you can
exp ect und er nor ma l op er atin g ci rc um sta nce s is ±3
counts for any input range.
One way to compensate for noise is to take a number of
samples and average the results. Taking 7 or more
samples does, in theory, cancel out any effects of noise.
A problem with this is noise tends to group together.
Taking 7 readings at one time might show no change
from the norm . Another 7 readings might be all high.
If possible, try to spread out readings over a period of
time.
Noise is, by definition, random . If you wer e to plot out
the deviations from a norm, it would roughly resemble a
bell shaped curve. Exper iments on the RPC-220 have
shown that 99% of the readings are w ithin the ±3 count
reading . N oise read ings wer e made with all inputs
shorted to ground.