Remote-processing RPC-220 Manuel d'utilisateur télécharger pdf (Page 35)

ANALOG INPUT SECTION 11

Page 12-2

EXTERNAL TRIGGER

External trigger (CPU signal STADC) at J1-16 causes an

A-D conversion on a positive or rising pulse. Tie this

line to ground 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

program.

See application program AIN220-3. C for programming

information.

NOISE

An input channel can appear to be noisy (change

readings at rando m) if unuse d inputs are allowed to float.

To minimize noise (and increase accur acy), connect all

unused inputs to ground.

A high impedance input is, by definition, sensitive to

voltage pickup. Noise is minimized by running wires

away from AC 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. Using 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 circ um stance 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 within the ±3 count

reading. N oise read ings wer e made with all inputs

shorted to ground.

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Remote-processing RPC-220 Manuel d'utilisateur Page 35

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