Remote-processing RPC-2350 Uživatelský manuál stáhnout pdf (Strana 46)

ANALOG I/O CHAPTER 8

8-4

divided 5 by 4095 to obtain K.

K = 5/4095

K = .001221

Your program could look something like:

1000 C = .001221 * AIN(N)

Example 2: You want to measure a 0 to 200 PSI

pressure transducer with a 0 to + 5V output. Divide 200

by 4095 to obtain the constant K.

K = 200 / 4095

K = .0488

The result is in PSI w hen used as follows:

1000 B = .0488*AIN(0)

Noise Notes

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 sensitive to voltage pickup.

Noise is minimized by running wires away from AC

power lines.

NOTE: Avoid running the cable over inductor L1.

This can increase noise when using 7-30V

input..

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. Try connecting the shield to ground

at only one point, not at both ends. You may need to

run a separate ground wire.

Wire pairs can also be twisted. 5-6 twists/foot provides

a reasonable 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. U nder ideal conditions,

noise contributes less than a count.

One way to compensate for noise is to take a number of

samples and average the results. Taking 7 or more

samples, in theory, cancels 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 (several seconds if possible).

Jumper block W13 is used to install filter capacitor s.

Generally, the higher the source impedance, the lower

the capacitor you will need. A 0.1 µF capacitor filters

noise nicely when impedance is 100K. While installing

capacitors filters noise, it also reduces the frequency

response. How much depends upon your source

impedance and capacitor values.

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-2350 have

shown tha t ov er 99% of th e r ead ing s ar e w ithi n the ±3

count reading. Noise r eadings w ere m ade with all inputs

shorted to ground and with no cable connected to J7.

CALIBRATION

The A/D converter is calibrated using an external

voltmeter. For 12 bit accuracy, you must use a

voltmeter with an accuracy of 0.02% or better.

To calibrate the RPC-2350:

1. Connect the digital voltmeter ground to U10,

pin 14. (Alternate ground pins are J7-2, 4, 6, 8,

10, 12 or 14).

2. Connect the digital voltmeter '+ ' lead to U9,

pin 6.

3. Adjust trim pot R19 for 5.000VDC.

You may increase the reference voltage to a higher

value, up to 5.12V. This will allow you to detect if an

input device is at or above its 5V output range.

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