Remote-processing RPC-210 Manual de usuario descargar pdf (Pagina 45)

ANALOG I/O BASIC SECTION 10

Page 10-3

A problem with offset and gain errors is that, at the

extreme ranges, readings are probably unreliable. The

upper and lower 100 mV should be deemed as unusable.

Offset

Offset is the initial, or base voltage the A/D reads with 0

volts input. It is also the output voltage of a D/A when

comm anded to outpu t 0 volts. Offset er rors are typically

±25 mV, but can be as high as ±50 mV. Offset errors

are reduced or eliminated simply by subtracting the error

from the current reading.

Gain

An offset error is specified at 0 volts while a gain error

is specified at full scale. Gain error, then, is the

difference betwe en what sho uld be read at full scale

(either 2.500V or 5.000V) and what is actually read after

offset is removed. This can be as high as ±25 mV.

Gain in the 5.000V range can be compensated for by

adjusting R16. Gain for the 2.500V range cannot be

adjusted. Typically it is 2.5V, but can range between

2.45 to 2. 55 volts.

Compensating for offset and gain errors

The best w ay to compensate for gain and offse t errors is

to account for them in your program. Typically some

kind of sensor or other device is connected to the board

anyway. Perform ing a 2 point calibration (offset and

gain) will remove most errors. See AIN-3.BAS for an

example of how these errors are compensated for.

NOISE

An input channel can appear to be noisy (change

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

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

unused inputs to ground.

RPBASIC-52 takes 8 reads and averages them before

returning the result. In theory, this filters out random

noise from the C PU and external sources. Noisy

environments may still require capacitors on the front

end.

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

expect under good operating cir cumstances 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 seems to 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-210 have

shown that 99% of readings can be w ithin ±3 counts.

The conversion clock is set for minimum noise.

CONVERTING ANALOG

MEASUREMENTS TO REAL WORLD

UNITS

Inputs are converted to engineering units of measurement

by performing scaling calculations in the program. The

AIN function returns values from 0 to 4095. To change

these numbers into something more meaningful, use the

following formula:

var = K * AIN(n)

n is the analog channel to read. K is the scaling

constant. K is obtained by dividing the highest number

in the range of units by the maximum AIN count (4095).

var result is in real world units (PSI, pounds, inches,

volts, etc.)

Example 1: To measure the results of an A/D

conversion in volts and the voltage range is 0 to 5V,

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.

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