Remote Processing RPC-2350 - Manual (Page 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: Y ou want to measure a 0 to 200 PSI
pressure transducer w ith 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)
N oise Notes
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 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 impe dance voltage sourc e 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 poin t, not a t both ends. You m ay 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 ci rc um sta nce s is ±3
counts for any input ra nge. U nder ideal conditions,
noise contr ibutes less than a c ount.
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 b lock 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 m uch 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
sho wn tha t ov er 99% of th e r ead ing s ar e w ithi n th e ±3
count rea ding. Noise r eadings w ere m ade with all inp uts
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. A d ju s t t r im p o t R 1 9 f o r 5 . 0 00 V D C .
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.