Arcom RC210, Operation Manual, Page: 54 / 68

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The Differences Between Accuracy And Precision
It is easy to become confused when the issues of precision and accuracy are raised. Let's take a few minutes to understand what
these terms mean in relation to the RC210. The RC210 can provide accurate measurements good to 3 decimal places of precision, if
you take the time to setup the inputs correctly. It might help to explain accuracy and precision to understand how to improve both:
Accuracy - The ability of a measurement to match the actual value of that which is being measured. For example, if you measure 3.32
volts, knowing the voltage you're measuring is actually 3.32 volts. The RC210 has a built-in reference voltage of 2.56 volts that is
supposed to be accurate. You can also provide an external reference voltage. The accuracy of the reference voltage, Vref, is the major
factor in determining the overall accuracy of the A/D conversion process.
Precision - The number of significant digits to which a value has been reliably measured. The precision of the RC210 is dependent on
the 10-bit A/D converter. With 10-bits, the A/D device can distinguish among 1024 separate values in the input, from 0 volts to Vref,
which is roughly about 5 volts). If your sensor puts out 0 volts at the lowest expected level of the quantity it is measuring, and Vref
(5.00 volts for example) at the highest expected level of the quantity you are measuring, you will have all 10 bits of precision available
to measure the physical quantity.
The next major issue in getting precise measurements is the characteristics of the sensor you use (design, buy, etc.) and how well it
matches the A/D converter measurement range (0 to Vref) when measuring physical quantities.
Suppose you want to measure 0 to 100 watts with a sensor and feed the sensor voltage to the controller to read back watts. You have a
sensor to monitor the power of your repeater transmitter. You have measured the output of your sensor while measuring the power
output on a calibrated watt meter and record the following values
At 0 watts the sensor produces 0.1 volts
At 100 watts the sensor produces 0.6 volts
As you can see, this is a small range of sensor voltage output -- only 0.5 volts. If Vref is 5 vdc, this is using only 10.0% of the full range
of the A/D input. Therefore, only 10.0% of the possible 1024 distinct measurement values is being used, or about 102 distinct
measurement points. The range of Watts being measured is 100 watts. Dividing this 100 watts by the 102 points of resolution means
the meter will be able to resolve
about 1 watt per “division” which is not bad! However if you wanted more precise readings, find a
sensor that produces a wider output voltage. For example, a sensor that produces 0.2 volts at 0 watts, and 4.2 volts at 100 watts. Now
the sensor voltage range is 4 volts of the 5.0 vdc Vref maximum. This is 80 percent of the 1024 points of resolution, or about 800
distinct measurement values. Now, for the same 100 watt range being measured, your improved sensor would now have a resolution of
(100 watts) / (800
divisions) = 0.125 watts per “division”. As you can see, a sensor that uses more of the Vref voltage range increases
the precision of your measurement.
In conclusion, the RC210 can resolve a sensor voltage change of as small as Vref / 1024. However, as you can see from the above
example, the resolution of the physical “thing” being measured is directly affected by the behavior of the sensor. For best results, try to
find sensors that produce sensor output voltages between 0 and Vref over the range of de
sired physical measurements. This isn’t an
absolute requirement to get useful and accurate readings, but it will help increase the precision of the measurements.
Programming the Reference Voltage Value for the A/D Converters
As it comes from the factory, the RC210 comes with the reference voltage value programmed to 5.00 volts. Since component
tolerances obviously vary, you will want to measure the actual reference voltage of your RC210 and then program that value into the
controller.
Note: This one reference voltage is used by all the A/D inputs. Setting it accurately will improve the measurement accuracy.
How to measure the Vref: Using a high impedance, precision voltmeter (a digital, not an analog one is recommended) and with power
applied to the RC210, carefully measure the voltage at pin 21 of the I/O connector, J1. Write this voltage down - it is only necessary to
include two decimal places). For example, if your meter displays "4.962", simply round it off to "4.96". Multiply this value by 100 as our
value to enter as Vref. The example below shows how to enter this measured Vref.
*2065 4 9 6
Note: NEVER apply voltages higher than 5.2 volts to any A/D channel input. It is best to keep any input voltages at or below Vref.