Measurement Computing CIO-DAS6402/12 - Manual (Page 29)

6 CALIBRATION AND TEST
Every board was fully tested and calibrated before being placed in finished goods inventory at the factory. For normal
environments a calibration interval of 6 months to one year is recommended. If frequent variations in temperature or humidity are
common then recalibrate at least once every three months. It takes less than 20 minutes to calibrate a CIO-DAS6402.
6.1 REQUIRED EQUIPMENT
Ideally, you will need a precision voltage source, a 4 1/2 digit digital voltmeter (5 ½ digit for the CIO-DAS-6402/16), a calculator
and some wire. If you do not have a precision voltage source, you will need a non-precision source and have to make a few
calculations.
You will not need an extender card to calibrate the board but you will need to have the cover off you computer with the power on,
so trim pots can be adjusted during calibration. For that reason a plastic screwdriver has been supplied with your CIO-DAS6402.
In the event that the screwdriver is dropped into the PC, no damage will result from short circuits.
6.2 CALIBRATING THE A/D & D/A CONVERTERS
The A/D is calibrated by applying a known voltage to an analog input channel and adjusting trim pots for offset and gain. There
are four trim pots requiring adjustment to calibrate the analog input section of the CIO-DAS6402. The entire procedure is
described in detail in the InstaCal
TM , calibration routine.
The CIO-DAS6402 should be calibrated for the range you intend to use it in. When the range is changed, slight variation in Zero
and Full Scale may result. These variations can be measured and removed in software if necessary.
7 SIGNAL CONDITIONING CIRCUITS
7.1 VOLTAGE DIVIDERS
An alternative method of measuring a signal which varies over a range greater than the input range of a digital input, is to use a
voltage divider. When correctly designed, it can drop the voltage of the input signal to a safe level the digital input can accept.
Ohm's law states:
Voltage = Current x Resistance
Kirkoff's law states:
The sum of the voltage drops around a circuit will be equal to the voltage drop for the entire circuit.
In a voltage divider, the voltage across one resistor in a series circuit is proportional to the total resistance divided by the one
resistor (see formula below).
The object in a voltage divider is to choose two resistors having the proportions of the maximum voltage of the input signal to the
maximum allowed input voltage.
The formula for attenuation is:
For example, if the signal varies between 0 and 20
volts and you wish to measure that with an analog
input with a full scale range of 0 to 10 volts, the
attenuation (A) is 2:1 or just 2.
2 = 10K + 10K
10K
Attenuation = R1 + R2
R2
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