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For example, if the CIO-DAS16 board is set up to read 0 to 5 volts, then:
5 volts / 0.02 Amps = 250 Ohm shunt resistor.
So a full 20 mA will register 5 volts and 4 mA will register 1 volt.
To hook up the CIO-DAS16 analog inputs to a 4-20 mA transducer or signal source,
place the shunt resistor across the plus and minus terminals or signal wires of the 4-20
mA. Once the resistor is in place, connect the analog input CH# Hi to the plus
terminal and the analog input CH# Lo to the minus input.
Figure 6-6. Current-to-Voltage Resistor
If they are backward, the A/D reading will be 0 or minus volts. Just reverse the
connection.
6.9 NOISE
Noise is unavoidable in PC based data acquisition systems. There is board-induced
noise which can be measured by shorting an analog input to ground and taking a series
of readings and plotting them in a histogram. There is EMI and RFI-induced noise
along the path of the signal wires. There is also noise at the signal source itself. All
these sources of noise combine to create a region of uncertainty around the signal
value.
Our objective here is to list the sources of noise and give some means to reduce it.
6.9.1 SOURCES OF NOISE
The first source of noise is the board itself. Manufacturers of A/D boards quote
component specifications in their data sheets but rarely quote a system specification
for general accuracy and noise. The reasons being that the system specification would
be less accurate than component specification and that system specifications must also
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