Measurement Specialties CIO-SSH16 user manual Sources of Noise

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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