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There is a need to isolate external ADCs.
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Electrical Isolation
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Electrical isolation is a method of physically and electrically separating two distinct parts of a circuit; meaning that current does not flow between the two parts of the system that are being isolated. In regards to measurement accuracy, the greatest advantage of electrical isolation is that isolation breaks ground loops.
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Ground loops are the most common source of noise, and occur when two connected terminals in a circuit are at different ground potentials. This causes current to flow between the two terminals and this potential difference is a source of error in the measured voltage.
| Vm=Vs+ΔVg | |
|---|---|
| Vm | Measured voltage |
| Vs | Signal voltage |
| ΔVg | Voltage difference between the signal source ground and the instrument ground |
Analog Versus Digital Isolation
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Because optical isolation uses light as the energy to transfer the measured signal across the isolation barrier, it gains the advantage of immunity from electrical- and magnetic-field interference. This can make optical isolation an effective technique in industrial areas where strong electric or magnetic fields could be present. The advantages gained by using light are balanced by some disadvantages. Optical isolation typically has slower data transfer rates, which are limited to the LED switching speed. It also has relatively high power dissipation when compared to capacitive and inductive isolation.
Isolation Types Summary
| Isolation Type | Advantages | Disadvantages |
|---|---|---|
| Capacitive |
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| Inductive |
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| Optical |
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Source: http://download.ni.com/evaluation/pxi/Isolation_Types.pdf
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