SIGNAL ISOLATION

Isolation is a method of conditioning a signal without having a galvanic connection between the input and the output. Isolation is often also required between the input, output, and power source.

Isolation is usually achieved by converting the input into a frequency and coupling the frequency signal to the output through a transformer or optical isolator.

Typical isolation circuits can withstand 1000V or more common mode signals on the input.

Isolation is useful in breaking ground loops (circuits where extraneous currents flow in the signal path and add to the signal) and providing high voltage common mode rejection.

The need for isolation depends on the signal source wiring and the load on the signal conditioner. If the signal conditioner is wired in a fixed system, in a consistent manner, isolation can probably be avoided through proper wiring practices.

If the signal conditioner is available for other user wiring, isolation should be considered if the other user can create ground loops or apply high common mode signals to the conditioner.

DC POWERED PRODUCTS

In the case of DC powered products, isolation may be required only between the input and the output with either the input or output being common with the power source.

If isolation is required between the power source and both input and output, the inverter that provides transformer isolation between the circuits is a little more complex and more costly.

RESPONSE TIME OF ISOLATION

Isolation circuits can be made with response times as short as 100 microseconds for a 99% recovery to a step change.

The isolation circuits have capacity coupling between the isolated circuits. This capacity, which is primarily transformer inter winding capacity, limits the frequency which the isolator rejects.

If a need exists for rejecting high frequency common mode signals, inverters can be used with small windings that reduce inter winding capacity.