Page 1196 - The Mechatronics Handbook

Isolation Amplifiers
Isolation amplifiers are useful in applications in which a voltage or current occurring in the presence of
a high common-mode voltage must be measured safely, accurately, and with a high CMR. They are also
useful when safety from DC and line-frequency leakage currents must be ensured, such as in biomedical
instrumentation.
The isolation amplifier can be thought of as consisting of three sections: an input stage, an output stage,
and a power circuit. All isolation amplifiers have their input stages galvanically isolated from their output
stages. Communication between the input and output stages is accomplished by modulation/demodulation.
An isolation amplifier is said to provide two-port isolation if there is a DC connection between its
power circuit and its output stage. If its power circuit is isolated from its output stage as well as its input
10
stage, then the amplifier is said to provide three-port isolation. Isolation impedances on the order of 10 Ω
are not atypical.
Isolation amplifiers are available in modular form with either two-port or three-port isolation. Both
single-channel and multichannel modules are offered.
Attenuation
Although the majority of transducers are low-level devices such as thermocouples, thermistors, resistance
temperature detectors (RTDs), strain gages, and so forth, whose outputs require amplification, there
are many measurement situations in which the input signal must be attenuated before introducing it
to the remainder of the system.
Voltage Scaling
Most typically, the signals to be attenuated take the form of voltages. Broadly, the attenuation is accom-
plished by either a voltage divider or a voltage transformer.
Voltage Dividers
In many cases a simple chain divider proves adequate. The transfer function of a two-element chain of
impedances Z 1 (s) and Z 2 (s) is

V o s() Z 1 s()
-------------- = -------------------------------
V in s() Z 1 s() + Z 2 s()
where the output voltage V o (s) is the voltage across Z 1 (s) and the input voltage V in is the voltage across
the two-element combination.
Of course, the impedances of the source (transducer) and the load (the remainder of the system) must
be taken into account when designing the divider network.

Resistive Dividers
If the elements in the chain are resistors, then the divider is useful from DC up through the frequencies
for which the impedances of the resistors have no significant reactive components. For Z 1 (s) = R 1 and
Z 2 (s) = R 2 ,

V o s() R 1
-------------- = -----------------
V in s() R 1 + R 2
Other configurations are available for resistive dividers. One example is the Kelvin–Varley divider,
which has several advantages that make it useful in situations requiring high accuracy. For a detailed
description, see Gregory (1973).

Capacitive Dividers
If the elements in the chain divider are capacitors, then the divider has as its transfer function

V o s() C 2
-------------- = ------------------
V in s() C 1 + C 2

©2002 CRC Press LLC