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HANDOUT #6: Isolation Amplifier Modeling
An isolation amplifier is a device with the primary function of providing ohmic isolation (break
the ohmic continuity of electrical signal) between the input signal/circuitry and the output of
the amplifiers. It usually consists of an input operational amplifier or instrumentation amplifier
(IA) followed by a unity-gain isolation stage.
The sole purpose of the unity-gain isolation stage is to completely isolate the input from the
output of the device. Ideally, the ohmic continuity of the input signal is broken (at the isolation
barrier) yet accurate signal transfer without any attenuation is achieved across the unity-gain
isolation stage. An important feature of an isolation amplifier is that it has a completely
floating input that helps eliminate cumbersome connections to source ground in several
applications. In addition, in the medical field, patient isolation is often required for safety
considerations.
Figure H6.1: Isolation amplifier schematic
Figure H6.1 shows a typical isolation amplifier model for a differential voltage source, VD.
Two sources of error are also shown; one associated with the common-mode voltage, VCM,
the other the isolation mode voltage, VISO. The isolation mode voltage is the voltage that
EE6913: Biomedical Instrumentation
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exists across the isolation barrier. The contribution of the output referred error caused by this
voltage is given by:
Verr = VISO ⋅
AD
IMRR
where:
VISO is the isolation barrier voltage
IMRR is termed the isolation mode rejection ratio
AD is the differential gain of the amplifier
The "Leakage Current" is defined as the current that flows across the isolation barrier with
some specified isolation voltage applied between the input and the output. In the hospital
environment, there are usually standards (eg. CSA) in place that that specifies the maximum
leakage current allowable. Naturally this varies depending on the intended use of the
instrumentation. Any systems which do not met this specification are deemed unsatisfactory
for medical applications. This is based on the possibility of micro and macro shock hazards.
This is generally based on the possibility of micro and macro shock hazards.
Common-mode Voltage and Isolation Voltage
Some manufacturers treat common-mode voltage and isolation voltages synonymously in
describing the use and/or specifications of isolation amplifiers. It is important to understand
the significance of these terms along with their differences.
When the input common is grounded, the input signal, VS, (see Figure H6.1) can be floated
by the amount VISO above the input ground. This common-mode voltage (CMV) is limited by
the rating of the input stage of the amplifier and is typically ± 10 volts. In applications
involving higher system common-mode voltages the input common terminal is not grounded
and the common-mode voltages are referenced across the isolation barrier to the output
common terminal.
The isolation voltage, VISO, is the potential difference between the input common and the
output common terminals. The isolation voltage rating describes the amount of voltage that
the isolation barrier can withstand without breakdown. This feature of the isolation amplifier
allows two distinct ground connections to be made when necessary. It allows the isolation
amplifier to be used in applications involving very-high common-mode voltages and in
applications of breaking ground loops. The latter application is mandatory in providing safe
electrical contacts to patients being monitored in a clinical setting.
Many applications involve a large "system common-mode voltage". In such applications, the
input common terminal of the isolation amplifier is not connected to any ground but the output
common terminal is connected to the system ground. In such a case, the term VCM shown in
Figure H6.1 becomes negligible and VISO determines the safe limit for the system commonmode voltage. In this manner, the isolation amplifier can accommodate common-mode
voltages of 2000 volts or more.
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DFL, 2002
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Common-mode Rejection and Isolation Rejection
Isolation-mode rejection (IMR) is another term that some other manufacturers refer to as
common-mode rejection (CMR). The above discussion on the common-mode voltage and
isolation voltage helps recognize the difference between CMR and the IMR. The CMR is the
measure of the ability of the input stage amplifier to reject common-mode input signals
(common-mode with reference to the output common) while transmitting the differential signal
across the isolation barrier. The isolation-mode rejection ratio (IMRR) is defined by the
equation shown in Figure H6.1 and is a measure of the effect of the isolation voltage, VISO, on
the output voltage. Thus, understanding the IMR capability of isolation amplifiers allows their
meaningful use in application in applications requiring very high common-mode rejection
ratios such as l00dB to 140dB.
Isolation Voltage Ratings
It is important to understand the significance of the continuous derated isolation voltage
specification and its relationship to the actual test voltage applied to the unit.
Since a
"continuous" test is impractical in a product manufacturing situation (implies infinite test
duration) it is generally accepted practice to perform a production test at a higher voltage
(higher than the continuous rating) for some shorter length of time.
The important consideration is then "what is the relationship between actual test conditions
and the continuous derated minimum specification?" There are several rules of thumb used
throughout the industry to establish this relationship. For most isolation amplifiers, BurrBrown, for example has chosen a very conservative one:
Vtest = (2 ⋅ Vcontinuous rating ) + 100 V
This relationship is appropriate for conditions where the system transient voltages are not
well defined. Where the real voltages are well defined or where the isolation voltage is not
continuous the user may choose to use a less conservative derating to establish a
specification from the test voltage.
Applications of Isolation Amplifiers
When one or more of the following conditions/requirements are present in an application, an
isolation amplifier would generally be the right choice as a signal conditioning device:
a) When ohmic isolation between the signal source and the output is a
requirement (isolation impedance between the input and the output >
10MΩ).
b) When excellent common-mode noise and voltage rejection is a requirement
(CMRR > 100 dB).
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c) When it is necessary to process signals in the presence of, or riding on,
high common-mode voltages (VCM > 10 Volts).
In general, most applications can be broadly categorized into the following four types:
•
Amplifying and measuring low-level signals in the presence of high
common-mode voltages.
•
Breaking ground loops and/or eliminating source ground connections. The
isolation amplifier provides a fully floating input, eliminating the need for
connections to source ground, and thus allows two-wire hook-up to the
signal sources.
•
Providing an interface between medical patient monitoring equipment and
the transducer/devices that may be in physical contact with the patients.
Such applications require high isolation voltage levels and very low
leakage currents.
•
Providing isolation protection to electronic instruments/equipment. Large
common-mode voltages occasionally cause hazardous electronic faults.
Low leakage currents and high isolation voltage capability of isolation
amplifiers help protect instruments against damage caused by such faults.
Isolation amplifier performance requirements vary significantly, depending on the type of
requirement. In applications where bandwidth and speed of response are more important
than gain accuracy and linearity, the optically coupled type of amplifiers will be the best
choice. For applications where gain accuracy and linearity are key parameters, then
transformer-coupled amplifiers may be the more suitable choice.
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GLOSSARY OF TERMS & DEFINITIONS
Current transfer ratio
The current transfer ratio is a key parameter of all optically coupled isolation devices.
The conventional optical coupler consists of a single light emitter, usually an LED, and a
single light sensor, which may be a photodiode, a phototransistor, or a photo-Darlington
device. The basic photodiode coupler has the lowest CTR, and the Darlington types
have the highest.
Isolation Amplifier
An isolation amplifier is a device that provides ohmic isolation between the input and the
output of the device. The method of coupling may be magnetic, optical, capacitive or
any means other than direct ohmic coupling. Such a device allows the input circuit to be
referenced separately and independent of the output circuitry.
Isolation Barrier
The isolation barrier is the region between the input and the output stage of an isolation
amplifier, where the signal-transfer is achieved between the input and the output.
Isolation Impedance
This isolation impedance is effective impedance between the input common terminal and
the output common terminal. It is the impedance of the isolation barrier that is usually
specified as a typical parameter. Leakage current is related to isolation impedance and
is usually specified with a maximum limit.
Isolation-Mode Rejection IMR
The IMR is the measure of the ability of an isolation amplifier to reject common-mode
input signals (common-mode with reference to the output common), while transmitting
the differential signal across the isolation barrier. It is the voltage or current that must be
applied to the input to force the output to zero when VISO is present
Isolation Voltage
The isolation voltage is the potential difference between the input stage common and the
output stage common terminals, VISO, of an isolation amplifier.
Isolation Voltage Rating
The amount: of voltage that can be impressed between the input common and the
output common terminals (across the isolation barrier) without resulting in breakdown.
Leakage Current
The leakage current is the current that flows between the input common terminal and the
output common terminal (across the isolation barrier) with a specified test voltage
applied. It is usually 100% tested and specified with a maximum limit.
EE6913: Biomedical Instrumentation
DFL, 2002