Handout #6 Isolation Amplifier Modeling 1/5 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 DFL, 2002 Handout #6 Isolation Amplifier Modeling 2/5 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. EE6913: Biomedical Instrumentation DFL, 2002 Handout #6 Isolation Amplifier Modeling 3/5 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). EE6913: Biomedical Instrumentation DFL, 2002 Handout #6 Isolation Amplifier Modeling 4/5 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. EE6913: Biomedical Instrumentation DFL, 2002 Handout #6 Isolation Amplifier Modeling 5/5 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