隔离运放的单电源供电方法

隔离运放的单电源供电方法 ------------------------------------------------------------------------------------------------ 隔离运放的单电源供电方法 ? Mailing Address: PO Box 11400 ? Tucson, AZ 85734 ? Street Address: 6730 S. Tucson Blvd. ? Tucson, AZ 85706Tel: (520) 746-1111 ? Twx: 910-952-111 ? Telex: 066-6491 ? FAX (520) 889-1510 ? Immediate Product Info: (800) 548-6132 SINGLE-SUPPLY OPERATION OF ISOLATION AMPLIFIERS By Rod Burt and R. Mark Stitt (602) 746-7445 For simplicity, many systems are designed to operate from asingle external power supply. In battery powered systemssuch as aircraft and automotive, it’s often a requirement.Isolation amplifiers such as the ISO120 and ISO122 can beeasily modified for input side single-supply operation withthe addition of an INA105 difference amplifier. With ISOamps, it’s the isolated input side power supply which mostoften needs to be single supply. The output side of the ISOamp uses a split ?15V power supply, allowing a full ?10Voutput swing. The difference amplifier has advantages as compared totraditional single-supply amplifiers. The inputs of a differ-ence amplifier can swing to both the positive and negativepower-supply rails. In fact, in the application shown inFigures 1 and 2, the input range of the circuit extendsapproximately 2V below ground (the negative power supplyrail). —————————————————————————————————————— ------------------------------------------------------------------------------------------------ This is because the resistors internal to the INA105divide the input level in half as seen by the op amp. The technique is illustrated in Figures 1 and 2 using theISO120 and ISO122. These ISO amps are specified foroperation from dual supplies as low as ?4.5V and can beoperated with a total single power supply voltage as low as9V. The circuit shown is designed for operation from a single+15V power supply. This allows a 0V to +5V input range.The most common application is for a single ended inputreferred to ground as shown. For a differential input, pin 2can be connected to a second input instead of ground. Thisprovides a 0V to 5V differential input with common-mode toeither rail. To understand how the circuit works, consider the operationof the INA105 difference amplifier. The difference amplifierforces its output (pin 6) relative to its reference (pin 1) to beequal to the differential input (pin 3 – pin 2). The differenceamplifier reference pin and the ISO amp common are held atapproximately 5.1V by the 10k? resistor and the zenerdiode. This pseudo ground establishes an arbitrary accept-able operating point for the ISO amp. The difference ampli-fier then translates its input, relative to true ground, up to the5.1V pseudo ground. In other words, a 0V to 5V inputbetween pins 3 and 2 of the INA105 is seen as a 0V to 5Vsignal at the ISO amp input. Isolated power is often at a premium and both the ISO120/122 and —————————————————————————————————————— ------------------------------------------------------------------------------------------------ the INA105 operate on relatively low power. Com-mon zener diodes, on the other hand, may require several mAfor proper operation. The 1N4689 zener diode specified is alow level type designed for applications requiring low oper-ating currents. It has a sharp breakdown voltage specified ata low 50μA. FIGURE 1. Single Supply Operation of the ISO122 Isolation Amplifier. ? Printed in U.S.A. August, 1991 1990 Burr-Brown Corporation 1AB-009A SBOA004 FIGURE 2. Single Supply Operation of the ISO120 Isolation Amplifier.The accuracy of the INA105 difference amplifier relies on careful resistor ratio matching (R3/R4 = R1/R2). Any sourceimpedance of the signal (RS) adds to the difference resistor(R3). For low source impedances, the error is acceptable. Forbetter accuracy at higher source impedances, a compensatingresistor (RC) can be added to restore the ratio matching. Theresistors in the INA105 are 25k?. For 0.1% gain accuracy,no compensating resistor is required with source impedancesup to 25?. For source impedances up to 2.5k?, use acompensating resistor which matches RS within 1%. If thesource impedance is not known exactly, a trim pot can beused to adjust gain —————————————————————————————————————— ------------------------------------------------------------------------------------------------ accuracy. For operation with source impedances greater than 2.5k?, aunity-gain-connected single-supply op amp can be added tobuffer the input as shown in Figure 3. Although the input range of the OPA1013 single-supply op amp includes thenegative rail, its output can not quite swing all the way to therail. The negative swing limit of this circuit is therefore?100mV—still adequate in many applications. For an instrumentation amplifier (IA) front end, the otherhalf of the OPA1013 can be connected to the inverting inputof the INA105 (pin 2) as shown in Figure 4. For a true single-supply ISO amp with high impedancedifferential inputs, the circuit shown in Figure 5 can be used.In this circuit, the inputs—and therefore the outputs—of theOPA1013s are level-shifted up a VBE with a matched pair ofPNP input transistors. The transistors are biased as emitterfollowers by a pair of 100μA current sources contained in theREF200 dual current source. FIGURE 3. Single Supply (almost—see text), High Input Impedance Isolation Amplifier. 2 The circuits shown in this bulletin were designed for 0V to5V operation from a single +15V power supply. With re-duced range, —————————————————————————————————————— ------------------------------------------------------------------------------------------------ operation from a lower voltage is feasible. Forhigher input range the circuit can be operated from a highersupply voltage. Table 1 shows the ranges obtainable forselected power supplies. VS(V)20+1512 INPUT RANGEFIGURES 1, 2 (V) (1) –2 to +10–2 to +5–2 to +2 INPUT RANGEFIGURES 3, 4 (V) (1) 0.1 to +100.1 to +50.1 to +2 INPUT RANGEFIGURE 5(V) (1) –0.3 to +10–0.3 to +5–0.3 to +2 Note: (1) Since the amplifier is unity gain, the input range is also the outputrange. The output can go to –2V since the output section of the ISO ampoperates from dual supplies. TABLE 1. Single-Supply ISO Amp Input Range vs Power Supply. FIGURE 4. Single Supply (almost—see text), Isolation Amplifier with High-Impedance Differential Inputs.. FIGURE 5. Single Supply Isolation Amplifier with High-Impedance Differential Inputs. 3 —————————————————————————————————————— ------------------------------------------------------------------------------------------------ For a single-supply ISO amp with higher common-mode-voltage differential inputs, an INA117 high common-modevoltage difference amplifier can be substituted for the INA105difference amplifier as shown in Figure 6. With a +15Vpower supply, the input common mode range is approxi-mately +125V, –50V. With a +12V supply, the input com-mon mode range is approximately ?50V. Differential inputrange remains as shown in Table I for Figures 1 and 2. FIGURE 6. Single Supply Isolation Amplifier with High Common-Mode Range Differential Inputs. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to changewithout notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrantany BURR-BROWN product for use in life support devices and/or systems. 4 IMPORTANT NOTICE Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinueany product or service —————————————————————————————————————— ------------------------------------------------------------------------------------------------ without notice, and advise customers to obtain the latest version of relevant informationto verify, before placing orders, that information being relied on is current and complete. All products are soldsubject to the terms and conditions of sale supplied at the time of order acknowledgment, including thosepertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale inaccordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extentTI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarilyperformed, except those mandated by government requirements. Customers are responsible for their applications using TI components. In order to minimize risks associated with the customer’s applications, adequate design and operatingsafeguards must be provided by the customer to minimize inherent or procedural hazards. TI assumes no liability for applications assistance or customer product design. TI does not warrant or representthat any license, either express or implied, is granted under any patent right, copyright, mask work right, or otherintellectual property right of TI covering or relating to —————————————————————————————————————— ------------------------------------------------------------------------------------------------ any combination, machine, or process in which suchsemiconductor products or services might be or are used. TI’s publication of information regarding any thirdparty’s products or services does not constitute TI’s approval, warranty or endorsement thereof. Copyright ? 2000, Texas Instruments Incorporated ——————————————————————————————————————