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APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (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 a single external power supply. In battery powered systems such as aircraft and automotive, it’s often a requirement. Isolation amplifiers such as the ISO120 and ISO122 can be easily modified for input side single-supply operation with the addition of an INA105 difference amplifier. With ISO amps, it’s the isolated input side power supply which most often needs to be single supply. The output side of the ISO amp uses a split ±15V power supply, allowing a full ±10V output swing. The most common application is for a single ended input referred to ground as shown. For a differential input, pin 2 can be connected to a second input instead of ground. This provides a 0V to 5V differential input with common-mode to either rail. To understand how the circuit works, consider the operation of the INA105 difference amplifier. The difference amplifier forces its output (pin 6) relative to its reference (pin 1) to be equal to the differential input (pin 3 – pin 2). The difference amplifier reference pin and the ISO amp common are held at approximately 5.1V by the 10kΩ resistor and the zener diode. This pseudo ground establishes an arbitrary acceptable operating point for the ISO amp. The difference amplifier then translates its input, relative to true ground, up to the 5.1V pseudo ground. In other words, a 0V to 5V input between pins 3 and 2 of the INA105 is seen as a 0V to 5V signal 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. Common zener diodes, on the other hand, may require several mA for proper operation. The 1N4689 zener diode specified is a low level type designed for applications requiring low operating currents. It has a sharp breakdown voltage specified at a low 50µA. The difference amplifier has advantages as compared to traditional single-supply amplifiers. The inputs of a difference amplifier can swing to both the positive and negative power-supply rails. In fact, in the application shown in Figures 1 and 2, the input range of the circuit extends approximately 2V below ground (the negative power supply rail). This is because the resistors internal to the INA105 divide the input level in half as seen by the op amp. The technique is illustrated in Figures 1 and 2 using the ISO120 and ISO122. These ISO amps are specified for operation from dual supplies as low as ±4.5V and can be operated with a total single power supply voltage as low as 9V. The circuit shown is designed for operation from a single +15V power supply. This allows a 0V to +5V input range. VS1 (+15V) 7 INA105 Difference Amp 2 5 R1 10kΩ +VS2 (+15V) R2 15 In 6 1 9 Signal Source VIN + RS R3 3 R4 ISO 122 (1) 1 RC Gnd 16 4 Reference IN4689 5.1V 7 8 VOUT = VIN 10 2 –VS1 Com 2 –VS2 (–15V) NOTE: (1) Select to match RS . FIGURE 1. Single Supply Operation of the ISO122 Isolation Amplifier. 16 © SBOA004 1990 Burr-Brown Corporation 1 AB-009A Printed in U.S.A. August, 1991 VS1 (+15V) 7 INA105 Difference Amp 2 5 R1 10kΩ 3 23 In ISO 15 11 120 10 22 Osc 21 Com 12 9 24 Gnd 16 4 –VS1 Com 2 6 Signal Source VIN + RS 4 (1) R4 R3 3 +VS2 (+15V) R2 1 RC Reference IN4689 5.1V VOUT = VIN –VS2 (–15V) NOTE: (1) Select to match RS . 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 source impedance of the signal (RS) adds to the difference resistor (R3). For low source impedances, the error is acceptable. For better accuracy at higher source impedances, a compensating resistor (RC) can be added to restore the ratio matching. The resistors in the INA105 are 25kΩ. For 0.1% gain accuracy, no compensating resistor is required with source impedances up to 25Ω. For source impedances up to 2.5kΩ, use a compensating resistor which matches RS within 1%. If the source impedance is not known exactly, a trim pot can be used to adjust gain accuracy. For operation with source impedances greater than 2.5kΩ, a unity-gain-connected single-supply op amp can be added to buffer the input as shown in Figure 3. Although the input range of the OPA1013 single-supply op amp includes the negative rail, its output can not quite swing all the way to the rail. The negative swing limit of this circuit is therefore ≈100mV—still adequate in many applications. For an instrumentation amplifier (IA) front end, the other half of the OPA1013 can be connected to the inverting input of the INA105 (pin 2) as shown in Figure 4. For a true single-supply ISO amp with high impedance differential inputs, the circuit shown in Figure 5 can be used. In this circuit, the inputs—and therefore the outputs—of the OPA1013s are level-shifted up a VBE with a matched pair of PNP input transistors. The transistors are biased as emitter followers by a pair of 100µA current sources contained in the REF200 dual current source. VS1 (+15V) 7 INA105 Difference Amp 2 5 R1 10kΩ +VS2 (+15V) R2 6 15 In 1 9 1/2 OPA1013 VIN R3 3 ISO 122 R4 1 Gnd 16 4 Reference IN4689 5.1V 7 8 10 2 –VS1 Com 2 –VS2 (–15V) FIGURE 3. Single Supply (almost—see text), High Input Impedance Isolation Amplifier. 2 VOUT = VIN The circuits shown in this bulletin were designed for 0V to 5V operation from a single +15V power supply. With reduced range, operation from a lower voltage is feasible. For higher input range the circuit can be operated from a higher supply voltage. Table 1 shows the ranges obtainable for selected power supplies. VS (V) INPUT RANGE FIGURES 1, 2 (V) (1) INPUT RANGE FIGURES 3, 4 (V) (1) INPUT RANGE FIGURE 5 (V) (1) 20+ 15 12 –2 to +10 –2 to +5 –2 to +2 0.1 to +10 0.1 to +5 0.1 to +2 –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 output range. The output can go to –2V since the output section of the ISO amp operates from dual supplies. TABLE 1. Single-Supply ISO Amp Input Range vs Power Supply. VS1 (+15V) 7 1/2 OPA1013 INA105 Difference Amp 2 5 VN R1 10kΩ +VS2 (+15V) R2 6 15 In 1 9 1/2 OPA1013 R4 R3 3 ISO 122 1 Gnd 16 VP 4 7 10 2 –VS1 Reference VOUT = VP – V N 8 Com 2 IN4689 5.1V –VS2 (–15V) FIGURE 4. Single Supply (almost—see text), Isolation Amplifier with High-Impedance Differential Inputs.. VS1 (+15V) 7 100µA REF200 1/2 OPA1013 INA105 Difference Amp 2 5 R1 10kΩ +VS2 (+15V) R2 6 15 In 1 9 VN PMI MAT03 1/2 OPA1013 3 1 Gnd 16 4 VP R4 R3 ISO 122 Reference IN4689 5.1V FIGURE 5. Single Supply Isolation Amplifier with High-Impedance Differential Inputs. 3 7 8 VOUT = VP – V N 10 2 –VS1 Com 2 –VS2 (–15V) power supply, the input common mode range is approximately +125V, –50V. With a +12V supply, the input common mode range is approximately ±50V. Differential input range remains as shown in Table I for Figures 1 and 2. For a single-supply ISO amp with higher common-modevoltage differential inputs, an INA117 high common-mode voltage difference amplifier can be substituted for the INA105 difference amplifier as shown in Figure 6. With a +15V VS1 (+15V) 7 V2 2 10kΩ V3 6 380kΩ 3 21.1kΩ 20kΩ INA117 4 8 +VS2 (+15V) 380kΩ 380kΩ 1 5 Reference IN4689 5.1V 3 23 In 15 11 ISO 10 21 120 22 Com 12 9 24 Gnd 16 4 –VS1 Com 2 VOUT = V3 –V2 –VS2 (–15V) 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 change without 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 warrant any 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 discontinue any product or service without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those pertaining to warranty, patent infringement, and limitation of liability. TI warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed, 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 operating safeguards 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 represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of TI covering or relating to any combination, machine, or process in which such semiconductor products or services might be or are used. TI’s publication of information regarding any third party’s products or services does not constitute TI’s approval, warranty or endorsement thereof. Copyright 2000, Texas Instruments Incorporated