ISO255 ® ISO 255 Precision, Powered, Three-Port Isolated INSTRUMENTATION AMPLIFIER FEATURES DESCRIPTION ● RATED 1500Vrms Continuous 2500Vrms for One Minute 100% Tested for Partial Discharge ● GAINS OF 1 TO 10,000 ● LOW NONLINEARITY: ±0.05% typ ● LOW INPUT BIAS CURRENT: ±10nA max ● LOW INPUT OFFSET VOLTAGE ISO255 is a precision three-port isolated instrumentation amplifier incorporating a novel duty cycle modulation-demodulation technique and has excellent accuracy. Internal input protection can withstand up to ±40V input differential without damage. The signal is transmitted digitally across a differential capacitive barrier. With digital modulation the barrier characteristics do not affect signal integrity. This results in excellent reliability and good high frequency transient immunity across the barrier. The DC/DC converter, amplifier and barrier capacitors are housed in a plastic DIP. ● ● ● ● INPUTS PROTECTED TO ±40V BIPOLAR OPERATION VO = ±10V SYNCHRONIZATION CAPABILITY 28-PIN PLASTIC DIP: 0.6" Wide A power supply range of 11V to 18V makes this amplifier ideal for a wide range of applications. APPLICATIONS ISO255 ● INDUSTRIAL PROCESS CONTROL Transducer Isolator, Thermocouple Isolator, RTD Isolator, Pressure Bridge Isolator, Flow Meter Isolator ● POWER MONITORING ● MEDICAL INSTRUMENTATION ● ANALYTICAL MEASUREMENTS ● BIOMEDICAL MEASUREMENTS ● DATA ACQUISITION 4 1 2 3 26 28 27 ● TEST EQUIPMENT ● GROUND LOOP ELIMINATION 25 +VIN +RG –RG VOUT INA 14 –VIN Com1 Com2 +VS1 +VS2 –VS1 –VS2 GND1 GND2 GND3 SYNC 17 16 13 12 11 10 +VS3 15 International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 • Twx: 910-952-1111 Internet: http://www.burr-brown.com/ • FAXLine: (800) 548-6133 (US/Canada Only) • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 1996 Burr-Brown Corporation PDS-1312B Printed in U.S.A. April, 1996 SPECIFICATIONS At TA = +25°C, VS3 = 15V, RL = 2kΩ, and 220nF capacitors on all generated supplies, unless otherwise noted. ISO255P PARAMETER ISOLATION Voltage Rated Continuous: AC 100% Test (AC 50Hz) Rated One Min Isolation-Mode Rejection DC AC 50Hz Barrier Impedance Leakage Current GAIN Gain Equation Gain Error Gain vs Temperature Nonlinearity CONDITIONS MIN TMIN to TMAX 1s; Partial Discharge ≤ 5pC 1500 2500 2500 TYP VISO = 240Vrms, 50Hz G=1 G = 10 G = 100 G = 1000 G=1 G = 10 G = 100 G = 1000 G=1 G = 10 G = 100 G = 1000 1 + 50kΩ/RG 0.15 0.15 0.15 0.2 ±50 ±50 ±50 ±50 ±0.05 ±0.05 ±0.05 ±0.05 INPUT OFFSET VOLTAGE Initial Offset vs Temperature CMRR vs Supply ± (1 + 520/G) 90 1 INPUT Voltage Range Bias Current vs Temperature Offset Current vs Temperature ±10 ±40 ±40 OUTPUT Voltage Range Current Drive Capacitive Load Drive Ripple Voltage FREQUENCY RESPONSE Small Signal Bandwidth Slew Rate Settling Time, 0.1% POWER SUPPLIES Rated Voltage Voltage Range Quiescent Current Rated Output Voltage ±0.35 V/V % ±0.95 % ±0.102 ppm/°C ppm/°C ppm/°C ppm/°C % ±0.104 % ± (0.125 + 101/G) mV µV/°C dB mV/V ±10 ±10 No Load 50mA Load On Two Supplies 11 25 13 12 0.1 25 50 50 30 4 0.2 20 20 30 240 kHz kHz kHz kHz V/µs µs µs µs µs 40 14.5 13.2 28 1 1 18 55 16 1.4 50 TEMPERATURE RANGE Operating Storage –40 –40 ® 2 V nA pA/°C nA pA/°C V mA µF mVp-p 15 Load Regulation Line Regulation SYNC Frequency Output Voltage Ripple ISO255 2 dB dB Ω || pF µArms ±10 ±5 G=1 G = 10 G = 100 G = 1000 G = 10 G=1 G = 10 G = 100 G = 1000 UNITS Vrms Vrms Vrms 120 95 1014 || 2 1.4 1500Vrms MAX 85 85 V V mA V V mV/mA V/V MHz mV °C °C PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS Supply Voltage ................................................................................... +18V VIN, Analog Input Voltage Range ....................................................... ±40V Com1 to GND1 .................................................................................... ±1V Com2 to GND2 .................................................................................... ±1V Continuous Isolation Voltage: .................................................... 1500Vrms ..................................................................................... 2500Vrms one min IMV, dv/dt ...................................................................................... 20kV/µs Junction Temperature ...................................................................... 150°C Storage Temperature ........................................................ –40°C to +85°C Lead Temperature (soldering, 10s) ................................................ +300°C Output Short Duration .......................................... Continuous to Common 28 +VS1 –RG 2 27 –VS1 –VIN 3 26 Com1 +VIN 4 25 GND1 –VS2 11 Any integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. +VS2 12 17 GND3 Com2 13 16 SYNC VOUT 14 ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet published specifications. 15 +VS3 PACKAGE INFORMATION PRODUCT ISO255P PACKAGE PACKAGE DRAWING NUMBER(1) 28-Pin Plastic DIP 335 NOTE: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. ORDERING INFORMATION ISO255P 1 GND2 10 ELECTROSTATIC DISCHARGE SENSITIVITY PRODUCT +RG PACKAGE 28-Pin Plastic DIP 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. ® 3 ISO255 TYPICAL PERFORMANCE CURVES (CONT) At TA = +25°C, +VS3 = 15V, RL = 2kΩ, and 220nF capacitors on all generated supplies, unless otherwise noted. ISOLATION LEAKAGE CURRENT vs FREQUENCY (V = 240Vrms) IMR vs FREQUENCY 1k 120 Rejection (dB) µAmps (rms) 100 100 10 80 60 1 40 10 1 100 1k 10k 100k 1 SIGNAL RESPONSE vs CARRIER FREQUENCY SINE RESPONSE (f = 1kHz, G = 1) Input (V) 0 –20 Signal (dB) 1k 10k 1500 2000 10 –10 –30 5 0 –5 –40 5 –50 Output (V) –60 –70 –80 –90 0 100k 200k 300k 400k 500k 600k 700k 0 –5 –10 800k 0 Frequency (Hz) 500 1000 Time (µs) SINE RESPONSE (f = 10kHz, G = 1) PULSE RESPONSE (f = 1kHz, G = 1) 10 10 Input (V) 5 0 5 0 –5 –5 5 5 Output (V) Input (V) 100 Frequency (Hz) 10 Output (V) 10 Frequency (Hz) 0 –5 –10 0 –5 –10 0 50 100 150 200 0 Time (µs) ® ISO255 500 Time (µs) 4 1000 TYPICAL PERFORMANCE CURVES (CONT) At TA = +25°C, +VS3 = 15V, RL = 2kΩ, and 220nF capacitors on all generated supplies, unless otherwise noted. PULSE RESPONSE (f = 10kHz, G = 1) GAIN vs FREQUENCY G = 1000 1 40 G = 100 Gain (dB) Input (V) 60 –1 Output (V) 1 20 G = 10 0 G=1 –20 –1 –40 0 20 40 60 80 1 100 100 1k 10k 100k Frequency (Hz) COMMON-MODE REJECTION vs FREQUENCY INPUT COMMON-MODE RANGE vs OUTPUT VOLTAGE 140 1M 15 120 Common-Mode Voltage (V) Common-Mode Rejection (dB) 10 Time (µs) G=1000 100 G=100 80 G=10 60 G=1 40 20 G ≥ 10 10 G ≥ 10 G=1 5 G=1 – VD/2 0 + – VD/2 –5 VO + VCM –10 All Gains 0 1 10 100 1k 10k –15 –15 100k –10 All Gains –5 Frequency (Hz) 0 5 10 15 Output Voltage (V) INPUT BIAS AND OFFSET CURRENT vs TEMPERATURE ISOLATION MODE VOLTAGE vs FREQUENCY 4 Max DC Rating IOS 3 2 Peak Isolation Voltage Input Bias and Offset Current (nA) 5 ±Ib 1 0 –1 –2 –3 2.1k Max AC Rating 1.0k Degraded Performance 100 Typical Performance –4 10 –5 –75 –50 –25 0 25 50 75 100 125 100 Temperature (°C) 1k 10k 100k 1M 10M 100M Frequency (Hz) ® 5 ISO255 BASIC OPERATION internal feedback resistors of the input amplifiers. These on chip metal film resistors are laser trimmed to accurate absolute values. The accuracy and temperature coefficient of these resistors are included in the gain accuracy and drift specifications of the ISO255. ISO255 is a precision, powered, three-port isolated instrumentation amplifier. The input and output sections are galvanically isolated by matched and EMI shielded capacitors built into the plastic package. The DC/DC converter input is also galvanically isolated from both the input and output supplies. INPUT COMMON-MODE RANGE The linear common-mode range of the input circuitry of the ISO255 is approximately ±14V (or 1V from the power supplies). As the output voltage increases. However, the linear input range will be limited by the output voltage swing of the internal amplifiers. Thus, the linear common-mode range is related to the output voltage of the complete input amplifier—see performance curves “Input Common-Mode Range vs Output Voltage.” A combination of common-mode and differential input voltage can cause the output voltage of the internal amplifiers to saturate. For applications where input common-mode range must be maximized, limit the output voltage swing by selecting a lower input gain. Input-overload can produce an output voltage that appears normal. For example, an input voltage of +20V on one input and +40V on the other input will exceed the linear commonmode range of both input amplifiers. Since both input amplifiers are saturated to nearly the same output voltage limit, the difference voltage measured by the output amplifier will be near zero. The output of the instrumentation amplifier will be near 0V even though both inputs are overloaded. SIGNAL AND POWER CONNECTIONS Figure 1 shows proper power and signal connections. The power supply input pin +VS3 should be bypassed with a 2.2µF tantalum capacitor and the outputs VS1 and VS2 with 220nF ceramic capacitors located as close to the amplifier as possible. All ground connections should be run independently to a common point. Signal Common on the input section provides a low-impedance point for sensing signal ground in noisy applications. Com1 and Com2 must have a path to ground for signal current return and should be maintained within ±1V of GND1 and GND2 respectively. SETTING THE GAIN Gain of the ISO255 is set by a single external resistor, RG, connected between pins 1 and 2: G = 1+ 50kΩ RG (1) The 50kΩ term in Equation 1 comes from the sum of the two ISO255 4 +VIN 1 +RG VIN RG 2 –RG VOUT 14 INA VOUT 3 –VIN +15VOUT –15VOUT 26 Com1 Com2 13 28 +VS1 +VS2 12 27 –VS1 –VS2 11 +15VOUT –15VOUT GND2 10 25 GND1 GND3 SYNC +VS3 17 VCM 220nF 16 220nF 15 2.2µF SYNC +15V FIGURE 1. Basic Connections. ® ISO255 6 220nF 220nF INPUT PROTECTION The inputs of the amplifier are individually protected for voltages up to ±40V. Internal circuitry on each input provides low series impedance under normal signal conditions. If the input is overloaded, the protection circuitry limits the input current to a safe value (approximately 1.5mA). The inputs are protected even if no power supply is present. When connecting up to eight ISO255’s without a driver the unit with the highest natural frequency will determine the synchronized running frequency. The SYNC pin is sensitive to capacitive loading: 150pF or less is recommended. If unused, the SYNC pin should be left open. Avoid shorting the SYNC pin directly to ground or supply potentials; otherwise damage may result. Soft start circuitry protects the MOSFET switches during startup. This is accomplished by holding the gate-to-source voltage of both MOSFET switches low until the free-running oscillator is fully operational. In addition, soft start circuitry and input current sensing also protects the switches. This current limiting keeps the MOSFET switches operating in their safe operating area under fault conditions or excessive loads. When either of these conditions occur, the peak input current exceeds a safe limit. The result is an approximate 5% duty cycle, 300µs drive period to the MOSFET switches. This protects the internal MOSFET switches as well as the external load from any thermal damage. When the fault or excessive load is removed, the converter resumes normal operation. A delay period of approximately 50µs incorporated in the current sensing circuitry allows the output filter capacitors to fully charge after a fault is removed. This delay period corresponds to a filter capacitance of no more than 1µF at either of the output pins. This provides full protection of the MOSFET switches and also sufficiently filters the output ripple voltage. The current sensing circuitry is designed to provide thermal protection for the MOSFET switches over the operating temperature range as well. When these conditions are exceeded, the unit will go into its shutdown mode. DC/DC CONVERTER ISO255 provides a reliable solution to the need for integral power. The high isolation rating being achieved by careful design and attention to the physical construction of the transformer. In addition to the high dielectric strength a low leakage coating increases the isolation voltage range. The soft start oscillator/driver design eliminates high inrush currents during turn-on. Input current sensing protects both the converter and the load from possible thermal damage during a fault condition. The DC/DC converter is synchronized to the amplifier and when multiple ISO255’s are used, each channel can be synchronized via the SYNC pin. The DC/DC converter consists of a free-running oscillator, control and switch driver circuitry, MOSFET switches, a transformer, rectifier diodes and filter capacitors all contained within the ISO255 package. The control circuitry consists of current limiting, soft start and synchronization features. In instances where several ISO255’s are used in a system, beat frequencies developed between the ISO255’s are a potential source of low frequency noise in the supply and ground paths. This noise may couple into the signal path and can be avoided by synchronizing the individual ISO255’s together by tying the SYNC pins together or using the circuit in Figure 2 to drive the SYNC pins from an external source. OUTPUT CURRENT RATINGS The total current which can be drawn from each output supply on the ISO255 is a function of the total power drawn from all outputs. For example if three outputs are not used then maximum current can be drawn from one output. In all cases, the total maximum current that can be drawn from any combination of outputs is: +15V MC1472 or Equivalent Peripheral +5V Driver 330Ω I1+ + I1– + I2+ + I2– ≤ 50mΑ 2N3904 TTL SYNC +VS3, GND3 The waveform of the ground return current is an 800kHz sawtooth. A capacitor between +VS3 and GND3 provides a bypass for the AC portion of this current. The power should never be instantaneously interrupted to the ISO255 (i.e., a break in the line to +VS3 either by accidental or switch means.) Normal power down of the +VS3 supply is not considered instantaneous. Should a rapid break in input power occur the internal transformers voltage will rapidly rise to maintain current flow and may cause internal damage to the ISO255. ISO255 620Ω 2N3904 16 15 17 100Ω ISO255 16 15 17 To Other ISO255’s SYNCHRONIZED OPERATION ISO255 can be synchronized to an external signal source. This capability is useful in eliminating troublesome beat FIGURE 2. External SYNC drive. ® 7 ISO255 frequencies in multi-channel systems and in rejecting AC signals and their harmonics. To use this feature, tie all sync pins together or apply an external signal to the SYNC pin. ISO255 can be synchronized to an external oscillator over the range 1-1.4MHz (this corresponds to a modulation frequency of 500kHz to 700kHz as SYNC is internally divided by 2). Leakage current is determined solely by the impedance of the barrier and transformer capacitance and is plotted in the “Isolation Leakage Current vs Frequency” curve. ISOLATION VOLTAGE RATINGS Because a long-term test is impractical in a manufacturing situation, the generally accepted practice is to perform a production test at a higher voltage for some shorter time. The relationship between actual test voltage and the continuous derated maximum specification is an important one. CARRIER FREQUENCY CONSIDERATIONS ISO255 amplifiers transmit the signal across the ISO-barrier by a duty-cycle modulation technique. This system works like any linear amplifier for input signals having frequencies below one half the carrier frequency, fC. For signal frequencies above fC/2, the behavior becomes more complex. The “Signal Response vs Carrier Frequency” performance curve describes this behavior graphically. Historically, Burr-Brown has chosen a deliberately conservative one: VTEST = (2 x ACrms continuous rating) + 1000V for 10 seconds, followed by a test at rated ACrms voltage for one minute. This choice was appropriate for conditions where system transients are not well defined. Recent improvements in high-voltage stress testing have produced a more meaningful test for determining maximum permissible voltage ratings, and Burr-Brown has chosen to apply this new technology in the manufacture and testing of the ISO255. It should be noted that for the ISO255, the carrier frequency is nominally 400kHz and the –3dB point of the amplifier is 60kHz. Spurious signals at the output are not significant under these circumstances unless the input signal contains significant components above 200kHz. When periodic noise from external sources such as system clocks and DC/DC converters are a problem, ISO255 can be used to reject this noise. The amplifier can be synchronized to an external frequency source, fEXT, placing the amplifier response curve at one of the frequency and amplitude nulls indicated in the “Signal Response vs Carrier Frequency” performance curve. Partial Discharge When an insulation defect such as a void occurs within an insulation system, the defect will display localized corona or ionization during exposure to high-voltage stress. This ionization requires a higher applied voltage to start the discharge and lower voltage to maintain it or extinguish it once started. The higher start voltage is known as the inception voltage, while the extinction voltage is that level of voltage stress at which the discharge ceases. Just as the total insulation system has an inception voltage, so do the individual voids. A voltage will build up across a void until its inception voltage is reached, at which point the void will ionize, effectively shorting itself out. This action redistributes electrical charge within the dielectric and is known as partial discharge. If, as is the case with AC, the applied voltage gradient across the device continues to rise, another partial discharge cycle begins. The importance of this phenomenon is that, if the discharge does not occur, the insulation system retains its integrity. If the discharge begins, and is allowed to continue, the action of the ions and electrons within the defect will eventually degrade any organic insulation system in which they occur. The measurement of partial discharge is still useful in rating the devices and providing quality control of the manufacturing process. The inception voltage for these voids tends to be constant, so that the measurement of total charge being redistributed within the dielectric is a very good indicator of the size of the voids and their likelihood of becoming an incipient failure. The bulk inception voltage, on the other hand, varies with the insulation system, and the number of ionization defects and directly establishes the absolute maximum voltage (transient) that can be applied across the test device before destructive partial discharge can begin. Measuring the bulk ISOLATION MODE VOLTAGE Isolation Mode Voltage (IMV) is the voltage appearing between isolated grounds GND1 and GND2. IMV can induce errors at the output as indicated by the plots of IMV vs Frequency. It should be noted that if the IMV frequency exceeds fC/2, the output will display spurious outputs, and the amplifier response will be identical to that shown in the “Signal Response vs Carrier Frequency” performance curve. This occurs because IMV-induced errors behave like inputreferred error signals. To predict the total IMR, divide the isolation voltage by the IMR shown in “IMR vs Frequency” performance curve and compute the amplifier response to this input-referred error signal from the data given in the “Signal Response vs Carrier Frequency” performance curve. Due to effects of very high-frequency signals, typical IMV performance can be achieved only when dV/dT of the isolation mode voltage falls below 1000V/µs. For convenience, this is plotted in the typical performance curves for the ISO255 as a function of voltage and frequency for sinusoidal voltages. When dV/dT exceeds 1000V/µs but falls below 20kV/µs, performance may be degraded. At rates of change above 20kV/µs, the amplifier may be damaged, but the barrier retains its full integrity. Lowering the power supply voltage below 15V may decrease the dV/dT to 500V/µs for typical performance, but the maximum dV/dT of 20kV/µs remains unchanged. ® ISO255 8 extinction voltage provides a lower, more conservative voltage from which to derive a safe continuous rating. In production, measuring at a level somewhat below the expected inception voltage and then de-rating by a factor related to expectations about system transients is an accepted practice. discharge. VDE in Germany, an acknowledged leader in high-voltage test standards, has developed a standard test method to apply this powerful technique. Use of partial discharge testing is an improved method for measuring the integrity of an isolation barrier. To accommodate poorly-defined transients, the part under test is exposed to voltage that is 1.6 times the continuousrated voltage and must display less than or equal to 5pC partial discharge level in a 100% production test. Partial Discharge Testing Not only does this test method provide far more qualitative information about stress-withstand levels than did previous stress tests, but it provides quantitative measurements from which quality assurance and control measures can be based. Tests similar to this test have been used by some manufacturers, such as those of high-voltage power distribution equipment, for some time, but they employed a simple measurement of RF noise to detect ionization. This method was not quantitative with regard to energy of the discharge, and was not sensitive enough for small components such as isolation amplifiers. Now, however, manufacturers of HV test equipment have developed means to quantify partial APPLICATIONS The ISO255 isolation amplifiers are used in three categories of applications: • Accurate isolation of signals from high voltage ground potentials • Accurate isolation of signals from severe ground noise and • Fault protection from high voltages in analog measurements +15V +15V –15V 1µF Tantalum 200kΩ 2 Offset +VS1 +V REF102 1MΩ 6 45kΩ OPA27 0.1µF 4 +RG –VS1 4 –RG +VIN 100kΩ VOUT 14 VOUT 10kΩ 22kΩ 22kΩ 22kΩ 3 ISO255 22kΩ 28 +VS1 27 220nF 2 3 –VIN +VS1 Com2 –VS1 +VS2 7 OPA27 6 4 220nF –VS2 26 25 11 220nF 10kΩ 0.1µF Com1 GND1 GND2 GND3 –VS1 12 10 220nF +VS3 2.2µF +15V FIGURE 3. Conditioning a Bridge Circuit. ® 9 ISO255