120 khz Bandwidth, Low Distortion, Isolation Amplifier AD215

Transcription

1 a FEATURES Isolation Voltage Rating:, V rms Wide Bandwidth: khz, Full Power ( db) Rapid Slew Rate: V/ s Fast Settling Time: 9 s Low Harmonic Distortion: 8 khz Low Nonlinearity:.% Wide Output Range: V, min (Buffered) Built-in Isolated Power Supply: V ma Performance Rated over C to +8 C APPLICATIONS INCLUDE High Speed Data Acquisition Systems Power Line and Transient Monitors Multichannel Muxed Input Isolation Waveform Recording Instrumentation Power Supply Controls Vibration Analysis khz Bandwidth, Low Distortion, Isolation Amplifier AD UNMITTED INPUT OP AMP FUNCTIONAL BLOCK DIAGRAM MODULATOR ISOLATED DC SUPPLY SIGNAL T POWER T DEMODULATOR LOW-PASS FILTER khz khz POWER OSCILLATOR AD R kω R 8.µF +V IN V IN GENERAL DESCRIPTION The AD is a high speed input isolation amplifier designed to isolate and amplify wide bandwidth analog signals. The innovative circuit and transformer design of the AD ensures wideband dynamic characteristics while preserving key dc performance specifications. The AD provides complete galvanic isolation between the input and output of the device including the user-available front-end isolated power supplies. The functionally complete design, powered by a ± V dc supply, eliminates the need for a user supplied isolated dc/dc converter. This permits the designer to minimize circuit overhead and reduce overall system design complexity and component costs. The design of the AD emphasizes maximum flexibility and ease of use in a broad range of applications where fast analog signals must be measured under high common-mode voltage (CMV) conditions. The AD has a ± V input/output range, a specified gain range of V/V to V/V, a buffered output with offset trim and a user-available isolated front-end power supply which produces ± V dc at ± ma. PRODUCT HIGHLIGHTS High Speed Dynamic Characteristics: The AD features a typical full-power bandwidth of khz ( khz min), rise time of µs and settling time of 9 µs. The high speed performance of the AD allows for unsurpassed galvanic isolation of virtually any wideband dynamic signal. Flexible Input and Buffered Output Stages: An uncommitted op amp is provided on the input stage of the AD to allow for input buffering or amplification and signal conditioning. The AD also features a buffered output stage to drive low impedance loads and an output voltage trim for zeroing the output offset where needed. High Accuracy: The AD has a typical nonlinearity of ±.% (B grade) of full-scale range and the total harmonic distortion is typically 8 db at khz. The AD provides designers with complete isolation of the desired signal without loss of signal integrity or quality. Excellent Common-Mode Performance: The ADBY (ADAY) provides, V rms (7 V rms) common-mode voltage protection from its input to output. Both grades feature a low common-mode capacitance of. pf inclusive of the dc/dc power isolation. This results in a typical common-mode rejection specification of db and a low leakage current of. µa rms max ( V rms, Hz). Isolated Power: An unregulated isolated power supply of ± V ± ma is available at the isolated input port of the AD. This permits the use of ancillary isolated front-end amplifiers or signal conditioning components without the need for a separate dc/dc supply. Even the excitation of transducers can be accomplished in most applications. Rated Performance over the C to +8 C Temperature Range: With an extended industrial temperature range rating, the AD is an ideal isolation solution for use in many industrial environments. Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Analog Devices, Inc., 99 One Technology Way, P.O. Box 9, Norwood, MA -9, U.S.A. Tel: 7/9-7 Fax: 7/-87

2 AD SPECIFICATIONS + C, V S = V dc, k output load, unless otherwise noted.) ADAY/BY Parameter Conditions Min Typ Max Units GAIN Range V/V Error G = V/V, No Load on V ISO ±. ± % vs. Temperature C to +8 C + ppm/ C C to C + ppm/ C vs. Supply Voltage ±(. V dc to. V dc) + ppm/v vs. Isolated Supply Load + ppm/ma Nonlinearity ADBY Grade ± V Output Swing, G = V/V ±. ±. % ± V Output Swing, G = V/V ±. % ADAY Grade ± V Output Swing, G = V/V ±. ±. % ± V Output Swing, G = V/V ±. % INPUT VOLTAGE RATINGS Input Voltage Rating G = V/V ± V Maximum Safe Differential Range or, to ± V CMRR of Input Op Amp db Isolation Voltage Rating Input to Output, AC, Hz ADBY Grade % Tested V rms ADAY Grade % Tested 7 V rms IMRR (Isolation Mode Rejection Ratio) R S Ω ( & ), G = V/V, Hz db R S Ω ( & ), G = V/V, khz db R S Ω ( & ), G = V/V, khz 8 db R S kω ( & ), G = V/V, Hz db R S kω ( & ), G = V/V, khz 8 db R S kω ( & ), G = V/V, khz db Leakage Current, Input to Output V rms, Hz µa rms INPUT IMPEDANCE Differential G = V/V MΩ Common Mode. GΩ pf INPUT OFFSET VOLTAGE + C ±. ±. mv vs. Temperature C to +8 C ± µv/ C C to C ± µv/ C OFFSET VOLTAGE + C, Trimmable to Zero 8 mv vs. Temperature C to +8 C ± µv/ C C to C ±8 µv/ C vs. Supply Voltage ± µv/v vs. Isolated Supply Load µv/ma INPUT BIAS CURRENT + C na vs. Temperature C to +8 C ± na INPUT DIFFERENCE CURRENT + C ± na vs. Temperature C to +8 C ± na INPUT VOLTAGE NOISE Input Voltage Noise Frequency > Hz nv/ Hz DYNAMIC RESPONSE ( kω Load) Full Signal Bandwidth ( db) G = V/V, V pk-pk Signal khz Transport Delay. µs Slew Rate ± V Output Swing V/µs Rise Time % to 9%, ± V Output Swing µs

3 AD ADAY/BY Parameter Conditions Min Typ Max Units DYNAMIC RESPONSE ( kω Load) Cont. Settling Time to ±.%, ± V Output Swing 9 µs Overshoot % Harmonic Distortion khz 8 khz db Overload Recovery Time G = V/V, ± V Drive µs Output Overload Recovery Time G > µs RATED Voltage Out HI to Out LO ± V Current kω Load ± ma Max Capacitive Load pf Output Resistance Ω Output Ripple and Noise 7 MHz Bandwidth mv pk-pk khz Bandwidth. mv pk-pk ISOLATED POWER 8 Voltage No Load ±. ± ±7. V vs. Temperature C to +8 C + mv/ C C to C + mv/ C Current at Rated Supply Voltage, 9 ± ma Regulation No Load to Full Load 9 mv/v Line Regulation 9 mv/v Ripple MHz Bandwidth, No Load mv rms POWER SUPPLY Supply Voltage Rated Performance ±. ± ±. V dc Operating ±. ±7 V dc Current Operating (+ V dc/ V dc Supplies) +/ 8 ma TEMPERATURE RANGE Rated Performance +8 C Storage +8 C NOTES The gain range of the AD is specified from to V/V. The AD can also be used with gains of up to V/V. With a gain of V/V a % reduction in the db bandwidth specification occurs and the nonlinearity degrades to ±.% typical. When the isolated supply load exceeds ± ma, external filter capacitors are required in order to ensure that the gain, offset, and nonlinearity specifications are preserved and to maintain the isolated supply full load ripple below the specified mv rms. A value of.8 µf is recommended. Nonlinearity is specified as a percent (of full-scale range) deviation from a best straight line. The isolation barrier (and rating) of every AD is % tested in production using a second partial discharge test with a failure detection threshold of pc. All B grade devices are tested with a minimum voltage of,8 V rms. All A grade devices are tested with a minimum voltage of 8 V rms. The AD should be allowed to warm up for approximately minutes before any gain and/or offset adjustments are made. Equivalent to a.8 degrees phase shift. 7 With the ± V dc power supply pins bypassed by. µf capacitors at the AD pins. 8 Caution: The AD design does not provide short circuit protection of its isolated power supply. A current limiting resistor may be placed in series with the isolated power terminals and the load in order to protect the supply against inadvertent shorts. 9 With an input power supply voltage greater than or equal ± V dc, the AD may supply up to ± ma from the isolated power supplies. Voltages less than. V dc may cause the AD to cease operating properly. Voltages greater than ±7. V dc may damage the internal components of the AD and consequently should not be used. Specifications subject to change without notice. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. WARNING! ESD SENSITIVE DEVICE

4 AD UNMITTED INPUT OP AMP SIGNAL MODULATOR DEMODULATOR T LOW-PASS FILTER khz R AD R 8 kω.µf POWER ISOLATED DC SUPPLY T khz POWER OSCILLATOR +V IN V IN Figure. Functional Block Diagram PIN CONFIGURATIONS BOTTOM VIEW OF FOOTPRINT 8 AD PIN DESIGNATIONS Pin Designation Function Noninverting Input Input Common Inverting Input Amplifier Feedback OUT Isolated V dc Power Supply OUT Isolated + V dc Power Supply Output Offset Trim Adjust Output Low 8 Output High + V IN + V dc Power ± V dc Power Supply Common V IN V dc Power ORDERING GUIDE Model Temperature Range V CMV Nonlinearity * ADAY C to +8 C 7.% ADBY C to +8 C.% + C, G = V/V. INSIDE THE AD The AD is a fully self-contained analog signal and power isolation solution. It employs a double-balanced amplitude modulation technique to perform transformer coupling of signals ranging in frequency from true dc values to those having frequencies of khz or less. To generate the power supplies used for the isolated front-end circuitry, an internal clock oscillator drives the primary winding of the integral dc/dc power supply s transformer, T. The resultant voltage developed across the secondary winding is then rectified and filtered for use as the isolated power supply. This built-in isolated dc/dc converter provides sufficient power for both the internal isolated circuit elements of the AD as well as any ancillary components supplied by the user. It saves onboard space and component cost where additional amplification or signal conditioning is required. After an input signal is amplified by the uncommitted op amp, it is modulated at a carrier frequency of approximately khz and applied across the primary winding of the signal isolation transformer T. The resultant signal induced on the secondary winding of the transformer is then demodulated and filtered using a low-pass Bessel response filter set at a frequency of khz. The function of the filter reconstructs the original signal as it appears on the input. The signal transformer design and construction allow nonlinearity to be independent of both the specified temperature and gain ranges. After complete reconstruction, the signal is subjected to an offset trim stage and final output buffer. The trim circuit allows the designer flexibility to adjust for any offset as desired.

5 Performance Characteristics AD. GAIN ERROR %.... CMR db 9 8 R S Ω R S kω TEMPERATURE C Figure. Gain Error vs. Temperature k k k FREQUENCY Hz Figure. Typical Common-Mode Rejection vs. Frequency NONLINEARITY mv 9 + % mv VOLTAGE Volts NONLINEARITY % GAIN db G = G = G =.. INPUT SIGNAL FREQUENCY khz Figure. Gain Nonlinearity vs. Output Voltage (G = V/V) Figure. Normalized Gain as a Function of Signal Frequency PHASE SHIFT Degrees TRANSPORT DELAY µs 9 G = G = G = G = G = G = FREQUENCY khz Figure. Phase Shift and Transport Delay vs. Frequency

6 AD Performance Characteristics 9 % mv V OVERSHOOT µs INPUT (+V STEP) Figure 7a. Overshoot to a Full-Scale Step Input (G = V/V) V ISO RIPPLE mv p-p.µf BYPASS CAPS 8 8.µF BYPASS CAPS.µF BYPASS CAPS 8 µf BYPASS CAPS V ISO LOAD ma Figure 9. ±V ISO Supply Ripple vs. Load. 9 V INPUT ( V STEP)..8 V S = ±V dc % mv UNDERSHOOT µs Figure 7b. Undershoot to a Full-Scale Input (G = V/V) V ISO ±V V ISO LOAD ±ma NOTE: THE GAIN AND OFFSET ERRORS WILL INCREASE WHEN THE ISOLATED POWER SUPPLY LOAD EXCEEDS ±ma Figure. ±V ISO Supply Voltage vs. Load V µs 9 % ±V, khz STEP RESPONSE (G=) Figure 8. Output Response to Full-Scale Step Input (G = V/V)

7 AD POWERING THE AD The AD is powered by a bipolar ± V dc power supply connected as shown in Figure. External bypass capacitors should be provided in bused applications. Note that a small signal-related current ( ma/v OUT ) will flow out of the OUT LO pin (Pin ). Therefore, the terminals should be bused together and referenced at a single Analog Star Ground to the ± V dc supply common as illustrated Figure. AD N AD ANALOG STAR GROUND N SIG N TH CHANNEL +V IN ST CHANNEL V IN.µF.µF +V dc V dc Figure. Typical Power Supply Connections Power Supply Voltage Considerations The rated performance of the AD remains unaffected for power supply voltages in the ±. V dc to ±. V dc range. Voltages below ±. V dc may cause the AD to cease operating properly. Note: Power supply voltages greater than ±7. V dc may damage the internal components and consequently should not be used. USING THE AD Unity Gain Input Configuration The basic unity gain configuration for input signals of up to ± V is shown in Figure. V SIGNAL R IN = kω AD FILTER, AND CIRCUITRY 8 Noninverting Configuration for Gain Greater Than Unity Figure shows how to achieve a gain greater than one while continuing to preserve a very high input impedance. A recommended PC board layout for multichannel applications is shown in Figure b. V SIGNAL R G R IN = kω C F 7pF AD FILTER, AND CIRCUITRY 8 Figure. Noninverting Input Configuration for Gain > V/V In this circuit, the gain equation is as follows: V O = ( + /R G ) V SIG where: V O V SIG R G = Output Voltage (V) = Input Signal Voltage (V) = Feedback Resistor Value (Ω) = Gain Resistor Value (Ω) The values for resistors and R G are subject to the following constraints: The total impedance of the gain network should be less than kω. The current drawn in is less than ma at ± V. Note that for each ma drawn by the feedback resistor, the isolated power supply drive capability decreases by ma. Amplifier gain is set by the feedback ( ) and gain resistor (R G ). It is recommended that is bypassed with a 7 pf capacitor as shown. Note: The kω input resistor (R IN ) in series with the input signal source and the terminal in Figures and is recommended to limit the current at the input terminals of the to. ma when the AD is not powered. Figure. Basic Unity Gain 7

8 AD Compensating the Uncommitted Input Op Amp The open-loop gain and phase versus frequency for the uncommitted input op amp are given in Figure. These curves can be used to determine appropriate values for the feedback resistor ( ) and compensation capacitor (C F ) to ensure frequency stability when reactive or nonlinear components are used. AVERAGE VOLTAGE GAIN db k GAIN M M FREQUENCY Hz PHASE M Figure. Open-Loop Gain and Frequency Response Inverting, Summing or Current Input Configuration Figure shows how the AD can measure currents or sum currents or voltages. IS R S V S R S V S C F 7pF AD FILTER, AND CIRCUITRY 8 Ø, EXCESS PHASE Degrees Figure. Noninverting Summing/Current Configuration For this circuit, the output voltage equation is: V O = (I S + V S /R S + V S /R S +...) where: V = Output Voltage (V) V S = Input Voltage Signal (V) V S = Input Voltage Signal (V) I S = Input Current Source (A) = Feedback Resistor (Ω) ( kω, typ) R S = Input Signal Source Resistance (Ω) R S = Input Signal Source Resistance (Ω) The circuit of Figure can also be used when the input signal is larger than the ± V input range of the isolator. For example, in Figure, if only V S, R S and were connected as shown with the solid lines, the input voltage span of V S could accommodate up to ± V when = kω and R S = kω. GAIN AND OFFSET ADJUSTMENTS General Comments The AD features an output stage pin useful for zeroing the output offset voltage through use of user supplied circuitry. When gain and offset adjustments are required, the actual compensation circuit ultimately used depends on the following: The input configuration mode of the isolation amplifier (noninverting or inverting). The placement of any adjusting potentiometer (on the isolator s input or output side). As a general rule: Gain adjustments should be accomplished at the gain-setting resistor network at the isolator s input. To ensure stability in the gain adjustment, potentiometers should be located as close as possible to the isolator s input and its impedance should be kept low. Adjustment ranges should also be kept to a minimum since their resolution and stability is dependent upon the actual potentiometers used. Output adjustments may be necessary where adjusting potentiometers placed near the input would present a hazard to the user due to the presence of high common-mode voltages during the adjustment procedure. It is recommended that input offset adjustments are made prior to gain adjustments. The AD should be allowed to warm up for approximately minutes before gain or offset adjustments are made. Input Gain Adjustments for Noninverting Mode Figure shows a suggested noninverting gain adjustment circuit. Note that the gain adjustment potentiometer R P is incorporated into the gain-setting resistor network. V SIGNAL R P R C RG R IN = kω C F.7pF AD FILTER, AND CIRCUITRY 8 Figure. Gain Adjustment for Noninverting Configuration For a ±% trim range: (R P kω), R C. R G R G + 8

9 AD Input Gain Adjustments for the Inverting Mode Figure 7 shows a suggested inverting gain adjustment circuit. In this circuit, gain adjustment is made using a potentiometer (R P ) in the feedback loop. The adjustments are effective for all gains in the to V/V range. V SIGNAL R IN kω R C C F 7pF AD FILTER, AND CIRCUITRY 8 Figure 7. Gain Adjustment for Inverting Configuration For an approximate ±% gain trim range, R X = R IN R IN + and select R C =. R IN while < kω C F = 7 pf Note: and R IN should have matched temperature coefficient drift characteristics. Output Offset Adjustments Figure 8 illustrates one method of adjusting the output offset voltage. Since the AD exhibits a nominal output offset of mv, the circuit shown was chosen to yield an offset correction of mv to +7 mv. This results in a total output offset range of approximately mv to +8 mv. AD LOW-PASS FILTER, (kω) kω 8 R T MΩ.µF +V IN V IN R P kω.µf.µf R S kω +V dc V dc Figure 8. Output Offset Adjustment Circuit Output Gain Adjustments Since the output amplifier stage of the AD is fixed at unity gain, any adjustments can be made only in a subsequent stage. USING ISOLATED POWER Each AD provides an unregulated, isolated bipolar power source of ± V ± ma, referred to the input common. This source may be used to power various ancillary components such as signal conditioning and/or adjustment circuitry, references, op amps or remote transducers. Figure 9 shows typical connections. LOAD.kΩ.kΩ C.8µF C.8µF ISOLATED DC SUPPLY FILTER, AND CIRCUITRY khz POWER OSCIL- LATOR AD 8 +V S V S.µF.µF Figure 9. Using the Isolated Power Supplies +V dc V dc PCB LAYOUT FOR MULTICHANNEL APPLICATIONS The pin out of the AD has been designed to easily facilitate multichannel applications. Figure a shows a recommended circuit board layout for a unity gain configuration V dc V dc.µf.µf.µf.µf SUPPLY BYPASS CAPACITORS FOR EVERY FOUR ADs Figure a. PCB Layout for Unity Gain ANALOG STAR GROUND CAUTION The AD design does not provide short-circuit protection of its isolated power supply. A current limiting resistor should be placed in series with the supply terminals and the load in order to protect against inadvertent shorts. 9

10 AD When gain setting resistors are used,." channel centers can still be achieved as shown in Figure b. IN +VISO VISO IN VISO RG R G C, C ARE V ISO FILTER CAPACITORS., R G ARE FEEDBACK, GAIN RESISTORS. C F IS A FEEDBACK BYPASS CAPACITOR. CF C CF C Figure b. PCB Layout for Gain Greater than Unity APPLICATIONS EXAMPLES Motor Control Figure shows an AD used in a dc motor control application. Its excellent phase characteristics and wide bandwidth are ideal for this type of application. MOTOR MAND ± VOLTS AD G = ISOLATED MOTOR MAND 8 ±V V C C C MOTOR CONTROL UNIT I MOTOR ENCODEEEDBACK OPTICAL SHAFT RESOLVER MOTOR OR θ TACHOMETER ENCODER Figure. Motor Control Application Multichannel Data Acquisition The current drive capabilities of the AD s bipolar ± V dc isolated power supply is more than adequate to meet the modest ±8 µa supply current requirements for the AD7 multiplexer. Digital isolation techniques should be employed to isolate the Enable (EN), A and A logic control signals. AC Transducer Applications In applications such as vibration analysis, where the user must acquire and process the spectral content of a sensor s signal rather than its dc level, the wideband characteristics of the AD prove most useful. Key specifications for ac transducer applications include bandwidth, slew rate and harmonic distortion. Since the transducer may be mechanically bonded or welded to the object under test, isolation is typically required to eliminate ground loops as well as protect the electronics used in the data acquisition system. Figure shows an isolated strain gage circuit employing the AD and a high speed operational amplifier (AD7). To alleviate the need for an instrumentation amplifier, the bridge is powered by a bipolar excitation source. Under this approach the common-mode voltage is ±V SPAN which is typically only a few millivolts, rather than the V EXC that would be achieved with a unipolar excitation source and Wheatstone bridge configuration. Using two strain gages with a gage factor of mv/v and a ±. V excitation signal, a ±. mv output signal will result. A gain setting of will scale this low level signal to ± V, which can then be digitized by a high speed, khz sampling ADC such as the AD787. The low voltage excitation is used to permit the front-end circuitry to be powered from the isolated power supplies of the AD, which can supply up to ± ma of isolated power at ± V. The bridge draws only. ma, leaving sufficient current to power the micropower dual BiFET ( µa quiescent current) and the high speed AD7 BiFET amplifier ( ma quiescent current). EN A A AD7 ( V) GND DTL/TTL TO CMOS LEVEL TRANSLATOR DECODER/DRIVER (+V) S S S S8 AD G = 8.8µF.8µF +V V Figure. Multichannel Data Acquisition Application

11 AD.8kΩ Ω AD89 9.7kΩ / AD8 kω / AD8 Ω Ω +ε Ω ε Ω Q N9 +.V.V Q N9 MΩ MΩ AD7.pF kω kω C C.8µF.8µF MOD ISOLATED DC SUPPLY AD DEMOD khz POWER OSC FILTER AND 8 +V V Figure. Strain Gage Signal Conditioning Application

12 AD OUTLINE DIMENSIONS Dimensions shown in inches and (mm). AD SIP PACKAGE.8 (.) MAX.8 (.7). (8.) MAX.8 (.) MAX C /9. (.). (.). (.) TYP.9 (.) TYP. (.). (.). (.). (.).. (.) (.). (.). (.). (.). (8.). (.8). (8.) BOTTOM VIEW OF. FOOTPRINT (.) MAX 8.7 (8.).7 (8.). (.8). (.) C L NOTE: PINS MEASURE. (.) x. (.) PRIOR TO TINNING. TINNING MAY ADD UP TO mils (.") TO THESE DIMENSIONS.. (.). (.) PRINTED IN U.S.A.