Low Power, Wide Supply Range, Low Cost Unity-Gain Difference Amplifiers AD8276/AD8277

Transcription

1 Data Sheet Low Power, Wide Supply Range, Low Cost Unity-Gain Difference Amplifiers AD87/AD877 FEATURES Wide input range beyond supplies Rugged input overvoltage protection Low supply current: μa maximum per channel Low power dissipation:. mw at VS =. V Bandwidth: khz CMRR: 8 db minimum, dc to khz Low offset voltage drift: ± μv/ C maximum (B Grade) Low gain drift: ppm/ C maximum (B Grade) Enhanced slew rate:. V/μs Wide power supply range: Single supply: V to 3 V Dual supplies: ± V to ±8 V APPLICATIONS Voltage measurement and monitoring Current measurement and monitoring Differential output instrumentation amplifier Portable, battery-powered equipment Test and measurement GENERAL DESCRIPTION The AD87/AD877 are general-purpose, unity-gain difference amplifiers intended for precision signal conditioning in power critical applications that require both high performance and low power. They provide exceptional common-mode rejection ratio (8 db) and high bandwidth while amplifying signals well beyond the supply rails. The on-chip resistors are laser-trimmed for excellent gain accuracy and high CMRR. They also have extremely low gain drift vs. temperature. The common-mode range of the amplifiers extends to almost double the supply voltage, making these amplifiers ideal for singlesupply applications that require a high common-mode voltage range. The internal resistors and ESD circuitry at the inputs also provide overvoltage protection to the op amps. The AD87/AD877 are unity-gain stable. While they are optimized for use as difference amplifiers, they can also be connected in high precision, single-ended configurations with G =, +, +. The AD87/AD877 provide an integrated precision solution that has smaller size, lower cost, and better performance than a discrete alternative. The AD87/AD877 operate on single supplies (. V to 3 V) or dual supplies (± V to ±8 V). The maximum quiescent supply current is μa per channel, which is ideal for batteryoperated and portable systems. FUNCTIONAL BLOCK DIAGRAM +VS 7 AD87 IN SENSE +IN 3 REF VS Figure. AD87 +VS AD877 INA SENSEA 79-3 A +INA 3 REFA INB SENSEB +INB 8 REFB VS Figure. AD877 Table. Difference Amplifiers by Category Low Distortion High Voltage Current Sensing 9 B 79- Low Power AD87 AD8 AD8 (U) AD87 AD87 AD9 AD83 (U) AD877 AD873 AD8 (B) AD878 AD87 AD8 (B) AMP3 AD8 (B) U = unidirectional, B = bidirectional. The AD87 is available in the space-saving 8-lead MSOP and SOIC packages, and the AD877 is offered in a -lead SOIC package. Both are specified for performance over the industrial temperature range of C to +8 C and are fully RoHS compliant. Rev. C 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 that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9, Norwood, MA -9, U.S.A. Tel: Fax: Analog Devices, Inc. All rights reserved.

2 AD87/AD877 TABLE OF CONTENTS Features... Applications... General Description... Functional Block Diagram... Revision History... Specifications... 3 Absolute Maximum Ratings... Thermal Resistance... Maximum Power Dissipation... Short-Circuit Current... ESD Caution... Pin Configurations and Function Descriptions... Typical Performance Characteristics... 8 Theory of Operation... Data Sheet Circuit Information... Driving the AD87/AD Input Voltage Range... Power Supplies... Applications Information... Configurations... Differential Output... Current Source... 7 Voltage and Current Monitoring... 7 Instrumentation Amplifier... 8 RTD... 8 Outline Dimensions... 9 Ordering Guide... REVISION HISTORY / Rev. B to Rev. C Change to Figure / Rev. A to Rev. B Changes to Figure Updated Outline Dimensions /9 Rev. to Rev. A Added AD Universal Changes to Features Section... Changes to General Description Section... Added Figure ; Renumbered Sequentially... Changes to Specifications Section... 3 Changes to Figure 3 and Table... Added Figure and Table 7; Renumbered Sequentially... 7 Changes to Figure... 8 Changes to Figure 3... Added Figure Changes to Input Voltage Range Section... Changes to Power Supplies Section and Added Figure... Added to Figure... Changes to Differential Output Section... Added Figure 7 and Changes to Current Source Section... 7 Added Voltage and Current Monitoring Section and Figure Moved Instrumentation Amplifier Section and Added RTD Section... 8 Changes to Ordering Guide... /9 Revision : Initial Version Rev. C Page of

3 Data Sheet AD87/AD877 SPECIFICATIONS VS = ± V to ± V, V REF = V, T A = C, RL = kω connected to ground, G = difference amplifier configuration, unless otherwise noted. Table. G = Grade B Grade A Parameter Conditions Min Typ Max Min Typ Max Unit INPUT CHARACTERISTICS System Offset µv vs. Temperature TA = C to +8 C µv Average Temperature Coefficient TA = C to +8 C. µv/ C vs. Power Supply VS = ± V to ±8 V µv/v Common-Mode Rejection Ratio (RTI) VS = ± V, VCM = ±7 V, RS = Ω 8 8 db Input Voltage Range (VS +.) +(VS.) (VS +.) +(VS.) V Impedance 3 Differential 8 8 kω Common Mode kω DYNAMIC PERFORMANCE Bandwidth khz Slew Rate V/µs Settling Time to.% V step on output, CL = pf µs Settling Time to.% µs Channel Separation f = khz 3 3 db GAIN Gain Error.... % Gain Drift TA = C to +8 C ppm/ C Gain Nonlinearity V = V p-p ppm PUT CHARACTERISTICS Output Voltage Swing VS = ± V, RL = kω, TA = C to +8 C VS +. +VS. VS +. +VS. V Short-Circuit Current Limit ± ± ma Capacitive Load Drive pf NOISE Output Voltage Noise f =. Hz to Hz μv p-p f = khz 7 7 nv/ Hz POWER SUPPLY Supply Current μa vs. Temperature TA = C to +8 C μa Operating Voltage Range 7 ± ±8 ± ±8 V TEMPERATURE RANGE Operating Range + + C Includes input bias and offset current errors, RTO (referred to output). The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation section for details. 3 Internal resistors are trimmed to be ratio matched and have ±% absolute accuracy. Output voltage swing varies with supply voltage and temperature. See Figure 8 through Figure for details. Includes amplifier voltage and current noise, as well as noise from internal resistors. Supply current varies with supply voltage and temperature. See Figure and Figure for details. 7 Unbalanced dual supplies can be used, such as VS =. V and +VS = + V. The positive supply rail must be at least V above the negative supply and reference voltage. Rev. C Page 3 of

4 AD87/AD877 Data Sheet VS = +.7 V to <± V, V REF = midsupply, TA = C, RL = kω connected to midsupply, G = difference amplifier configuration, unless otherwise noted. Table 3. G = Grade B Grade A Parameter Conditions Min Typ Max Min Typ Max Unit INPUT CHARACTERISTICS System Offset µv vs. Temperature TA = C to +8 C µv Average Temperature Coefficient TA = C to +8 C. µv/ C vs. Power Supply VS = ± V to ±8 V µv/v Common-Mode Rejection Ratio (RTI) VS =.7 V, VCM = V to. V, RS = Ω 8 8 db VS = ± V, VCM = V to +7 V, RS = Ω 8 8 db Input Voltage Range (VS +.) +(VS.) (VS +.) +(VS.) V Impedance 3 Differential 8 8 kω Common Mode kω DYNAMIC PERFORMANCE Bandwidth khz Slew Rate.. V/µs Settling Time to.% 8 V step on output, CL = pf, VS = V µs Channel Separation f = khz 3 3 db GAIN Gain Error.... % Gain Drift TA = C to +8 C ppm/ C PUT CHARACTERISTICS Output Swing RL = kω, TA = C to +8 C VS +. +VS. VS +. +VS. V Short-Circuit Current ± ± ma Limit Capacitive Load Drive pf NOISE Output Voltage Noise f =. Hz to Hz μv p-p f = khz nv/ Hz POWER SUPPLY Supply Current TA = C to +8 C μa Operating Voltage V Range TEMPERATURE RANGE Operating Range + + C Includes input bias and offset current errors, RTO (referred to output). The input voltage range may also be limited by absolute maximum input voltage or by the output swing. See the Input Voltage Range section in the Theory of Operation section for details. 3 Internal resistors are trimmed to be ratio matched and have ±% absolute accuracy. Output voltage swing varies with supply voltage and temperature. See Figure 8 through Figure for details. Includes amplifier voltage and current noise, as well as noise from internal resistors. Supply current varies with supply voltage and temperature. See Figure 3 and Figure for details. Rev. C Page of

5 Data Sheet ABSOLUTE MAXIMUM RATINGS Table. Parameter Rating Supply Voltage ±8 V Maximum Voltage at Any Input Pin VS + V Minimum Voltage at Any Input Pin +VS V Storage Temperature Range C to + C Specified Temperature Range C to +8 C Package Glass Transition Temperature (TG) C Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. THERMAL RESISTANCE The θja values in Table assume a -layer JEDEC standard board with zero airflow. Table. Package Type θja Unit 8-Lead MSOP 3 C/W 8-Lead SOIC C/W -Lead SOIC C/W MAXIMUM POWER DISSIPATION The maximum safe power dissipation for the AD87/AD877 is limited by the associated rise in junction temperature (TJ) on the die. At approximately C, which is the glass transition temperature, the properties of the plastic change. Even temporarily exceeding this temperature limit may change the stresses that the package exerts on the die, permanently shifting the parametric performance of the amplifiers. Exceeding a temperature of C for an extended period may result in a loss of functionality. MAXIMUM POWER DISSIPATION (W) LEAD MSOP θ JA = 3 C/W -LEAD SOIC θ JA = C/W AD87/AD877 7 AMBIENT TEMERATURE ( C) T J MAX = C 8-LEAD SOIC θ JA = C/W Figure 3. Maximum Power Dissipation vs. Ambient Temperature SHORT-CIRCUIT CURRENT The AD87/AD877 have built-in, short-circuit protection that limits the output current (see Figure for more information). While the short-circuit condition itself does not damage the part, the heat generated by the condition can cause the part to exceed its maximum junction temperature, with corresponding negative effects on reliability. Figure 3 and Figure, combined with knowledge of the supply voltages and ambient temperature of the part, can be used to determine whether a short circuit will cause the part to exceed its maximum junction temperature. ESD CAUTION 79- Rev. C Page of

6 AD87/AD877 Data Sheet PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS REF IN +IN 3 VS AD87 TOP VIEW (Not to Scale) NC = NO CONNECT 8 7 NC +VS SENSE Figure. AD87 8-Lead MSOP Pin Configuration 79-3 REF IN +IN 3 VS AD87 TOP VIEW (Not to Scale) NC = NO CONNECT 8 7 NC +VS SENSE Figure. AD87 8-Lead SOIC Pin Configuration 79- Table. AD87 Pin Function Descriptions Pin No. Mnemonic Description REF Reference Voltage Input. IN Inverting Input. 3 +IN Noninverting Input. VS Negative Supply. SENSE Sense Terminal. Output. 7 +VS Positive Supply. 8 NC No Connect. Rev. C Page of

7 Data Sheet AD87/AD877 NC INA Table 7. AD877 Pin Function Descriptions Pin No. Mnemonic Description NC No Connect. INA Channel A Inverting Input. 3 +INA Channel A Noninverting Input. VS Negative Supply. +INB Channel B Noninverting Input. INB Channel B Inverting Input. 7 NC No Connect. 8 REFB Channel B Reference Voltage Input. 9 B Channel B Output. SENSEB Channel B Sense Terminal. +VS Positive Supply. SENSEA Channel A Sense Terminal. 3 A Channel A Output. REFA Channel A Reference Voltage Input. REFA 3 A +INA 3 AD877 SENSEA VS TOP VIEW +VS (Not to Scale) +INB SENSEB INB 9 B NC 7 8 REFB NC = NO CONNECT Figure. AD877 -Lead SOIC Pin Configuration 79-3 Rev. C Page 7 of

8 AD87/AD877 Data Sheet TYPICAL PERFORMANCE CHARACTERISTICS VS = ± V, T A = C, RL = kω connected to ground, G = difference amplifier configuration, unless otherwise noted. N = MEAN =.8 SD = NUMBER OF HITS 3 SYSTEM OFFSET (µv) SYSTEM OFFSET VOLTAGE (µv) Figure 7. Distribution of Typical System Offset Voltage TEMPERATURE ( C) Figure. System Offset vs. Temperature, Normalized at C 79-8 N = MEAN =.87 SD =. NUMBER OF HITS 3 GAIN ERROR (µv/v) CMRR (µv/v) Figure 8. Distribution of Typical Common-Mode Rejection 79- REPRESENTATIVE DATA TEMPERATURE ( C) Figure. Gain Error vs. Temperature, Normalized at C 79-9 V S = ±V CMRR (µv/v) GAIN (db) 3 V S = +.7V REPRESENTATIVE DATA TEMPERATURE ( C) Figure 9. CMRR vs. Temperature, Normalized at C 79-7 k k k M M FREQUENCY (Hz) Figure. Gain vs. Frequency, VS = ± V, +.7 V 79- Rev. C Page 8 of

9 Data Sheet AD87/AD877 V S = ±V 8 V REF = MIDSUPPLY CMRR (db) 8 COMMON-MODE VOLTAGE (V) V S =.7V V S = V k k k M FREQUENCY (Hz) Figure 3. CMRR vs. Frequency PUT VOLTAGE (V) Figure. Input Common-Mode Voltage vs. Output Voltage, V and.7 V Supplies, VREF = Midsupply 79-8 V REF = V V S = V PSRR (db) 8 +PSRR PSRR COMMON-MODE VOLTAGE (V) V S =.7V k k k M FREQUENCY (Hz) 3 Figure. PSRR vs. Frequency PUT VOLTAGE (V) Figure 7. Input Common-Mode Voltage vs. Output Voltage, V and.7 V Supplies, VREF = V +V S 79- V S = ±V. COMMON-MODE VOLTAGE (V) V S = ±V PUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES T A = C T A = + C T A = +8 C T A = + C 3 PUT VOLTAGE (V) Figure. Input Common-Mode Voltage vs. Output Voltage, ± V and ± V Supplies 79-3 V S 8 8 SUPPLY VOLTAGE (±V S ) Figure 8. Output Voltage Swing vs. Supply Voltage Per Channel and Temperature, RL = kω 79- Rev. C Page 9 of

10 AD87/AD877 Data Sheet PUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES +V S T A = C T A = + C T A = +8 C T A = + C V S 8 8 SUPPLY VOLTAGE (±V S ) Figure 9. Output Voltage Swing vs. Supply Voltage Per Channel and Temperature, RL = kω +V S 79-7 SUPPLY CURRENT (µa) SUPPLY VOLTAGE (±V) Figure. Supply Current Per Channel vs. Dual Supply Voltage, VIN = V PUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES T A = C T A = + C T A = +8 C T A = + C SUPPLY CURRENT (µa) 7 3 V S k k k LOAD RESISTANCE (Ω) Figure. Output Voltage Swing vs. RL and Temperature, VS = ± V SUPPLY VOLTAGE (V) Figure 3. Supply Current Per Channel vs. Single-Supply Voltage, VIN = V, VREF = V 79- +V S. V REF = MIDSUPPLY PUT VOLTAGE SWING (V) REFERRED TO SUPPLY VOLTAGES T A = C T A = + C T A = +8 C T A = + C SUPPLY CURRENT (µa) V S = ±V V S = +.7V +. V S PUT CURRENT (ma) Figure. Output Voltage Swing vs. I and Temperature, VS = ± V TEMPERATURE ( C) Figure. Supply Current Per Channel vs. Temperature 79- Rev. C Page of

11 Data Sheet AD87/AD877 3 SHORT-CIRCUIT CURRENT (ma) I SHORT+ I SHORT V/DIV.%/DIV. µs TO.% 3.8µs TO.% TEMPERATURE ( C) 79-3 TIME (µs) µs/div 79- Figure. Short-Circuit Current Per Channel vs. Temperature Figure 8. Large-Signal Pulse Response and Settling Time, V Step, VS = ± V.. SLEW RATE (V/µs)..8.. SR +SR V/DIV.%/DIV.3 µs TO.%.µs TO.% TEMPERATURE ( C) Figure. Slew Rate vs. Temperature, VIN = V p-p, khz 79- TIME (µs) µs/div Figure 9. Large-Signal Pulse Response and Settling Time, V Step, VS =.7 V NONLINEARITY (ppm/div) V/DIV PUT VOLTAGE (V) Figure 7. Gain Nonlinearity, VS = ± V, RL kω 79- µs/div Figure 3. Large-Signal Step Response 79-8 Rev. C Page of

12 AD87/AD877 Data Sheet PUT VOLTAGE (V p-p) 3 V S = ±V V S = ±V OVERSHOOT (%) 3 3 ±V ±V ±V ±8V k k k M FREQUENCY (Hz) Figure 3. Maximum Output Voltage vs. Frequency, VS = ± V, ± V CAPACITIVE LOAD (pf) Figure 3. Small-Signal Overshoot vs. Capacitive Load, RL kω 79-. k. V S = V. PUT VOLTAGE (V p-p) V S =.7V.. k k k M FREQUENCY (Hz) Figure 3. Maximum Output Voltage vs. Frequency, VS = V,.7 V 79-3 mv/div NOISE (nv/ Hz). k k k FREQUENCY (Hz) Figure 3. Voltage Noise Density vs. Frequency µv/div 79-3 C L = pf C L = pf C L = 3pF C L = 7pF µs/div Figure 33. Small-Signal Step Response for Various Capacitive Loads 79- s/div Figure 3.. Hz to Hz Voltage Noise 79-3 Rev. C Page of

13 Data Sheet AD87/AD877 CHANNEL SEPARATION (db) 8 NO LOAD kω LOAD kω LOAD kω LOAD k k k FREQUENCY (Hz) Figure 37. Channel Separation 79- Rev. C Page 3 of

14 AD87/AD877 THEORY OF OPERATION CIRCUIT INFORMATION Each channel of the AD87/AD877 consists of a low power, low noise op amp and four laser-trimmed on-chip resistors. These resistors can be externally connected to make a variety of amplifier configurations, including difference, noninverting, and inverting configurations. Taking advantage of the integrated resistors of the AD87/AD877 provides the designer with several benefits over a discrete design, including smaller size, lower cost, and better ac and dc performance. DC Performance +VS 7 IN SENSE IN+ 3 REF VS AD87 Figure 38. Functional Block Diagram Much of the dc performance of op amp circuits depends on the accuracy of the surrounding resistors. Using superposition to analyze a typical difference amplifier circuit, as is shown in Figure 39, the output voltage is found to be V R R + V R + R R3 = VIN + IN 79-3 R R3 This equation demonstrates that the gain accuracy and commonmode rejection ratio of the AD87/AD877 is determined primarily by the matching of resistor ratios. Even a.% mismatch in one resistor degrades the CMRR to db for a G = difference amplifier. The difference amplifier output voltage equation can be reduced to ( V V ) = R V + IN I N R3 as long as the following ratio of the resistors is tightly matched: R R = R R3 The resistors on the AD87/AD877 are laser trimmed to match accurately. As a result, the AD87/AD877 provide superior performance over a discrete solution, enabling better CMRR, gain accuracy, and gain drift, even over a wide temperature range. Data Sheet AC Performance Component sizes and trace lengths are much smaller in an IC than on a PCB, so the corresponding parasitic elements are also smaller. This results in better ac performance of the AD87/ AD877. For example, the positive and negative input terminals of the AD87/AD877 op amps are intentionally not pinned out. By not connecting these nodes to the traces on the PCB, the capacitance remains low, resulting in improved loop stability and excellent common-mode rejection over frequency. DRIVING THE AD87/AD877 Care should be taken to drive the AD87/AD877 with a low impedance source: for example, another amplifier. Source resistance of even a few kilohms (kω) can unbalance the resistor ratios and, therefore, significantly degrade the gain accuracy and common-mode rejection of the AD87/AD877. Because all configurations present several kilohms of input resistance, the AD87/AD877 do not require a high current drive from the source and so are easy to drive. INPUT VOLTAGE RANGE The AD87/AD877 are able to measure input voltages beyond the supply rails. The internal resistors divide down the voltage before it reaches the internal op amp and provide protection to the op amp inputs. Figure 39 shows an example of how the voltage division works in a difference amplifier configuration. For the AD87/AD877 to measure correctly, the input voltages at the input nodes of the internal op amp must stay below. V of the positive supply rail and can exceed the negative supply rail by. V. Refer to the Power Supplies section for more details. R R + R (V IN+) V IN V IN+ R3 R R R R R + R (V IN+) Figure 39. Voltage Division in the Difference Amplifier Configuration The AD87/AD877 have integrated ESD diodes at the inputs that provide overvoltage protection. This feature simplifies system design by eliminating the need for additional external protection circuitry, and enables a more robust system. The voltages at any of the inputs of the parts can safely range from +VS V up to VS + V. For example, on ± V supplies, input voltages can go as high as ±3 V. Care should be taken to not exceed the +VS V to VS + V input limits to avoid risking damage to the parts Rev. C Page of

15 Data Sheet POWER SUPPLIES The AD87/AD877 operate extremely well over a very wide range of supply voltages. They can operate on a single supply as low as V and as high as 3 V, under appropriate setup conditions. For best performance, the user must exercise care that the setup conditions ensure that the internal op amp is biased correctly. The internal input terminals of the op amp must have sufficient voltage headroom to operate properly. Proper operation of the part requires at least. V between the positive supply rail and the op amp input terminals. This relationship is expressed in the following equation: R V REF < + V S. V R + R For example, when operating on a +VS = V single supply and VREF = V, it can be seen from Figure that the input terminals of the op amp are biased at V, allowing more than the required. V headroom. However, if VREF = V under the same conditions, the input terminals of the op amp are biased at. V, barely allowing the required. V headroom. This setup does not allow any practical voltage swing on the noninverting input. Therefore, the user needs to increase the supply voltage or decrease VREF to restore proper operation. AD87/AD877 The AD87/AD877 are typically specified at single- and dualsupplies, but they can be used with unbalanced supplies, as well; for example, VS = V, +V S = V. The difference between the two supplies must be kept below 3 V. The positive supply rail must be at least V above the negative supply and reference voltage. R R + R (V REF) R3 R R R V REF R R + R (V REF ) Figure. Ensure Sufficient Voltage Headroom on the Internal Op Amp Inputs Use a stable dc voltage to power the AD87/AD877. Noise on the supply pins can adversely affect performance. Place a bypass capacitor of. µf between each supply pin and ground, as close as possible to each supply pin. Use a tantalum capacitor of µf between each supply and ground. It can be farther away from the supply pins and, typically, it can be shared by other precision integrated circuits Rev. C Page of

16 AD87/AD877 APPLICATIONS INFORMATION CONFIGURATIONS The AD87/AD877 can be configured in several ways (see Figure to Figure ). All of these configurations have excellent gain accuracy and gain drift because they rely on the internal matched resistors. Note that Figure 3 shows the AD87/AD877 as difference amplifiers with a midsupply reference voltage at the noninverting input. This allows the AD87/AD877 to be used as a level shifter, which is appropriate in single-supply applications that are referenced to midsupply. As with the other inputs, the reference must be driven with a low impedance source to maintain the internal resistor ratio. An example using the low power, low noise OP77 as a reference is shown in Figure. V INCORRECT IN +IN AD87 REF V CORRECT + OP77 AD87 REF Figure. Driving the Reference Pin IN +IN 3 V = V IN+ V IN Figure. Difference Amplifier, Gain = 3 V = V IN+ V IN V REF = MIDSUPPLY Figure 3. Difference Amplifier, Gain =, Referenced to Midsupply IN 3 V = V IN Figure. Inverting Amplifier, Gain = IN 3 V = V IN Figure. Noninverting Amplifier, Gain = IN 3 V = V IN Figure. Noninverting Amplifier, Gain = Data Sheet DIFFERENTIAL PUT Certain systems require a differential signal for better performance, such as the inputs to differential analog-to-digital converters. Figure 7 shows how the AD87/AD877 can be used to convert a single-ended output from an AD8 instrumentation amplifier into a differential signal. The internal matched resistors of the AD87 at the inverting input maximize gain accuracy while generating a differential signal. The resistors at the noninverting input can be used as a divider to set and track the common-mode voltage accurately to midsupply, especially when running on a single supply or in an environment where the supply fluctuates. The resistors at the noninverting input can also be shorted and set to any appropriate bias voltage. Note that the VBIAS = VCM node indicated in Figure 7 is internal to the AD87 because it is not pinned out. +IN AD8 V S IN V REF R AD87 R R R V BIAS = V CM Rev. C Page of V S Figure 7. Differential Output With Supply Tracking on Common-Mode Voltage Reference 79-3

17 Data Sheet The differential output voltage and common-mode voltage of the AD8 is shown in the following equations: VDIFF_ = V+ V = GainAD8 (V+IN V IN) VCM = (VS+ VS )/ = VBIAS Refer to the AD8 data sheet for additional information. IN +IN +VS AD VS 9 + Figure 8. AD877 Differential Output Configuration The two difference amplifiers of the AD877 can be configured to provide a differential output, as shown in Figure 8. This differential output configuration is suitable for various applications, such as strain gage excitation and single-ended-to-differential conversion. The differential output voltage has a gain of as shown in the following equation: VDIFF_ = V+ V = (V+IN V IN) CURRENT SOURCE The AD87 difference amplifier can be implemented as part of a voltage-to-current converter or a precision constant current source as shown in Figure 9. Using an integrated precision solution such as the AD87 provides several advantages over a discrete solution, including space-saving, improved gain accuracy, and temperature drift. The internal resistors are tightly matched to minimize error and temperature drift. If the external resistors, R and R, are not well-matched, they become a significant source of error in the system, so precision resistors are recommended to maintain performance. The ADR8 provides a precision voltage reference and integrated op amp that also reduces error in the signal chain. The AD87 has rail-to-rail output capability that allows higher current outputs. 79- V+ V 3 REF ADR V 3 V+ 7 AD87 AD87/AD877 Figure 9. Constant Current Source N39 I O =.V(/ + /R) R = R R R R LOAD VOLTAGE AND CURRENT MONITORING Voltage and current monitoring is critical in the following applications: power line metering, power line protection, motor control applications, and battery monitoring. The AD87/ AD877 can be used to monitor voltages and currents in a system, as shown in Figure. As the signals monitored by the AD87/AD877 rise above or drop below critical levels, a circuit event can be triggered to correct the situation or raise a warning. I R AD87 I 3 I C R AD87 V R AD87 V 3 R AD87 V C R AD87 8: OP77 ADC Figure.Voltage and Current Monitoring in 3-Phase Power Line Protection Using the AD87 Figure shows an example of how the AD87 can be used to monitor voltage and current on a 3-phase power supply. I through I3 are the currents to be monitored, and V through V3 are the voltages to be monitored on each phase. IC and VC are the common or zero lines. Couplers or transformers interface the power lines to the front-end circuitry and provide attenuation, isolation, and protection. On the current monitoring side, current transformers (CTs) step down the power-line current and isolate the front-end circuitry from the high voltage and high current lines. Across the inputs of each difference amplifier is a shunt resistor that converts the coupled current into a voltage. The value of the Rev. C Page 7 of

18 AD87/AD877 resistor is determined by the characteristics of the coupler or transformer and desired input voltage ranges to the AD87. On the voltage monitoring side, potential transformers (PTs) are used to provide coupling and galvanic isolation. The PTs present a load to the power line and step down the voltage to a measureable level. The AD87 helps to build a robust system because it allows input voltages that are almost double its supply voltage, while providing additional input protection in the form of the integrated ESD diodes. Not only does the AD87 monitor the voltage and currents on the power lines, it is able to reject very high common-mode voltages that may appear at the inputs. The AD87 also performs the differential-to-single-ended conversion on the input voltages. The 8 kω differential input impedance that the AD87 presents is high enough that it should not load the input signals. I SH R SH AD87 V = I SH R SH Figure. AD87 Monitoring Current Through a Shunt Resistor Figure shows how the AD87 can be used to monitor the current through a small shunt resistor. This is useful in power critical applications such as motor control (current sense) and battery monitoring. INSTRUMENTATION AMPLIFIER The AD87/AD877 can be used as building blocks for a low power, low cost instrumentation amplifier. An instrumentation amplifier provides high impedance inputs and delivers high common-mode rejection. Combining the AD87 with an Analog Devices, Inc., low power amplifier (see Table 8) creates a precise, power efficient voltage measurement solution suitable for power critical systems. IN A R F 79-8 Data Sheet Table 8. Low Power Op Amps Op Amp (A, A) Features AD8 Dual micropower op amp AD87 Precision dual micropower op amp AD87 Low cost CMOS micropower op amp AD87 Dual precision CMOS micropower op amp It is preferable to use dual op amps for the high impedance inputs because they have better matched performance and track each other over temperature. The AD87 difference amplifiers cancel out common-mode errors from the input op amps, if they track each other. The differential gain accuracy of the inamp is proportional to how well the input feedback resistors (RF) match each other. The CMRR of the in-amp increases as the differential gain is increased ( + RF/RG), but a higher gain also reduces the common-mode voltage range. Note that dual supplies must be used for proper operation of this configuration. Refer to A Designer s Guide to Instrumentation Amplifiers for more design ideas and considerations. RTD Resistive temperature detectors (RTDs) are often measured remotely in industrial control systems. The wire lengths needed to connect the RTD to a controller add significant cost and resistance errors to the measurement. The AD87 difference amplifier is effective in measuring errors caused by wire resistance in remote 3-wire RTD systems, allowing the user to cancel out the errors introduced by the wires. Its excellent gain drift provides accurate measurements and stable performance over a wide temperature range. R T I EX R L R L R L3 AD87 V Σ-Δ ADC Figure 3. 3-Wire RTD Cable Resistance Error Measurement 79-9 R G V R F AD87 A REF +IN V = ( + R F /R G ) (V IN+ V IN ) Figure. Low Power Precision Instrumentation Amplifier 79- Rev. C Page 8 of

19 Data Sheet AD87/AD877 LINE DIMENSIONS PIN IDENTIFIER. BSC COPLANARITY.... MAX MAX.3.9 COMPLIANT TO JEDEC STANDARDS MO-87-AA Figure. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters B. (.98).8 (.89). (.7) 3.8 (.97) 8. (.).8 (.8). (.98). (.) COPLANARITY. SEATING PLANE.7 (.) BSC.7 (.88).3 (.3). (.).3 (.) 8. (.98).7 (.7). (.9). (.99).7 (.). (.7) COMPLIANT TO JEDEC STANDARDS MS--AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure. 8-Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-8) Dimensions shown in millimeters and (inches) 7-A Rev. C Page 9 of

20 AD87/AD877 Data Sheet 8.7 (.3) 8. (.33). (.7) 3.8 (.9) 8 7. (.).8 (.83). (.98). (.39) COPLANARITY..7 (.) BSC. (.).3 (.).7 (.89).3 (.3) SEATING PLANE 8. (.98).7 (.7). (.97). (.98).7 (.). (.7) COMPLIANT TO JEDEC STANDARDS MS--AB CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure. -Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-) Dimensions shown in millimeters and (inches) -A ORDERING GUIDE Model Temperature Range Package Description Package Option Branding AD87ARMZ C to +8 C 8-Lead MSOP RM-8 HP AD87ARMZ-R7 C to +8 C 8-Lead MSOP, 7" Tape and Reel RM-8 HP AD87ARMZ-RL C to +8 C 8-Lead MSOP, 3" Tape and Reel RM-8 HP AD87ARZ C to +8 C 8-Lead SOIC_N R-8 AD87ARZ-R7 C to +8 C 8-Lead SOIC_N, 7" Tape and Reel R-8 AD87ARZ-RL C to +8 C 8-Lead SOIC_N, 3" Tape and Reel R-8 AD87BRMZ C to +8 C 8-Lead MSOP RM-8 HQ AD87BRMZ-R7 C to +8 C 8-Lead MSOP, 7" Tape and Reel RM-8 HQ AD87BRMZ-RL C to +8 C 8-Lead MSOP, 3" Tape and Reel RM-8 HQ AD87BRZ C to +8 C 8-Lead SOIC_N R-8 AD87BRZ-R7 C to +8 C 8-Lead SOIC_N, 7" Tape and Reel R-8 AD87BRZ-RL C to +8 C 8-Lead SOIC_N, 3" Tape and Reel R-8 AD877ARZ C to +8 C -Lead SOIC_N R- AD877ARZ-R7 C to +8 C -Lead SOIC_N, 7" Tape and Reel R- AD877ARZ-RL C to +8 C -Lead SOIC_N, 3" Tape and Reel R- AD877BRZ C to +8 C -Lead SOIC_N R- AD877BRZ-R7 C to +8 C -Lead SOIC_N, 7" Tape and Reel R- AD877BRZ-RL C to +8 C -Lead SOIC_N, 3" Tape and Reel R- Z = RoHS Compliant Part. 9 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D79--/(C) Rev. C Page of