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| Precision amplifier for bridge circuits AM457 PRINCIPLE FUNCTION Adjustable amplification of differential voltage signals (5 to 100mV FS) to an adjustable, ratiometric output voltage of between 0.2 and Vcc-0.2Volt with a diagnostic unit VCC = 5V 5% Differential input voltages ( 5... 100mV FS) Diagnostic unit AM457 e.g. VOUT = 0.2...VCC-0.2V (adjustable) e.g. 0.5...4.5V ratiometric Iout = 2mA TYPICAL APPLICATIONS * Precision amplifiers for bridge circuits with low sensitivity: for ceramic measurement cells for DMS sensing elements for GMR sensing elements * Sensor amplification with functional control analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 1/10 Rev. 2.2 Precision amplifier for bridge circuits TABLE OF CONTENTS Features Block diagram General description Electrical specifications Circuit Description of functions Example application Block diagram and pinout Delivery options Further reading AM457 3 3 3 4 5 6 7 9 10 10 TABLE OF FIGURES AND TABLES Table 1: Specifications Table 2: Circuit Table 3: Pin configuration Figure 1: Block diagram Figure 2: Schematic circuit diagram for resistance bridges with low sensitivity Figure 3: Block diagram of AM457 Figure 4: AM457 Pin out Figure 5 : SOP 8 Package dimensions 4 5 9 3 7 9 9 10 analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 2/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 * High input sensitivity * Wide differential input voltage range (5...100mV FS) * Low offset * Low offset drift * Low input noise * High CMRR: > 120dB * Wide operating temperature range: -40... +125C * Adjustable output voltage * Rail-to-rail output stage: VOUT = 0.2V ... Vcc - 0.2V * Sink/source output * Single ratiometric supply: VCC = 5V * Integrated current source: 2mA * Integrated diagnostic unit * Separate function groups * Integrated EMV protective circuitry * Short-circuit-proofing * Small-scale design FEATURES GENERAL DESCRIPTION AM457 is a high-precision integrated amplifier which has been developed to condition signals from small differential input voltages within a range of a few millivolts (5 to 100mV FS). The chief component of the IC is a highly accurate, low-noise amplifier circuit (AMP). An additional integrated operational amplifier (OP) can be used to supply a sensing element with current or record temperature for sensor bridges powered by voltage, for example. The entire system (sensing element plus amplifier) is monitored by the integrated diagnostic unit which logs any sensing element errors or defects, system undervoltage, AMP overloads and excessive temperature. The various function groups are not mutually dependent on one another; the AMP can be operated independently of the OP, for example. The functionality of both component groups (AMP and OP) is autonomous with regard to the diagnostic unit. AM457 is a combination of high-impedance precision amplifier, additional OP and extensive diagnostic unit, enabling a self-monitoring sensor system to be assembled using very few components. BLOCK DIAGRAM AM457 OUTOP 7 8 VCC OP 1k 9k VCC IN+ _ INOP 6 AMP Temp Diagnostic unit 5 DIAG 1 OUT 4 GND Figure 1: Block diagram analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 3/10 Rev. 2.2 Precision amplifier for bridge circuits ELECTRICAL SPECIFICATIONS Tamb = 25C, VCC = 5V (unless otherwise stated) Parameter Voltage Range Quiescent Current Temperature Specifications Operating Storage Junction Thermal Resistance Amplifier AMP Offset Voltage VOS vs. Temperature VOS vs. Temperature Input Bias Current IB vs. Temperature Differential Input Voltage Input Offset Current IOS vs. Temperature Input Resistance Input Capacitance Common Mode Input Range Common Mode Rejection Ratio Open Loop Gain Adjustable Gain Output Voltage Range Output Current Output Load Capacitance Output Load Resistance Power Supply Rejection Ratio Gain Bandwidth Product VOS dVOS/dT dVOS/dT IB dIB/dT VIN IOS dIOS/dT RIN CIN CMIR CMRR G0 G VOUT IOUT CL RL PSRR GBW G = 10; C1 = 1nF; RL = 20k Including filters for EMC protection Non Linearity Slew Rate Input Voltage Noise SR en G 100 C1 = 1nF; RL = 20k Tamb, ;Rs = 1k; VCC = 5V Rs = Source Impedance 0.3 0.8 30 10-4 Sink and source Sink and source 1 120 120 10 0.2 250 1 20 90 110 30 5 135 140 Tamb= -45...105C Tamb= 105...125C VCM = 2.5V Tamb= -45...125C VIN =VIN+ - VINVCM = 2.5V Tamb= -45...125C VCM / IB,typ (VCM = 2.5V) 5 0.5 2.5 100 90 30 -130 0.1 0.5 0.3 1.2 120 200 -600 100 5 30 Tamb Tst TJ ja ja DIL8 plastic package SO8 narrow plastic package 111 181 -45 -55 125 150 150 Symbol VCC ICC Tamb = -40...+85C Conditions Min. 4.5 Typ. 5 1.5 Max. 5.5 2.5 AM457 Unit V mA C C C C/W C/W mV V/C V nA pA/C mV nA pA/C M pF VCC - 1.3 V dB dB VCC - 0.2 V A nF k dB kHz V/s nV/Hz analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 4/10 Rev. 2.2 Precision amplifier for bridge circuits Error Monitoring (Diagnosis) Output Voltage on Error (high) Output Voltage without Error (low) Output Current on Error Output Current without Error VDIAG VDIAG IERR INOERR VDIAG = 2.5V VDIAG = 2.5V 1.5 -6 105 250 3.8 With regard to the sensing element VCC - 1.8 0.4 VCC - 0.2 GND+ 0.2 3.5 -3.5 6 -1.5 143 800 4.3 AM457 V V A A C A V V V Threshold for Temperature Error Monitor- TERR ing Threshold for Output Current (AMP) Error Monitoring Threshold for Supply Voltage Error Monitoring Threshold for High Input Voltage (AMP) Error Monitoring Threshold for Low Input Voltage (AMP) Error Monitoring Operational Amplifier (OP) Offset Voltage VOS vs. Temperature VOS vs. Temperature Input Bias Current IB vs. Temperature Input Resistance Input Capacitance Common Mode Input Range Open Loop Gain Output Voltage Range Output Source Current Output Load Capacitance Gain Bandwidth Product Slew Rate VOS dVOS/dT dVOS/dT IB dIB/dT RIN CIN CMIR G0 G0.2mA VOUT IOUT C1 GBW SR GGAIN = 10; C1 = 1nF; RL = 1.25k GGAIN = 100; C1 = 1nF; RL = 25k PNP Open Collector Output I0ut =0.1mA I0ut =2mA Tamb= -45...125C VCM / IB,typ (VCM = 0.5V) Tamb= -45...105C Tamb= 105...125C IIA,ERR VCC,ERR VIN,high,ERR VIN,low,ERR VCC - 0.8 0.9 4 10 80 100 6.25 30 0 90 75 0 1 0.9 0.2 1.4 0.7 0.5 110 90 8 40 320 300 400 mV V/C V nA pA/C M pF 1 V dB dB VCC - 0.2 2 5 V mA nF MHz V/s Table 1: Specifications Currents flowing into the IC are negative. Positive input of the operational amplifier (fixed internally): 0.1VCC VCM Input Common Mode Voltage CIRCUIT Parameter AMP Output Capacitor Compensation Capacitor DIAG Output Capacitor (optional) OP Output Capacitor (optional) Stabilization Capacitor (optional) EMV Protection Capacitor (optional) Load Resistor Symbol C1 C2 C3 C4 C5 C6 RL 20 1 100 470 Conditions Min. 1 100 3.3 5 Typ. Max. 5 Unit nF pF nF nF nF pF k Table 2: Circuit Components Recommendation: Ceramic capacitor analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 5/10 Rev. 2.2 Precision amplifier for bridge circuits DESCRIPTION OF FUNCTIONS AM457 AM457 is an integrated precision circuit for the high-impedance amplification and conditioning of signals and monitoring of sensor bridges with low sensitivity, particularly for ceramic or DMS sensing elements. The IC generates a rail-to-rail output signal of 0.2V to Vcc-0.2V. With suitable dimensioning an output voltage of 0.5...4.5V can also be provided. As these values are standard the following description refers to this output voltage. Besides amplifying and conditioning signals the IC can also display a range of system errors ("system" in this context refers to the sensor bridge and the IC plus the voltage supply). AM457 works on the principle of ratiometry with a supply voltage of 5V 5%. The IC is distinguished by its low offset, extremely low thermal offset drift and its ability to cope with a wide range of temperature, enabling it to be classified as a precision amplifier. FUNCTIONS AM457 is made up of three modular function blocks (see Figure 1) which can be accessed individually or operated in suitable combinations. These are: 1. A precision amplifier (AMP) with a sink/source facility for the amplification and conditioning of bridge signals within a range of 5...100mV FS including the output stage. The output stage is protected against short circuits. The amplification and offset can be fixed using external resistors. 2. An operational amplifier (OP) whose positive input is set internally to reference point 0.1VCC. The negative input is accessible externally, with which the OP is set as an inverter. The OP may be used as a current source to power the bridge, for example, as an output line driver or to record temperature when a sensing element is supplied by voltage. The latter application allows the temperature dependency of the sensing element bridge resistors to be exploited, for example. 3. A diagnostic unit which monitors system features. With a signal source in bridge configuration this displays the following faults and errors: * * * * * * * * Lack of bridge voltage Lack of bridge ground Missing signal connection Two missing signal connections At least one of the four bridge resistors is of high impedance Malfunction of the signal conditioning unit due to an AMP overload Malfunction of the signal conditioning unit due to undervoltage Detection of excessive temperature by the signal conditioning unit. In the event of one or more disruptions output DIAG (pin 5) switches to a logic high (see the electrical specifications). A low is otherwise indicated. The precision amplifier (AMP) and additional operational amplifier (OP) can be operated independently of one another without the diagnostic unit. analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 6/10 Rev. 2.2 Precision amplifier for bridge circuits EXAMPLE APPLICATION AM457 Offset and span compensation for a Wheatstone bridge with discrete adjusting resistors. AM457 is suitable for the amplification of all resistance bridge circuits (four-resistor bridges) with low sensitivity. To compensate for or calibrate the offset and span and to amplify the signal as necessary a network was developed (see Figure 2) for a four-resistor resistance bridge circuit (such as a DMS sensing element, for example) consisting of the resistance bridge, an AM457 IC, four resistors and a maximum of six capacitors (see [1]). AM457's characteristics define the network and the IC amplifies the bridge output signal to the required output values. R4 C2 R1 7 AM457 OUTOP VCC 8 OP 1k 9k VCC VCC=5V C5 C4 3 IN+ ININOP C6 R3 2 6 AMP Temp Diagnostic unit DIAG 5 OUT 1 VDIAG C1 VOUT 4 GND GND Figure 2: Schematic circuit diagram for resistance bridges with low sensitivity The formulae used here to calibrate the offset and span of a bridge circuit are calculated using a model which is based on the classic Wheatstone bridge circuit of four resistors. On the basis of this model three simple equations were approximated for the setting of the resistors; they are also suitable for calibration using a minimum of data pertinent to the sensing element and without any need for further correction. analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 7/10 Rev. 2.2 Precision amplifier for bridge circuits The formulae used are: AM457 R1 = R 2 = 2 R B (k) 5 d RS Span adjustment (1) R1 (k) 100 ( -8 d RO + d RS ) R 4 (k) R3 = 8 d RO +9 d RS R4 = Auxiliary resistor (2) Offset adjustment (3) whereby the following applies to the individual sensing elements: RB - Bridge resistance in ohms dRS - Span/supply voltage in mV/V dRO - Offset/supply voltage in mV/V In order to calculate the discrete adjusting resistors R1, R2, R3 and R4 the offset, span and bridge resistance of the sensing element to be calibrated are first determined. The sensing element is then connected up to an AM457 to form the circuit illustrated in Figure 2. The resistors are dimensioned and applied according to the given formulae. The values of the capacitors are selected according to Table 1. (Capacitors C3...C6 are designated as decoupling capacitors; their use is therefore optional.) Using the calculated resistances and the given capacitors the offset and span (0.5...4.5V) should now be set. Discrepancies between the resistors used and the theoretical value enter the offset error proportionally. The calibration suggested in this article is suitable and has a sufficient accuracy for all sensing elements * * * * which consist of four bridge resistors which permit calibration with discrete resistors which have a sensitivity of 5...100mV FS where the span signal is greater than the offset. Depending on the level of accuracy required, when using sensing elements which have additional resistors (such as ceramic sensing elements for offset correction, for example) the discrete adjusting resistors must be corrected due to the discrepancy between the model and the sensing element (see [2]). analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 8/10 Rev. 2.2 Precision amplifier for bridge circuits BLOCK DIAGRAM AND PINOUT AM457 AM457 7 OUTOP OP VCC 1k 9k 8 VCC Diagnostic unit IN+ IN3 Temp 5 DIAG 1 OUT AMP 2 6 4 GND Figure 3: Block diagram of AM457 PIN 1 2 3 4 5 6 7 8 NAME OUT ININ+ EXPLANATION Output AMP Negative Input AMP Positive Input AMP IC Ground Output Diagnosis Negative Input OP Output OP Supply Voltage OUT ININ+ GND 1 2 3 4 8 7 6 5 VCC OUTOP INOP DIAG GND DIAG INOP OUTOP VCC Figure 4: AM457 Pin out Table 3: Pin configuration analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 9/10 Rev. 2.2 Precision amplifier for bridge circuits DELIVERY OPTIONS AM457 is available as: * An SOP 8 * Dice on 5" blue foil (on request) AM457 A1 e D b A L w E H Figure 5: SOP 8 package dimensions FURTHER READING www.analogmicro.de [1] [2] Application notes AN1011: the AM457 amplifier IC for DMS sensing elements Application notes AN1012: the AM457 amplifier IC for ceramic sensing elements analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz Internet: www.analogmicro.de Phone: Fax: Email: +49 (0)6131/91 073 - 0 +49 (0)6131/91 073 - 30 info@analogmicro.de May 2005 10/10 Rev. 2.2 |
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