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% Diagnostic unit Differential input voltages ( 5... 100mV FS) 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 [email protected] May 2005 1/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 TABLE OF CONTENTS Features Block diagram General description Electrical specifications Circuit Description of functions Example application Block diagram and pinout Delivery options Further reading 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 4 5 9 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 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 [email protected] 3 7 9 9 10 May 2005 2/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 GENERAL DESCRIPTION FEATURES • 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... +125°C • 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 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 8 VCC OP 7 IN+ AMP _ INOP VCC 9k 1k Temp Diagnostic unit 5 DIAG 1 OUT 6 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 [email protected] May 2005 3/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise stated) Parameter Symbol Voltage Range VCC Quiescent Current ICC Conditions Min. 4.5 Tamb = -40...+85°C Typ. Max. Unit 5 5.5 V 1.5 2.5 mA 125 °C Temperature Specifications Operating Tamb -45 Storage Tst -55 Junction TJ Thermal Resistance Θja DIL8 plastic package 111 °C/W Θja SO8 narrow plastic package 181 °C/W ±0.1 ±0.3 mV Tamb= -45…105°C ±0.5 ±1.2 µV/°C ±120 µV 200 nA –600 pA/°C ±100 mV 150 °C 150 °C Amplifier AMP Offset Voltage VOS VOS vs. Temperature dVOS/dT VOS vs. Temperature dVOS/dT Tamb= 105…125°C Input Bias Current IB VCM = 2.5V IB vs. Temperature dIB/dT Tamb= -45…125°C Differential Input Voltage VIN VIN =VIN+ – VIN- 30 –130 ±5 Input Offset Current IOS VCM = 2.5V ±0.5 ±5 nA IOS vs. Temperature dIOS/dT Tamb= -45…125°C ±2.5 ±30 pA/°C Input Resistance RIN VCM / IB,typ (VCM = 2.5V) 100 MΩ Input Capacitance CIN 90 pF Common Mode Input Range CMIR 1 Common Mode Rejection Ratio CMRR 120 135 dB Open Loop Gain G0 120 140 dB Adjustable Gain G 10 Output Voltage Range VOUT 0.2 Output Current IOUT Output Load Capacitance CL Output Load Resistance RL Power Supply Rejection Ratio PSRR Gain Bandwidth Product GBW Sink and source VCC – 1.3 VCC – 0.2 250 5 20 90 G = 10; C1 = 1nF; RL = 20kΩ V µA 1 Sink and source V nF kΩ 110 dB 30 kHz Including filters for EMC protection Non Linearity Slew Rate Input Voltage Noise 10-4 G ≤ 100 SR en C1 = 1nF; RL = 20kΩ 0.3 Tamb, ;Rs = 1kΩ; VCC = 5V 0.8 V/µs 30 nV/√Hz Rs = Source Impedance 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 [email protected] May 2005 4/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 Error Monitoring (Diagnosis) Output Voltage on Error (high) VDIAG VCC – 0.2 Output Voltage without Error (low) VDIAG GND+ 0.2 V Output Current on Error IERR VDIAG = 2.5V 1.5 3.5 6 µA Output Current without Error INOERR VDIAG = 2.5V -6 -3.5 -1.5 µA V Threshold for Temperature Error Monitor- TERR ing 105 143 °C Threshold for Output Current (AMP) Error Monitoring IIA,ERR 250 800 µA Threshold for Supply Voltage Error Monitoring VCC,ERR 3.8 4.3 V Threshold for High Input Voltage (AMP) Error Monitoring VIN,high,ERR VCC – 1.8 VCC – 0.8 V Threshold for Low Input Voltage (AMP) Error Monitoring VIN,low,ERR 0.4 0.9 V With regard to the sensing element Operational Amplifier (OP) Offset Voltage VOS VOS vs. Temperature dVOS/dT Tamb= -45…105°C ±4 ±8 mV ±10 ±40 µV/°C VOS vs. Temperature dVOS/dT Tamb= 105…125°C Input Bias Current IB ±320 µV IB vs. Temperature dIB/dT Input Resistance RIN Input Capacitance CIN Common Mode Input Range CMIR 0 0.5 Open Loop Gain G0 I0ut =0.1mA 90 110 dB G0.2mA I0ut =2mA 75 90 dB 80 300 nA Tamb= -45…125°C 100 400 pA/°C VCM / IB,typ (VCM = 0.5V) 6.25 MΩ 30 pF 0 1 V Output Voltage Range VOUT Output Source Current IOUT Output Load Capacitance C1 Gain Bandwidth Product GBW GGAIN = 10; C1 = 1nF; RL = 1.25kΩ 0.9 1.4 MHz Slew Rate SR GGAIN = 100; C1 = 1nF; RL = 25kΩ 0.2 0.7 V/µs PNP Open Collector Output 1 VCC – 0.2 V 2 mA 5 nF Table 1: Specifications Currents flowing into the IC are negative. Positive input of the operational amplifier (fixed internally): 0.1⋅VCC VCM Input Common Mode Voltage CIRCUIT Parameter AMP Output Capacitor Symbol Conditions C1 Min. Typ. 1 Compensation Capacitor C2 100 DIAG Output Capacitor (optional) C3 3.3 OP Output Capacitor (optional) C4 Stabilization Capacitor (optional) C5 EMV Protection Capacitor (optional) C6 Load Resistor RL Table 2: Circuit Components 1 470 5 nF pF nF nF nF pF kΩ Recommendation: Ceramic capacitor analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Internet: www.analogmicro.de Unit 5 100 20 Max. Phone: Fax: Email: +49 (0)6131/91 073 – 0 +49 (0)6131/91 073 – 30 [email protected] May 2005 5/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 DESCRIPTION OF FUNCTIONS 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 0.1⋅VCC. 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 [email protected] May 2005 6/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 EXAMPLE APPLICATION 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 AM457 R1 7 VCC 8 OP OUTOP 1k C4 3 C6 2 6 IN+ IN- AMP Temp 9k VCC Diagnostic unit DIAG 5 OUT R3 VDIAG 1 INOP 4 VCC=5V C5 GND C1 VOUT 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 [email protected] May 2005 7/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 The formulae used are: R1 = R 2 = 2 ⋅R B (kΩ) 5 ⋅d RS Span adjustment R1 (kΩ) 100 ( −8 ⋅d RO + d RS ) ⋅R 4 (kΩ) R3 = 8 ⋅d RO +9 ⋅d RS R4 = (1) 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 [email protected] May 2005 8/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 BLOCK DIAGRAM AND PINOUT AM457 OP 7 8 VCC 1k VCC 9k OUTOP 5 Diagnostic unit IN+ IN- 3 DIAG Temp 1 AMP 2 OUT 6 4 GND Figure 3: Block diagram of AM457 PIN 1 2 3 4 5 6 7 8 NAME OUT ININ+ GND DIAG INOP OUTOP VCC EXPLANATION OUT Output AMP Negative Input AMP Positive Input AMP IC Ground Output Diagnosis Negative Input OP Output OP Supply Voltage ININ+ 1 2 3 GND 4 8 7 6 5 VCC OUTOP INOP DIAG 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 [email protected] May 2005 9/10 Rev. 2.2 Precision amplifier for bridge circuits AM457 DELIVERY OPTIONS AM457 is available as: • An SOP 8 • Dice on 5" blue foil (on request) A1 e D A w b L E H Figure 5: SOP 8 package dimensions FURTHER READING www.analogmicro.de [1] Application notes AN1011: the AM457 amplifier IC for DMS sensing elements [2] 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 [email protected] May 2005 10/10 Rev. 2.2