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Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 1/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de 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 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 AM457 V = 5 V 5 %CC ± I = 2 m Aout Differential input voltages (5 . . .1 0 0 m V F S ) (adjustable) ratiometric OUT CC Diagnostic unit

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 3/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de

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:
  • 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 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 OP AMP Temp VCC AM457 Diagnostic unit OUTOP IN+ INOP GND OUT DIAG VCC 1k 9k Figure 1: Block diagram

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 4/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise stated) Parameter Symbol Conditions Min. Typ. Max. Unit Voltage Range VCC 4.5 5 5.5 V Temperature Specifications Operating Tamb -45 125 °C Storage Tst -55 150 °C Junction TJ 150 °C Thermal Resistance Θja DIL8 plastic package 111 °C/W Θja SO8 narrow plastic package 181 °C/W Amplifier AMP Offset Voltage VOS ±0.1 ±0.3 mV VOS vs. Temperature d VOS/dT Tamb= -45…105°C ±0.5 ±1.2 µV/°C VOS vs. Temperature d VOS/dT Tamb= 105…125°C ±120 µV Input Bias Current IB 30 200 nA IB vs. Temperature d IB/dT VCM = 2.5V Tamb= -45…125°C –130 –600 pA/°C Differential Input Voltage VIN V IN =VIN+ – VIN- ±5 ±100 mV Input Offset Current IOS ±0.5 ±5 nA IOS vs. Temperature d IOS/dT VCM = 2.5V Tamb= -45…125°C ±2.5 ±30 pA/°C Input Resistance RIN V CM / IB,typ (VCM = 2.5V) 100 MΩ Input Capacitance CIN 90 pF Common Mode Input Range CMIR 1 VCC – 1.3 V 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 VCC – 0.2 V Output Current IOUT Sink and source 250 µA Output Load Capacitance CL 1 5 nF Output Load Resistance RL Sink and source 20 kΩ Power Supply Rejection Ratio PSRR 90 110 dB Gain Bandwidth Product GBW G = 10; C1 = 1nF; RL = 20kΩ Including filters for EMC protection 30 kHz Non Linearity G ≤ 100 10 -4 Slew Rate SR C1 = 1nF; RL = 20kΩ 0.3 0.8 V/µs Input Voltage Noise en Tamb, ;Rs = 1kΩ; VCC = 5V Rs = Source Impedance 30 nV/√Hz

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 5/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de Error Monitoring (Diagnosis) Output Voltage on Error (high) VDIAG VCC – 0.2 V Output Voltage without Error (low) VDIAG GND+ 0.2 V Output Current on Error IERR V DIAG = 2.5V 1.5 3.5 6 µA Output Current without Error INOERR V DIAG = 2.5V -6 -3.5 -1.5 µA Threshold for Temperature Error Monitor- ing TERR 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 With regard to the sensing element VCC – 1.8 VCC – 0.8 V Threshold for Low Input Voltage (AMP) Error Monitoring VIN,low,ERR 0.4 0.9 V Operational Amplifier (OP) Offset Voltage VOS ±4 ±8 mV VOS vs. Temperature d VOS/dT Tamb= -45…105°C ±10 ±40 µV/°C VOS vs. Temperature d VOS/dT Tamb= 105…125°C ±320 µV Input Bias Current IB 80 300 nA IB vs. Temperature d IB/dT Tamb= -45…125°C 100 400 pA/°C Input Resistance RIN V CM / IB,typ (VCM = 0.5V) 6.25 MΩ Input Capacitance CIN 30 pF Common Mode Input Range CMIR 0 0.5 1 V Open Loop Gain G0 I 0ut =0.1mA 90 110 dB G0.2mA I 0ut =2mA 75 90 dB Output Voltage Range VOUT 0 VCC – 0.2 V Output Source Current IOUT PNP Open Collector Output 2 mA Output Load Capacitance C1 1 5 nF 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 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 Symbol Conditions Min. Typ. Max. Unit AMP Output Capacitor C1 1 5 nF Compensation Capacitor C2 100 pF DIAG Output Capacitor (optional) C3 3.3 nF OP Output Capacitor (optional) C4 1 5 nF Stabilization Capacitor (optional) C5 100 nF EMV Protection Capacitor (optional) C6 470 pF Load Resistor RL 20 kΩ Table 2: Circuit Components Recommenda tion: Ceramic capacitor

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 6/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de 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, pa rticularly for ceramic or DMS sensing elements. The IC generate s a rail-to-rail output signal of 0.2V to Vcc-0.2V. With suitable 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 br idge 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 f acility 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 amplifi cation and offset can be fixed using external resistors. 2. An operational amplifier (OP) whose positive in put 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.

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 7/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de EXAMPLE APPLICATION Offset and span compensation for a Wheatstone bridge with discrete adjusting resistors. AM457 is suitable for the amplification of all res istance 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 resist ance bridge, an AM457 IC , four resistors and a maximum of six capacitors (see [1]). AM457's char acteristics define the network and the IC amplifies the bridge output signal to the required output values. 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 minimu m of data pertinent to the sensing element and without any need for further correction. OP AMP Temp VCC AM457 Diagnostic unit OUTOP IN+ INOP GND OUT DIAG VCC 1k 9k R4 R1 V= 5 VCC VDIAG VOUT GND IN-

Table 1. (Capacitors C3… C6 are designated as decoupling capacitors; their use is therefore optional.)

  • 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]).

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 9/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de BLOCK DIAGRAM AND PINOUT PIN NAME EXPLANATION

1 OUT Output AMP

2 IN- Negative Input AMP

3 IN+ Positive Input AMP

4 GND IC Ground

5 DIAG Output Diagnosis

6 INOP Negative Input OP

7 OUTOP Output OP

8 VCC Supply Voltage

Table 3: Pin configuration IN+ IN- AM457 OUTOP OP7 AMP VCC Temp VCC DIAG OUT GND 1k 9k Diagnostic unit Figure 3: Block diagram of AM457 4 5 8OUT IN- IN+ GND DIAG INOP OUTOP VCC Figure 4: AM457 Pin out

Precision amplifier for bridge circuits AM457 analog microelectronics May 2005 Analog Microelectronics GmbH P hone: +49 (0)6131/91 073 – 0 10/10 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 2.2 Internet: www.analogmicro.de Email: info@analogmicro .de DELIVERY OPTIONS AM457 is available as:

  • An SOP 8
  • Dice on 5" blue foil (on request) 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 amp lifier IC for ceramic sensing elements E H e D b A1 A L w