AM417 ANALOGMICRO | Alldatasheet

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AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 1/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de PRINCIPLE FUNCTION Integrated instrumentation amplifier with an output stage for the amplification of differential signals and with an internal current source fo r the supply of external signal sources. The output signal is a voltage between 0.5 and 4.5V, ratiometrical to the supply voltage. The output span could be adjusted by the changeable gain of the output stage. TYPICAL APPLICATIONS

  • Amplification of resistor bridge signals
  • Voltage measurement e.g. temperature sensors
  • Current measurement via Shunt resistors
  • Amplification circuitry for sensing elements e.g. silicon pressure sensing elements
  • Differential input circuit for microprocessors/ADC-applications
  • Automotive bridge signal conditioning IBR = 1mA AM417 VCC = 5V 5% differential input voltage 0...200mV VOUT = 0,5...4,5V ratiometric

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 2/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de

CONTENTS

ELECTRICAL SPECIFICATIONS 4 BOUNDARY CONDITIONS / EXTERNAL COMPONENTS 5 DESCRIPTION OF FUNCTIONS 6 Instrumentation amplifier 6 Current source 6 Output stage 6 CALIBRATION WITH A RESISTOR BRIDGE CIRCUIT 7 Setting the output span 7 Setting the output offset 7 EXAMPLES 10 Example 1: Piezoresistive pressure sensing element in a bridge circuit with a positive offset 10 Example 2: Piezoresistive pressure sensing element in a bridge circuit with a negative offset 11 Example 3: Piezoresistive pressure sensing element in a bridge circuit with a high positive offset 12 TEMPERATURE COMPENSATION OF THE OUTPUT SPAN 13 EXAMPLE 15 Example 4: TCS compensation of a piezoresistive pressure sensing element 15 BLOCK DIAGRAM AND PINOUT 16 DELIVERY 16 EXAMPLE APPLICATIONS 17 FURTHER READING 18

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 3/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de

FEATURES

  • Instrumentation amplifier input for positive input voltages: 0...200mV
  • Adjustable gain
  • Common mode input range (CMIR): 1.3…V CC - 2.2V
  • Output voltage ratiometric to the
  • Low offset
  • Low offset drift
  • Supply voltage range: 5V ± 5% (ratiometric range)
  • Wide operating temperature range: -40°C...+125°C
  • Ratiometric current source for the supply of external measuring cells
  • Output driver (PNP open collector): IOUT = +11mA
  • No limited resolution
  • Output current limitation
  • Low internal noise
  • Integrated EMC protection
  • Small SO8 package
  • Low cost

DESCRIPTION

AM417 is a low-cost ratiometric interface IC which has been specifically designed for the conditioning of differential signals. The IC is particularly suitable for the signal evaluation of sensor elements which have to be powered by an internal current source (OP). These include piezoresistive and magnetoresistive silicon measuring cells and temperature sensing elements based on a resistor setup. In essence AM417 consists of a precision instrumentation amplifier, a ratiometric operational amplifier and a protected voltage output which has been configured as a driver stage. The amplifier can be adjusted across a wide range using two external resistors and the offset of an additional resistor affixed to the measuring bridge. Precision amplifier AM417 has been engineered in such a way that it can be used as an instrumentation amplifier for follow-on processors or A/D converters to make optimum use of the converter range. BLOCK DIAGRAM AM417 Output- st age OP IA IIB IN+ IB IN- RB VCC VOUT VR R 9 R GND Figure 1: Block diagram of AM417.

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 4/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise stated). Currents flowing into the IC are negative. Symbols in the table refer to Figure 1 and Figure 2. Parameter Symbol Conditions Min. Typ. Max. Unit System Parameters* Supply Voltage Range VCC Ratiometric range 4.75 5 5.25 V Maximum Supply Voltage VCCmax 6 V Quiescent Current ICC VCC = 5V, R1 = 500Ω, IIB = 1mA 7.6 mA Temperature Specifications Operating temperature Tamb -40 125 °C Storage temperature Tst -55 125 °C Junction temperature TJ 150 °C Parameter Symbol Conditions Min. Typ. Max. Unit OP (Ratiometric Current Source) Input Voltage VRB Ratiometric with VCC = 5V 0.5 V Input current IRB 100 nA Output Current Range IIB 0.50 1.25 mA Output Current accuracy IIB Ratiometric with VCC = 5V, R1 = 500 Ω 0.98 1 1.02 mA Ratiometric Error RAT@IB RAT@IB = 1.05 VRB (VCC = 5V) – VRB (VCC = 5.25V) -1 1 mV IIB vs. temperature d IIB/dT IIB = 1mA -45 -25 -5 ppm/°C IRB vs. temperature d IRB/dT IIB = 1mA -20 + 20 ppm/°C Output Voltage Range VIB IIB = 1.25mA 2.0 VCC–0.2V V Output Resistance RIB RIB = VIB/IIB, VIB = 2V, ∆VIB = 2.8V, IIB = 1mA, 1.5 30 M Ω Instrumentation Amplifier Common Mode Input Voltage Range CMIR 1.3 VCC–2.2V V Differential Input Voltage Range ∆VIN 0 200 mV Internal Gain GIA 9.8 10.0 10.2 Input Bias Current IIN+;– 25 75 nA Input Offset Voltage VOIA -3 3 mV VOS vs. temperature dVOIA/dT T amb = -40…100°C -10 10 µV/°C VOS vs. temperature dVOIA/dT T amb = 100…125°C -30 30 µV/°C Output Voltage Range VVIA 0.05 VCC–2V V Nonlinearity NLIA V IN– = 1.3V, ∆VIN = 100mV, 200mV 0.15 % FS Common Mode Rejection Ratio CMRR V IN– = 1.3V, ∆VIN = 100mV 80 90 dB Power Supply Rejection Ratio PSRR V IN– = 1.3V, ∆VIN = 100mV 74 80 dB Input Voltage Noise en G IA = 10 35 nV/√Hz

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 5/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Parameter Symbol Conditions Min. Typ. Max. Unit Voltage Output Stage Adjustable Gain GOUT 2 11 Input Voltage Range VVR 0.05 VCC– 2.25V V Input Current IIN V IN– = 2V, ∆VIN = 50mV 20 75 nA Input Offset Voltage VOS -3 3 mV VOS vs. temperature d VOS/dT V IN– = 2V, ∆VIN = 50mV, Tamb = -40…100°C -15 15 µV/°C VOS vs. temperature d VOS/dT V IN– = 2V, ∆VIN = 50mV, Tamb = 100…125°C -100 0 µV/°C Output Current IVOUT Pin VOUT 65 150 350 µA Output Voltage Range VOUT With external transistor* 0.5 4.5 V Output Current IOUT With external transistor* 11 mA Output Resistance ROUT With external transistor* 0.1 0.85 Ω Power Supply Rejection Ratio PSRR -72 -90 dB Current Limitation Threshold VTHRESH VTHRESH = VVCC – VVOUTmin R2 = 27Ω, IOUT ≈ 14mA 1.00 1.15 V System Parameters Input Voltage Range ∆VIN ∆VIN @ VOUTmax = 4.5V and GOUT = 10 @ VOUTmax = 4.5V and GOUT = 2 200 mV mV Gain Bandwidth Product GBW C OUT = 1nF 400 1,500 kHz Nonlinearity NL 0.15 %FS Table 1: Electrical specifications System parameters: specifications which refer to the AM417 circuit as a whole. * Output current dependent on resistor R2 (see Equation 4). BOUNDARY CONDITIONS / EXTERNAL COMPONENTS Parameter Symbol Conditions Min. Typ. Max. Unit Resistor Adjustment Current Source R1 400 1000 Ω Resistor Sense Current Limitation R2 0 50 Ω Gain Resistor Sum R3 + R4 VOUT = (R3 + R4)/R4 GIA 0:41 2.1 k Ω Capacitor Power Supply C1 100 330 nF Capacitor Frequency Compensation C2 X7R capacitor , ±10% 4.7 4,7 nF Capacitor Load C3 X7R capacitor , ±10% 1.0 10.0 nF Output PNP Transistor βT1 e.g. BCW68H or BC557C, low drop, high β for Tamb = -40….125°C 180 Table 2: Electrical boundary conditions

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 6/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de DESCRIPTION OF FUNCTIONS AM417 is a ratiometric, adjustable interface IC which has been specially developed for the conditioning of bridge signals for automotive appli cations. With its integrated, ratiometric current source it is particularly suitable for the excitation of piezoresistive bridge devices in a constant current mode. The IC enables simple calibration and temperature compensation of the input signals. AM417 consist of three functional units: Instrumentation amplifier Using the input stage of the instrumentation am plifier (IA) the input signal is preamplified by GIA = 10. The IA can only process positive input si gnals. A negative input voltage or negative input offset must be balanced by using additional resistor at positive input pin V IN+ (c.f. Setting the output offset). Current source The additional operational amplifier (OP) is linked internally to supply voltage VCC via a voltage divider (10:1). With the OP acting as a ratiometric current source a resistor measuring cell can be supplied with constant current within a range of 0.5 – 1.25mA. The supply current of the external sensing element IIB can be set by varying resistor R1 at the minus input of the OP (VIN-) using the following ratio: 110 R VI VCC IB = (1) Output stage A voltage amplifier with an exte rnal PNP open collector stage ( T1) acts as a voltage output and can provide a maximum current of IOUT = 11mA. Using external resistors R3 and R4 the Gain GOUT can be adjusted between 2.and 11. R RRGOUT += (2) The gain of the entire circuit AM417 is thus: GSYS = GIA GOUT. A current limitation has been integrated into th e output stage. The limit circuit restricts output voltage VOUTmin with reference to VCC, where VBE is the basic emitter voltage of external transistor T1. ( )1min 5.1 TVVV BEVCCVOUT −= (3) With this the maximum output current can be adjusted using resistor R2 in series with the T1 transistor emitter (see Figure 2). The current is thus calculated as: max mV380 RR TVVI BETHRESH OUT ≈−= (4)

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 7/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de where VTHRFSH is current limitation threshold. Should no current limit be necessary, the T1 transistor emitter can be directly connected up to pin VCC ( R2 = 0). Good thermal coupling between T1 and the IC reduces the temperature drift of output current IOUT, thus raising the quality of the current limit. The output stage is not protected against reverse polarity. Reverse polarity of VCC referenced to ground can be realized using a simple additional circuit, see [3]. CALIBRATION WITH A RESISTOR BRIDGE CIRCUIT Setting the output span The output signal span can be set using gain GOUT of the output stage (see Equation 2): IAOUTME SPAN OUT GV VG ⋅= ( 5 ) where VSPAN = VOUTmax – VOUT min and VOUTME is the output voltage of the sensing element. Setting the output offset In a Wheatstone bridge circuit, such as those frequently used with piezoresistive sensors, the offset of the output voltage VOUTmin must be calibrated depending on the required degree of accuracy and with reference to the offset of both the sensi ng element and the IC. To this end, a compensating AM417 VOUT VS C2 R3 6 C3 Ground R 9 R OP IA IIB Output- stage RB1 RB2 RB3 RB4 Ro VIN- VIN+ Figure 2: Measuring a constant-current sensing element using a Whea t- VOUTME - = negative bridge output signal VOUTME + = positive bridge output signal VOUTME + - VOUTME - = VOUTME VOUTME + = VIN+ VOUTME - = VIN- VIN+ - VIN_ = VIN

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 8/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de resistor RO is inserted into the measuring bridge (see Figure 2). By using this compensating resistor the instrumentation amplifier input voltage ∆VIN is set in such a way, that output voltage VOUTmin has a value of 0.5V, for example. The voltage drop VRO across resistor RO is given by: +− +−+−∆= 4 1/ BB B BB B BB B BRINRO RR R RR R RR RVVV (6) where VBR is the voltage drop across the entire sensing element, RBR the total bridge resistance and RB1,2,3,4 the individual bridge resistors. Assuming that the four separate bridge resistors have the same value, the following approximation formula is valid: VRO = 2∆VIN ( 7 ) ∆VIN is the voltage to be set at the input of the instrumentation amplifier where there are no offsets. OUTIA OUT SYS OUT IN GG V G VV ⋅==∆ minmin ( 8 ) Taking the offset of the sensing element ( VOSME) and that of the IC ( VOSIC) into account (VOSIC = VOSIA + 0.1 VOSOUT, where VOSIA is the instrumentation amplifier offset and VOSOUT the output stage offset), the adjustable voltage is calculated as: ∆VIN’ = ∆VIN - VOSIC - VOSME ( 9 ) From (9) and (8) it follows that: OSMEOSIC OUTIA OUT IN VVGG Applying (7) and (10), the necessary voltage drop across RO required to calibrate the offset of the output voltage VOUTmin is expressed thus: OUTIA OUT RO VVGG VV min2 (11) On condition, the sensing element offset is low referenced to the sensing element output voltage (VOSME < 10 VOUTME), the resistor RO is calculated with sufficient accuracy as: IB RO O I VR ⋅= 2 ( 1 2 ) Applying (11) and the condition, that the voltage drop across Ro may only be positive, the maximum compensatable offset is computed thus: OUTIA OUT OSMEOSIC GG VVV ⋅≤+ min ( 1 3 )

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 9/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de If when calculating VRO (Equation 11) a negative value is produced, the resistor R0 must be placed in the left arm of the bridge (RO’; see Figure 3): Doing so changes the effective direction of RO and its resistance is now expressed as: IB RO O I AM417 VOUT VS C2 R3 6 C3 Ground R 9 R OP IA IIB Output- stage RB1 RB2 RB3 RB4 VIN- VIN+ RO´ Figure 3: Circuit as in Figure 2 with R0’ (instead R0) at input pin 5 (IN-)

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 10/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de EXAMPLES By way of example the equations shall be calculate d using typical values for piezoresistive sensing elements [2] in an attempt to illustrate how various sensing elements can be calibrated and compensated with very few external components. The aim of the exercise is to calibrate the output voltage of AM417 to VOUTmin = 0.5V and VOUTmax = 4.5V Example 1: Piezoresistive pressure sensing element in a bridge circuit with a positive offset

  • VOUTME = 160mV at VBR = 5V
  • VCC = 5V
  • VOSIC = -2mV
  • VOSME= +10mV at VBR = 5V
  • RBR = 3KΩ The sensing element is to be supplied with cons tant current as this provides a simple way of compensating the temperature behavior of the span (see: TEMPERATURE COMPENSATION OF THE OUTPUT SPAN). Taking the maximum output voltage at pin 2 ( IB) into account the supply current is selected as IIB = 1mA (R1 = 500Ω). At pin 2 (IB) the voltage is: VVmAkVIRV VRBRBRIB 5.35.013 =+⋅Ω=+⋅= . Considering a typical positive temperature coefficient of the sensing element bridge resistor RBR of TCR = +0.0028/°C the maximum voltage at pin 2 (IB) is not overshot (VIBmax = 4.8V at VCC = 5V). The bridge voltage is: VkmARIV BRBRBR 331' =Ω⋅=⋅= . The output voltage of the sensing element given for VBR = 5V must be corrected by the ratio of the bridge voltages: mVV VmVVOUTME 965 3160' =⋅= The offset voltage of the sensing element given for VBR = 5V must be corrected by the ratio of the bridge voltages: mVV VmVVOSME 65 310' =⋅= Applying Equation 5 the following is accrued: 166.41096 4 =⋅= mV VGOUT

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 11/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de and from Equation 11 we are presented with: mVmVmVVVRO 1662166.410 5.02 =   −+⋅⋅= Referring to Equation 12 the resistance for offset calibration is thus: Ω=⋅= 322 0 IB R I VR If RO is set to 32 Ω and if we take the offsets of sensing element and IC into consideration, the output signal offset of the overall circuit is set to VOUTmin = 0.5V and the maximum output signal is VOUTmax = 4.5V. Example 2: Piezoresistive pressure sensing element in a bridge circuit with a negative offset

  • VOUTME = 100mV at VBR = 5V
  • VCC = 5V
  • VOSIC = 2mV
  • VOSME= -10mV at VBR = 5V The sensing element is supplied with constant current. Taking the maximum output voltage of the OP into account (see Example 1) IB is again selected as IIB = 1mA (R1 = 500Ω). The bridge voltage is: VkmARIV BRBRBR 331' =Ω⋅=⋅= . The output voltage of the sensing element is corrected by the ratio of the bridge voltages: mVV VmVVOUTME 605 3100' =⋅= The offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: mVV VmVVOSME 65 310' −=⋅−= Applying Equation 5 the following is accrued: 67.61060 4 =⋅= mV VGOUT

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 12/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de and from Equation 11 we are presented with: mVmVmVVVRO 236267.610 5.02 =   +−⋅⋅= Referring to Equation 12 the resistance for offset calibration is thus: Ω=⋅= 462 0 IB R I VR If RO is set to 46 Ω and if we take the offsets of sensing element and IC into consideration, the output signal offset of the overall circuit is set to VOUTmin = 0.5V and the maximum output signal is VOUTmax = 4.5V. Example 3: Piezoresistive pressure sensing element in a bridge circuit with a high positive offset

  • VOUTME = 100mV at VBR = 5V
  • VCC = 5V
  • VOSIC = 2mV
  • VOSME= 10mV at VBR = 5V
  • RBR = 3KΩ The sensing element is supplied with constant current. Taking the maximum output voltage of the OP into account IB is again selected as IIB = 1mA (R1 = 500Ω). The bridge voltage is: VkmARIV BRBRBR 331' =Ω⋅=⋅= . The output voltage of the sensing element is corrected by the ratio of the bridge voltages: mVV VmVVOUTME 605 3100' =⋅= The offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: mVV VmVVOSME 65 310' =⋅= Applying Equation 5 the following is accrued: 67.61060 4 =⋅= mV VGOUT

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 13/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de and from Equation 11 we are presented with: mVmVmVVVRO 5.66267.610 5.02 −= Referring to Equation 12a the resistance for offset calibration is thus: () Ω=⋅−⋅= 132' 0 IB R I VR If RO’ (resistor on the left) is set to 13 Ω and if we take the offsets of sensing element and IC into consideration, the output signal offset of the overall circuit is set to VOUTmin = 0.5V and the maximum output signal is VOUTmax = 4.5V. TEMPERATURE COMPENSATION OF THE OUTPUT SPAN Supplying a piezoresistive sensing element with constant current makes compensation of the temperature of the span a relatively simple affa ir. With a constant current supply the negative temperature coefficient of sensor sensitivity S can be compensated by the positive temperature coefficient of bridge resistor RBR. IIB RTSC Input pin AM417 IIB´ RBR Figure 4: Bridge array for the compensation of TC with RBR = bridge resistor

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 14/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de The output signal of a piezoresistive sensing element is accrued from: BRIBBROUTME RIPSVPSV ⋅⋅⋅=⋅⋅= ( 1 4 ) S is the sensor sensitivity of the sensing element and P is the applied pressure. Sensor sensitivity S and bridge resistor RBR are the dominant temperature-depende nt variables in Equation 14. The following applies: ( )() oBRBR TTTCRRR −⋅+⋅= 10 ( 1 6 ) S0 is the basic value of the sensitivity and RBRO the basic value of the bridge resistance at T0 (usually room temperature). T is the actual temperature. TCS and TCR are the linear temperature coefficients of sensitivity and bridge resistance. Typical values are: TCS = -0.0019/°C and TCR = +0.0028/°C [3]. Good temperature compensation of sensing element output signal VOUTME would be automatically achieved if both temperature coefficients had the same value. If both are different, however, an attempt is made to equalize them. This is done by adding an additional compensatory TCS resistor RTCS which is inserted parallel to the sensing element (see Figure 4). The TCR value of the entire system is thus amended so that it is the same as TCS of the sensing element. In the temperature compensation of the sens ing element output signal described above the following applies to the compensatory TCS resistor: TCSTCR TCSRR BRTCS −⋅= ( 1 7 ) As part of the set bridge supply current IIB´ flows through the shunt resistor RTCS the circuit output signal is reduced after TCS compensation according to the following equation: () BRTCS TCS IB IB RR R I I +=' ( 1 8 ) In order to reinstate the original output signal of the circuitry the circuit gain must be increased by the reciprocal ratio: ( ) TCS BRTCS IB IB R RR I ITCSFactor +== ' ( 1 9 )

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 15/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de In order to achieve a maximum a sensing element output signal it is best to increase set bridge supply current IIB by the TCS Factor. Gain GOUT can also be increased by the same factor if, for example, maximum bridge current IIBmax = 1.25mA or if the maxi mum voltage at pin 2 ( IB) is overshot during an increase. EXAMPLE Example 4: TCS compensation of a piezoresistive pressure sensing element [2]

  • TCS = -0.0019/°C
  • VCC = 5V
  • RBR = 3KΩ
  • TCR = +0.0028/°C
  • Temperature range: -20°C – 80°C Bridge supply current IIB is selected according to the fo llowing. Assuming that the maximum operating temperature of the circuit is 80°C, th e maximum bridge resistance is calculated using Equation (16): ( )() Ω=°−°⋅°+⋅Ω= kCCCkRRB 46,32580/0028,013max With a bridge current of IIB = 0.8mA, at 80°C and VCC = 5V, pin 2 (IB) has a potential of: VVmAkVIB 27.35.08.046.3 =+⋅Ω= Applying Equation (17): R TCS = 6.33KΩ Using Equation (19) the following is calculated for T0: TCS Factor = 1.47 If bridge current IIB is now increased by a factor of TCSFactor, the result is a new amended bridge current of: IIBnew = 1.18mA The original output signal of the sensing element is thus reinstated following TCS compensation. Output stage gain GOUT could also be increased by a factor of TCSFactor by adjusting resistors R3 and R4 according to Equation (2).

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 16/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de BLOCK DIAGRAM AND PINOUT PIN NAME FUNCTION

1 GND IC Ground

2 IB Current Source Output

3 RB Current Source Set

4 IN+ Positive IA Input

5 IN– Negative IA Input

6 VR Gain Set

7 VOUT Voltage Output

8 VCC Supply Voltage

Table 3: Pin out DELIVERY AM417 is available as:

  • An SOP08
  • Dice on 5“ blue foil AM417 Output- stage OP IA IIB IN+ IB IN- RB VCC VOUT VR R

9 R VCC

Figure 5: Circuit diagram of AM417 1 8 2 7 4 5 GND IB RB IN+ VCC VOUT VR IN- AM 417 Figure 6: AM417 Pin out

AM417 – Ratiometric instrumentation amplifier with adjustable output stage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2008 –Rev 3.1- Page 18/18 Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de FURTHER READING [1] The Analog Microelectronics GmbH website: http://www.analogmicro.de/ [2] On the AMSYS GmbH website: http://www.amsys.info/products/ms54xx.htm [3] Reverse polarity protection for a ratiometric application using AM417: Analog Microelectronics reserves the right to make amendments to any dimensions, technical data or other information herein without further notice.