AM417 – Ratiometric instrumentation amplifier with adjustable output stage PRINCIPLE FUNCTION Integrated instrumentation amplifier with an output stage for the amplification of differential signals and with an internal current source for 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. VCC = 5V differential input voltage 0...200mV 5% AM417 VOUT = 0,5...4,5V ratiometric IBR = 1mA 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 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 1/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage CONTENTS PRINCIPLE FUNCTION 1 TYPICAL APPLICATIONS 1 FEATURES 3 BLOCK DIAGRAM 3 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 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 2/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage DESCRIPTION FEATURES • Instrumentation amplifier input for positive input voltages: 0...200mV • Adjustable gain • Common mode input range (CMIR): 1.3…VCC - 2.2V • Output voltage ratiometric to the supply: 0.5...4.5V • 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 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 IIB IB 2 IN+ 4 9R OP R Outputstage IA IN- 5 RB 3 1 AM417 8 VCC 7 VOUT 6 VR GND Figure 1: Block diagram of AM417. Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 3/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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 4.75 5 5.25 V 6 V 7.6 mA °C System Parameters* Supply Voltage Range VCC Maximum Supply Voltage VCCmax Quiescent Current ICC Ratiometric range VCC = 5V, R1 = 500Ω, IIB = 1mA Temperature Specifications Operating temperature Tamb -40 125 Storage temperature Tst -55 125 °C Junction temperature TJ 150 °C Max. Unit Parameter Symbol Conditions Min. Typ. OP (Ratiometric Current Source) Input Voltage VRB Input current IRB Output Current Range IIB Output Current accuracy IIB Ratiometric with VCC = 5V, R1 = 500 Ω Ratiometric Error RAT@IB RAT@IB = 1.05 VRB (VCC = 5V) – VRB (VCC = 5.25V) IIB vs. temperature dIIB/dT IRB vs. temperature Ratiometric with VCC = 5V 0.5 100 0.50 dIRB/dT V 0.98 1 -1 mA 1.02 mA 1 mV IIB = 1mA -45 -5 ppm/°C IIB = 1mA -20 + 20 ppm/°C VCC–0.2V V Output Voltage Range VIB IIB = 1.25mA 2.0 Output Resistance RIB RIB = VIB/IIB, VIB = 2V, ∆VIB = 2.8V, IIB = 1mA, 1.5 -25 nA 1.25 30 MΩ Instrumentation Amplifier Common Mode Input Voltage Range CMIR Differential Input Voltage Range ∆VIN 0 Internal Gain GIA 9.8 Input Bias Current IIN+;– Input Offset Voltage VOIA -3 3 mV VOS vs. temperature dVOIA/dT Tamb = -40…100°C -10 10 µV/°C VOS vs. temperature dVOIA/dT Tamb = 100…125°C -30 30 µV/°C Output Voltage Range VVIA 0.05 VCC–2V V Nonlinearity NLIA VIN– = 1.3V, ∆VIN = 100mV, 200mV 0.15 % FS Common Mode Rejection Ratio CMRR VIN– = 1.3V, ∆VIN = 100mV 80 Power Supply Rejection Ratio PSRR VIN– = 1.3V, ∆VIN = 100mV 74 Input Voltage Noise en GIA = 10 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz 1.3 VCC–2.2V V 200 mV 10.0 10.2 25 75 90 nA dB 80 dB 35 nV/√Hz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 4/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage Parameter Symbol Conditions Min. Typ. Max. Unit Voltage Output Stage Adjustable Gain GOUT 2 11 Input Voltage Range VVR 0.05 VCC– 2.25V Input Current IIN Input Offset Voltage VOS VOS vs. temperature dVOS/dT VIN– = 2V, ∆VIN = 50mV 20 VIN– = 2V, ∆VIN = 50mV, V 75 nA -3 3 mV -15 15 µV/°C -100 0 µV/°C 350 µA 4.5 V 11 mA Tamb = -40…100°C VOS vs. temperature dVOS/dT VIN– = 2V, ∆VIN = 50mV, Output Current IVOUT Pin VOUT 65 Output Voltage Range VOUT With external transistor* 0.5 Output Current IOUT With external transistor* Output Resistance ROUT With external transistor* Power Supply Rejection Ratio PSRR Current Limitation Threshold VTHRESH VTHRESH = VVCC – VVOUTmin R2 = 27Ω, IOUT ≈ 14mA 1.00 1.15 V VTRESH vs. Temperature dVTHRESH/dT -40...+125°C without external transistor* -4.2 -1.8 mV/°C Tamb = 100…125°C 150 0.1 -72 0.85 -90 Ω dB System Parameters ∆VIN @ VOUTmax = 4.5V and GOUT = 10 0 40 mV ∆VIN @ VOUTmax = 4.5V and GOUT = 2 0 200 mV Gain Bandwidth Product GBW COUT = 1nF Nonlinearity NL 0.15 %FS Max. Unit Input Voltage Range 400 1,500 kHz 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. Resistor Adjustment Current Source R1 400 1000 Ω Resistor Sense Current Limitation R2 0 50 Ω 2.1 kΩ Gain Resistor Sum R3 + R4 VOUT = (R3 + R4)/R4 GIA Capacitor Power Supply C1 Capacitor Frequency Compensation C2 X7R capacitor , ±10% Capacitor Load C3 Output PNP Transistor βT1 0:41 100 330 nF 4.7 4,7 nF X7R capacitor , ±10% 1.0 10.0 nF e.g. BCW68H or BC557C, low drop, high β for Tamb = -40….125°C 180 Table 2: Electrical boundary conditions Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 5/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage DESCRIPTION OF FUNCTIONS AM417 is a ratiometric, adjustable interface IC which has been specially developed for the conditioning of bridge signals for automotive applications. 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 amplifier (IA) the input signal is preamplified by GIA = 10. The IA can only process positive input signals. A negative input voltage or negative input offset must be balanced by using additional resistor at positive input pin VIN+ (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: I IB = VVCC 10 R1 (1) Output stage A voltage amplifier with an external 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. GOUT = R3 + R4 R4 (2) The gain of the entire circuit AM417 is thus: GSYS = GIA GOUT. A current limitation has been integrated into the output stage. The limit circuit restricts output voltage VOUTmin with reference to VCC, where VBE is the basic emitter voltage of external transistor T1. VVOUT min = VVCC − 1.5 VBE (T1 ) (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: I OUT max = VTHRESH − VBE (T1 ) 380mV ≈ R2 R2 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz (4) Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 6/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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 9R IIB RB1 RB3 OP R 2 RB2 RB4 Ro 4 VIN + 5 V IN - 8 Outputstage IA 3 R1 1 AM417 7 6 VS C1 VOUTME + = positive bridge output signal R2 T1 C2 R3 VOUTME - = negative bridge output signal VOUT C3 R4 VOUTME + - VOUTME - = VOUTME VOUTME + = VIN+ VOUTME - = VINVIN+ - VIN_ = VIN Ground Figure 2: Measuring a constant-current sensing element using a Wheat- Setting the output span The output signal span can be set using gain GOUT of the output stage (see Equation 2): GOUT = VSPAN VOUTME ⋅ GIA (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 sensing element and the IC. To this end, a compensating Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 7/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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: RB 4 RB 2 RB 3 / 1 − VRO = ∆VIN − VBR − RB 3 + RB 4 RB1 + RB 2 RB 3 + RB 4 (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. ∆VIN = VOUT min V = OUT min G SYS G IA ⋅ GOUT (8) Taking the offset of the sensing element (VOSME) and that of the IC (VOSIC) into account (VOSIC = VOSIA + 0.1VOSOUT, where VOSIA is the instrumentation amplifier offset and VOSOUT the output stage offset), the adjustable voltage is calculated as: ∆VIN’ = ∆VIN - VOSIC - VOSME From (9) and (8) it follows that: ∆VIN ´= (9) VOUT min − VOSIC − VOSME G IA ⋅ GOUT (10) Applying (7) and (10), the necessary voltage drop across RO required to calibrate the offset of the output voltage VOUTmin is expressed thus: V V RO = 2 ⋅ OUT min − VOSIC − VOSME G IA ⋅ GOUT (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: RO = 2 ⋅ V RO I IB (12) Applying (11) and the condition, that the voltage drop across Ro may only be positive, the maximum compensatable offset is computed thus: VOSIC + VOSME ≤ Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz VOUT min GIA ⋅ GOUT (13) Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 8/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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): 9R IIB RB1 4 RB3 OP R 2 RB2 RB4 RO´ 5 8 VS C1 R2 VIN + Outputstage IA VIN - 3 R1 1 AM417 7 6 T1 C2 R3 VOUT C3 R4 Ground Figure 3: Circuit as in Figure 2 with R0’ (instead R0) at input pin 5 (IN-) Doing so changes the effective direction of RO and its resistance is now expressed as: 2 ⋅ (−V RO ) RO ' = (12a) I IB Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 9/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage EXAMPLES By way of example the equations shall be calculated 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 VOUT = 0.5...4.5V, => VSPAN = 4V, VOUTmin = 0.5V VOSIC = -2mV VOSME= +10mV at VBR = 5V RBR = 3KΩ The sensing element is to be supplied with constant 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: VIB = RBR ⋅ I BR + VVR = 3kΩ ⋅ 1mA + 0.5V = 3.5V . 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: V BR ' = I BR ⋅ RBR = 1mA ⋅ 3kΩ = 3V . The output voltage of the sensing element given for VBR = 5V must be corrected by the ratio of the bridge voltages: VOUTME ' = 160mV ⋅ 3V = 96mV 5V The offset voltage of the sensing element given for VBR = 5V must be corrected by the ratio of the bridge voltages: VOSME ' = 10mV ⋅ 3V = 6mV 5V Applying Equation 5 the following is accrued: GOUT = 4V = 4.166 96mV ⋅ 10 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 10/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage and from Equation 11 we are presented with: 0.5V + 2mV − 6mV = 16mV VRO = 2 ⋅ 10 ⋅ 4.166 Referring to Equation 12 the resistance for offset calibration is thus: R0 = 2 ⋅ VR 0 = 32Ω I IB If RO is set to 32Ω 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 VOUT = 0.5...4.5V, => VSPAN = 4V, VOUTmin = 0.5V 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: V BR ' = I BR ⋅ R BR = 1mA ⋅ 3kΩ = 3V . The output voltage of the sensing element is corrected by the ratio of the bridge voltages: VOUTME ' = 100mV ⋅ 3V = 60mV 5V The offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: VOSME ' = − 10mV ⋅ 3V = −6mV 5V Applying Equation 5 the following is accrued: GOUT = 4V = 6.67 60mV ⋅ 10 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 11/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage and from Equation 11 we are presented with: 0.5V − 2mV + 6mV = 23mV VRO = 2 ⋅ 10 ⋅ 6.67 Referring to Equation 12 the resistance for offset calibration is thus: R0 = 2 ⋅ VR 0 = 46Ω I IB If RO is set to 46Ω 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 VOUT = 0.5...4.5V, => VSPAN = 4V, VOUTmin = 0.5V 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: V BR ' = I BR ⋅ R BR = 1mA ⋅ 3kΩ = 3V . The output voltage of the sensing element is corrected by the ratio of the bridge voltages: VOUTME ' = 100mV ⋅ 3V = 60mV 5V The offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: VOSME ' = 10mV ⋅ 3V = 6mV 5V Applying Equation 5 the following is accrued: GOUT = 4V = 6.67 60mV ⋅ 10 Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 12/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage and from Equation 11 we are presented with: 0.5V − 2mV − 6mV = −6.5mV VRO = 2 ⋅ 10 ⋅ 6.67 Referring to Equation 12a the resistance for offset calibration is thus: R0 ' = 2 ⋅ (− V R 0 ) ⋅ = 13Ω I IB If RO’ (resistor on the left) is set to 13Ω 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 affair. 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 IIB´ RBR 2 4 RTSC 5 Input pin AM417 3 R1 Figure 4: Bridge array for the compensation of TC with RBR = bridge resistor Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 13/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage The output signal of a piezoresistive sensing element is accrued from: VOUTME = S ⋅ P ⋅ VBR = S ⋅ P ⋅ I IB ⋅ RBR (14) 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-dependent variables in Equation 14. The following applies: S = S 0 ⋅ (1 + TCS ⋅ (T − T0 )) (15) R BR = RBR 0 ⋅ (1 + TCR ⋅ (T − To )) (16) 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 sensing element output signal described above the following applies to the compensatory TCS resistor: RTCS = RBR ⋅ − TCS TCR + TCS (17) 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: RTCS I IB ' = (18) I IB (RTCS + R BR ) In order to reinstate the original output signal of the circuitry the circuit gain must be increased by the reciprocal ratio: (R + RBR ) I (19) TCSFactor = IB = TCS I IB ' RTCS Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 14/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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 maximum 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 following. Assuming that the maximum operating temperature of the circuit is 80°C, the maximum bridge resistance is calculated using Equation (16): R RB max = 3kΩ ⋅ (1 + 0,0028 / °C ⋅ (80°C − 25°C )) = 3,46kΩ With a bridge current of IIB = 0.8mA, at 80°C and VCC = 5V, pin 2 (IB) has a potential of: V IB = 3.46kΩ ⋅ 0.8mA + 0.5V = 3.27V Applying Equation (17): RTCS = 6.33KΩ Using Equation (19) the following is calculated for T0: TCSFactor = 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). Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 15/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage BLOCK DIAGRAM AND PINOUT 9R I IB 2 IN+ 4 OP IA IN- 5 RB 3 VC C 8 VCC 7 VOUT 6 VR R Outputstage 1 AM417 GND IB RB IN+ 1 2 3 4 AM 417 IB 8 7 6 5 VCC VOUT VR IN- GND Figure 5: Circuit diagram of AM417 PIN 1 2 3 4 5 6 7 8 NAME GND IB RB IN+ IN– VR VOUT VCC Figure 6: AM417 Pin out FUNCTION IC Ground Current Source Output Current Source Set Positive IA Input Negative IA Input Gain Set Voltage Output Supply Voltage Table 3: Pin out DELIVERY AM417 is available as: • An SOP08 • Dice on 5“ blue foil Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 16/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage EXAMPLE APPLICATIONS • Interface IC for sensing elements in a resistor bridge circuit (e.g. piezoresistive pressure sensing elements) with electronic compensation of errors via an external microcontroller. In this application AM417 is used as a preamplifier to set the operating point. VC C = 5V + 5% 0,5-1,25mA AM417 VO U T = 0,2V...V CC -0,2V µP Figure 7: Application for sensing elements with an external microcontroller or ADC • Signal conditioning IC with an external, analog compensation network, in which the offset can be adjusted using additional resistors on the sensing element and the gain using AM417. VC C = 5V + 5% 0,5 - 1,25mA AM417 VO U T = 0,5...4,5V / 11mA Figure 8: Application as a signal conditioning IC with an external compensation network How to protect the output of the AM467 against reverse polarity see [3] Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 17/18 AM417 – Ratiometric instrumentation amplifier with adjustable output stage 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: http://www.analogmicro.de/products/info/english/analogmicro.de.an1019.pdf Analog Microelectronics reserves the right to make amendments to any dimensions, technical data or other information herein without further notice. Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Phone:+49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: [email protected] July 2008 –Rev 3.1- Page 18/18