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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 ou tput 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 i b r = 1ma AM417 v cc = 5v 5 % differential input voltage 0...200mv v out = 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 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 piezo resistive 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 supply: 0.5...4.5v ? low offset ? low offset drift ? supply voltage range: 5v 5% (ratiometric range) ? wide operating temperature range: -40c...+125c ? ratiometric current source for the supply of external measuring cells ? output driver (pnp open collector): i out = +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 1 7 AM417 8 output- stage 6 2 3 op ia i ib 4 5 in+ ib in- rb vcc vo ut vr r 9 r gnd f i g ure 1: block dia g ram 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 t amb = 25c, v cc = 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 v cc ratiometric range 4.75 5 5.25 v maximum supply voltage v cc max 6 v quiescent current i cc v cc = 5v, r 1 = 500 ? , i ib = 1ma 7.6 ma temperature specifications operating temperature t amb -40 125 c storage temperature t st -55 125 c junction temperature t j 150 c parameter symbol conditions min. typ. max. unit op (ratiometric current source) input voltage v rb ratiometric with v cc = 5v 0.5 v input current i rb 100 na output current range i ib 0.50 1.25 ma output current accuracy i ib ratiometric with v cc = 5v, r 1 = 500 ? ? 0.98 1 1.02 ma ratiometric error rat @ ib rat @ ib = 1.05 v rb ( v cc = 5v) ? v rb ( v cc = 5.25v) -1 1 mv i ib vs. temperature d i ib /dt i ib = 1ma -45 -25 -5 ppm/c i rb vs. temperature d i rb /dt i ib = 1ma -20 + 20 ppm/c output voltage range v ib i ib = 1.25ma 2.0 v cc ?0.2v v output resistance r ib r ib = v ib / i ib , v ib = 2v, ? v ib = 2.8v, i ib = 1ma, 1.5 30 m ? instrumentation amplifier common mode input voltage range cmir 1.3 v cc ?2.2v v differential input voltage range ? v in 0 200 mv internal gain g ia 9.8 10.0 10.2 input bias current i in +;? 25 75 na input offset voltage v oia -3 3 mv v os vs. temperature dv oia /dt t amb = -40?100c -10 10 v/c v os vs. temperature dv oia /dt t amb = 100?125c -30 30 v/c output voltage range v via 0.05 v cc ?2v v nonlinearity nl ia v in ? = 1.3v, ? v in = 100mv, 200mv 0.15 % fs common mode rejection ratio cmrr v in ? = 1.3v, ? v in = 100mv 80 90 db power supply rejection ratio psrr v in ? = 1.3v, ? v in = 100mv 74 80 db input voltage noise e n 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 g out 2 11 input voltage range v vr 0.05 v cc ? 2.25v v input current i in v in? = 2v, ? v in = 50mv 20 75 na input offset voltage v os -3 3 mv v os vs. temperature d v os /dt v in? = 2v, ? v in = 50mv, t amb = -40?100c -15 15 v/c v os vs. temperature d v os /dt v in? = 2v, ? v in = 50mv, t amb = 100?125c -100 0 v/c output current i vout pin vout 65 150 350 a output voltage range v out with external transistor* 0.5 4.5 v output current i out with external transistor* 11 ma output resistance r out with external transistor* 0.1 0.85 ? power supply rejection ratio psrr -72 -90 db current limitation threshold v thresh v thresh = v vcc ? v vout min r 2 = 27 ? , i out 14ma 1.00 1.15 v v tresh vs. temperature d v thresh /dt -40...+125c without external tr ansistor* -4.2 -1.8 mv/c system parameters input voltage range ? v in ? v in @ v outmax = 4.5v and g out = 10 @ v outmax = 4.5v and g out = 2 0 0 40 200 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 r 2 (see equation 4). boundary conditions / external components parameter symbol conditions min. typ. max. unit resistor adjustment current source r 1 400 1000 ? resistor sense current limitation r 2 0 50 ? gain resistor sum r 3 + r 4 v out = ( r 3 + r 4 )/ r 4 g ia 0:41 2.1 k ? capacitor power supply c 1 100 330 nf capacitor frequency compensation c 2 x7r capacitor , 10% 4.7 4,7 nf capacitor load c 3 x7r capacitor , 10% 1.0 10.0 nf output pnp transistor t 1 e.g. bcw68h or bc557c, low drop, high for t amb = -40?.125c 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 a nd 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 g ia = 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 v cc 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 i ib can be set by varying resistor r 1 at the minus input of the op ( v in - ) using the following ratio: 1 10 r v i vcc ib = (1) output stage a voltage amplifier with an exte rnal pnp open collector stage ( t 1 ) acts as a voltage output and can provide a maximum current of i out = 11ma. using external resistors r 3 and r 4 the gain g out can be adjusted between 2.and 11. 4 4 3 r r r g out + = (2) the gain of the entire circuit AM417 is thus: g sys = g ia g out . a current limitation has been integrated into th e output stage. the limit circuit restricts output voltage v outmin with reference to v cc , where v be is the basic emitter voltage of external transistor t 1 . ( ) 1 min 5 . 1 t v v v be vcc vout ? = (3) with this the maximum output current can be adjusted using resistor r 2 in series with the t 1 transistor emitter (see figure 2 ). the current is thus calculated as: () 2 2 1 max mv 380 r r t v v i be thresh 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 v thrfsh is current limitation threshold. should no current limit be necessary, the t 1 transistor emitter can be directly connected up to pin vcc ( r 2 = 0). good thermal coupling between t 1 and the ic reduces the temperature drift of output current i out , 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 g out of the output stage (see equation 2): ia outme span out g v v g ? = (5) where v span = v outmax ? v out min and v outme 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 v outmin 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 1 7 AM417 c 1 v out 8 v s t 1 r 2 c 2 r 3 r 4 6 c 3 ground r 9 r 2 3 op ia i ib output- stage r 1 4 5 r b1 r b2 r b3 r b4 r o v in- v in+ figure 2: measuring a constant - current sensing element using a whea t - v outme - = negative bridge output signal v outme + = positive bridge output signal v outme + - v outme - = v outme v outme + = v in+ v outme - = v in- v in+ - v in_ = v 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 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 r o is inserted into the measuring bridge (see figure 2 ). by using this compensating resistor the instrumentation amplifier input voltage ? v in is set in such a way, that output voltage v outmin has a value of 0.5v, for example. the voltage drop v ro across resistor r o is given by: ? ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + ? + ? ? = 4 3 3 2 1 2 4 3 4 1 / b b b b b b b b b br in ro r r r r r r r r r v v v (6) where v br is the voltage drop across the entire sensing element, r br the total bridge resistance and r b1,2,3,4 the individual bridge resistors. assuming that the four separate bridge resistors have the same value, the following approximation formula is valid: v ro = 2 ? v in (7) ? v in is the voltage to be set at the input of the instrumentation amplifier where there are no offsets. out ia out sys out in g g v g v v ? = = ? min min (8) taking the offset of the sensing element ( v osme ) and that of the ic ( v osic ) into account ( v osic = v osia + 0.1 v osout , where v osia is the instrumentation amplifier offset and v osout the output stage offset), the adjustable voltage is calculated as: ? v in ? = ? v in - v osic - v osme (9) from (9) and (8) it follows that: osme osic out ia out in v v g g v v ? ? ? = ? min (10) applying (7) and (10), the necessary voltage drop across r o required to calibrate the offset of the output voltage v outmin is expressed thus: ? ? ? ? ? ? ? ? ? ? ? ? = osme osic out ia out ro v v g g v v min 2 (11) on condition, the sensing element offset is low referenced to the sensing element output voltage ( v osme < 10 v outme ), the resistor r o is calculated with sufficient accuracy as: ib ro o i v r ? = 2 (12) applying (11) and the condition, that the voltage drop across ro may only be positive, the maximum compensatable offset is computed thus: out ia out osme osic g g v v v ? + min (13)
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 v ro (equation 11) a negative value is produced, the resistor r 0 must be placed in the left arm of the bridge ( r o ? ; see figure 3 ): doing so changes the effective direction of r o and its resistance is now expressed as: ib ro o i v r ) ( 2 ' ? ? = (12a) 1 7 AM417 c 1 v out 8 v s t 1 r 2 c 2 r 3 r 4 6 c 3 ground r 9 r 2 3 op ia i ib output- stage r 1 4 5 r b1 r b2 r b3 r b4 v in- v in+ r o figure 3: circuit as in figure 2 with r 0 ? (instead r 0 ) 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 v outmin = 0.5v and v outmax = 4.5v example 1: piezoresistive pressure sensing elemen t in a bridge circuit with a positive offset ? v outme = 160mv at v br = 5v ? v cc = 5v ? v out = 0.5...4.5v, => v span = 4v, v outmin = 0.5v ? v osic = - 2mv ? v osme = +10mv at v br = 5v ? r br = 3k ? 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 i ib = 1ma ( r 1 = 500 ? ). at pin 2 ( ib ) the voltage is: v v ma k v i r v vr br br ib 5 . 3 5 . 0 1 3 = + ? ? = + ? = . considering a typical positive temperature coefficient of the sensing element bridge resistor r br of tcr = +0.0028/c the maximum voltage at pin 2 ( ib) is not overshot ( v ibmax = 4.8v at v cc = 5v). the bridge voltage is: v k ma r i v br br br 3 3 1 ' = ? ? = ? = . the output voltage of the sensing element given for v br = 5v must be corrected by the ratio of the bridge voltages: mv v v mv v outme 96 5 3 160 ' = ? = the offset voltage of the sensing element given for v br = 5v must be corrected by the ratio of the bridge voltages: mv v v mv v osme 6 5 3 10 ' = ? = applying equation 5 the following is accrued: 166 . 4 10 96 4 = ? = mv v g 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 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: mv mv mv v v ro 16 6 2 166 . 4 10 5 . 0 2 = ? ? ? ? ? ? ? + ? ? = referring to equation 12 the resistance for offset calibration is thus: ? = ? = 32 2 0 0 ib r i v r if r o is set to 32 ? and if we take the offsets of sensing element and ic into consideration, the output signal offset of the overall circuit is set to v outmin = 0.5v and the maximum output signal is v outmax = 4.5v. example 2: piezoresistive pressure sensing element in a bridge circuit with a negative offset ? v outme = 100mv at v br = 5v ? v cc = 5v ? v out = 0.5...4.5v, => v span = 4v, v outmin = 0.5v ? v osic = 2mv ? v osme = -10mv at v br = 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 i ib = 1ma ( r 1 = 500 ? ). the bridge voltage is: v k ma r i v br br br 3 3 1 ' = ? ? = ? = . the output voltage of the sensing element is corrected by the ratio of the bridge voltages: mv v v mv v outme 60 5 3 100 ' = ? = the offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: mv v v mv v osme 6 5 3 10 ' ? = ? ? = applying equation 5 the following is accrued: 67 . 6 10 60 4 = ? = mv v g 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 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: mv mv mv v v ro 23 6 2 67 . 6 10 5 . 0 2 = ? ? ? ? ? ? + ? ? ? = referring to equation 12 the resistance for offset calibration is thus: ? = ? = 46 2 0 0 ib r i v r if r o is set to 46 ? and if we take the offsets of sensing element and ic into consideration, the output signal offset of the overall circuit is set to v outmin = 0.5v and the maximum output signal is v outmax = 4.5v. example 3: piezoresistive pressure sensing elemen t in a bridge circuit with a high positive offset ? v outme = 100mv at v br = 5v ? v cc = 5v ? v out = 0.5...4.5v, => v span = 4v, v outmin = 0.5v ? v osic = 2mv ? v osme = 10mv at v br = 5v ? r br = 3k ? the sensing element is supplied with constant current. taking the maximum output voltage of the op into account ib is again selected as i ib = 1ma ( r 1 = 500 ? ). the bridge voltage is: v k ma r i v br br br 3 3 1 ' = ? ? = ? = . the output voltage of the sensing element is corrected by the ratio of the bridge voltages: mv v v mv v outme 60 5 3 100 ' = ? = the offset voltage of the sensing element is also corrected by the ratio of the bridge voltages: mv v v mv v osme 6 5 3 10 ' = ? = applying equation 5 the following is accrued: 67 . 6 10 60 4 = ? = mv v g 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 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: mv mv mv v v ro 5 . 6 6 2 67 . 6 10 5 . 0 2 ? = ? ? ? ? ? ? ? ? ? ? = referring to equation 12a the resistance for offset calibration is thus: () ? = ? ? ? = 13 2 ' 0 0 ib r i v r if r o ? (resistor on the left) is set to 13 ? and if we take the offsets of sensing element and ic into consideration, the output signal offset of the overall circuit is set to v outmin = 0.5v and the maximum output signal is v outmax = 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 r br . 2 i ib r 1 4 5 r tsc 3 input pin AM417 i ib r br figure 4 : bridge array for the compensation of tc with r br = 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: br ib br outme r i p s v p s v ? ? ? = ? ? = (14) s is the sensor sensitivity of the sensing element and p is the applied pressure. sensor sensitivity s and bridge resistor r br are the dominant temperature-depende nt variables in equation 14. the following applies: ( ) () 0 0 1 t t tcs s s ? ? + ? = (15) ( ) () o br br t t tcr r r ? ? + ? = 1 0 (16) s 0 is the basic value of the sensitivity and r bro the basic value of the bridge resistance at t 0 (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 v outme 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 r tcs 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: tcs tcr tcs r r br tcs + ? ? = (17) as part of the set bridge supply current i ib flows through the shunt resistor r tcs the circuit output signal is reduced after tcs compensation according to the following equation: () br tcs tcs ib ib r r r i i + = ' (18) in order to reinstate the original output signal of the circuitry the circuit gain must be increased by the reciprocal ratio: ( ) tcs br tcs ib ib r r r i i tcsfactor + = = ' (19)
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 i ib by the tcs factor . gain g out can also be increased by the same factor if, for example, maximum bridge current i ibmax = 1.25ma or if the maxi mum voltage at pin 2 ( ib ) is overshot during an increase. example example 4: tcs compensation of a piez oresistive pressure sensing element [2] ? tcs = -0.0019/c ? v cc = 5v ? r br = 3k ? ? tcr = +0.0028/c ? temperature range: -20c ? 80c bridge supply current i ib is selected according to the fo llowing. assuming that the maximum operating temperature of the circuit is 80c, th e maximum bridge resistance is calculated using equation (16): ( ) () ? = ? ? + ? ? = k c c c k r rb 46 , 3 25 80 / 0028 , 0 1 3 max with a bridge current of i ib = 0.8ma, at 80c and v cc = 5v, pin 2 ( ib) has a potential of: v v ma k v ib 27 . 3 5 . 0 8 . 0 46 . 3 = + ? ? = applying equation (17): r tcs = 6.33k ? using equation (19) the following is calculated for t 0 : tcs factor = 1.47 if bridge current i ib is now increased by a factor of tcsfactor, the result is a new amended bridge current of: i ibnew = 1.18ma the original output signal of the sensing element is thus reinstated following tcs compensation. output stage gain g out could also be increased by a factor of tcsfactor by adjusting resistors r 3 and r 4 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 1 7 AM417 8 output- stage 6 2 3 op ia i ib 4 5 in+ ib in- rb vcc vout vr r 9 r v cc gnd figure 5: circuit diagram of AM417 1 8 2 7 36 4 5 gnd ib rb in+ vcc vout vr in- a m 4 1 7 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 17/18 phone:+49 (0)6131/91 073-0 fax: +49 (0)6131/91 073-30 internet: http://www.analogmicro.de email: info@analogmicro . de example applications ? interface ic for sensing elements in a resistor br idge 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. ? signal conditioning ic with an external, analog co mpensation network, in which the offset can be adjusted using additional resistors on the sensing element and the gain using AM417. how to protect the output of the am467 against reverse polarity see [3] p v out = 0,2v...v -0,2v cc 0,5-1,25ma v cc = 5v 5% + AM417 figure 7: application for sensing elements with an external microcontroller or adc AM417 v out = 0,5...4,5v 11ma v cc = 5v 5 % + 0,5 - 1,25ma / figure 8: application as a signal conditioning ic with an external compensation network
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: 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 wit hout further notice.

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