precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 1/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de principle function adjustable amplificat ion of differential in put voltage signals (from 0 to 5 mv fs up to 0 to 100 mv fs) to an adjustable, ratiometric out put voltage of 0.2v to vcc-0.2 v typical applications ? amplifiers for ceramic sensing elements ? amplifiers for dms sensing elements ? amplifiers for gmr sensing elements ? for industrial and automotive applications AM467 v = 5v 5 % cc differential input voltages ( 5... 100mv fs) v0,2v-0,2v (adjustable) e.g.0,5...4,5v ratiometric out cc = ... _ + _ + _ +
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 2/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de table of contents principle function 1 typical applications 1 features 3 general description 3 block diagram 3 electrical specifications 4 description of functions 5 example application 5 the procedure in detail (for calibration with discrete resistors) 6 dimensioning 7 notes 8 block diagram and pinout 8 further reading 9
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 3/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de features ? wide differential input voltage range ( 5 mv fs... 100 mv fs) ? low offset ? low offset drift ? low input noise ? high cmrr: > 120 db ? wide operating temperature range: ? 40 c... +125 c ? adjustable output voltage ? rail-to-rail output stage: v out = 0.2 v ... v cc ? 0.2 v ? sink/source output ? single ratiometric supply: v cc = 5 v ? integrated emv protective functions ? overvoltage protection 12 v ? integrated esd protective functions ? 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 100 mv fs). the chief component of the ic is a highly accurate, low-noise amplifier circuit which can be adjusted to the sensing elements using an external network of resistors. with the exception of the additional operational amplifier and the diag nostic unit ic AM467's pins and functions are compatible with the am457. the rail-to-rail output makes the ic suitable for the amplification of dms and piezoceramic sensing elements and gm r measuring cells with respect to the specific calibration network. block diagram figure 1: block diagram for AM467 AM467 input- amplifier biasing unit output stage short circuit protection current limitation vcc out gnd 1 4 8 in+ 3 in- 2
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 4/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de electrical specifications (with reference to the example application) t amb = 25c, v cc = 5v (unless otherwise stated) parameter symbol conditions min. typ. max. unit voltage range v cc 4.5 5 5.5 v quiescent current i cc t amb = ? 40 ... +125c 170 320 530 a temperature specifications operating t amb ?45 125 c storage t st ?55 150 c junction t j 150 c amplifier amp offset voltage v os 0.1 0.3 mv v os vs. temperature d v os /d t t amb = -45 ? 105c 0.5 3 v/c v os vs. temperature d v os /d t t amb = 105 ? 125c 6 v/c input bias current i b 30 200 na i b vs. temperature d i b /d t v cm = 2.5v t amb = -45?125c ?0.13 ?0.6 na/c differential input voltage v in v in = vout + ? vout- see figure 2 5 100 mv input offset current i os 0.5 5 na i os vs. temperature d i os /d t v cm = 2.5v t amb = -45 ? 125c 2.5 30 pa/c input resistance r in v cm / i b,typ ( v cm = 2.5v) 80 m ? input capacitance c in by design 90 pf common mode input range cmir 1 3.7 v common mode rejection ratio cmrr 100 120 db open loop gain g 0 c1 = 1nf; iout = 1a 120 140 db adjustable gain g 10 output voltage range v out 0.2 vcc ? 0.2 v guaranteed max output current i out sink and source 250 a output load resistance r l source and sink , / out out i v = 20 k ? power supply rejection ratio psrr 90 110 db gain bandwidth product gbw r1,2=1k rk=10k, c1 =1nf, ck =100pf, no rl 190 310 khz non linearity nl = g/go, g 100 10 -4 slew rate sr c 1 = 1nf; r l = 20k ? 0.2 0.3 v/s input voltage noise e n rs = 100 ? ; v cc = 5v ; fg=1khz with rs = source impedance 13.5 20 nv/ hz overvoltage protection 12 v table 1: specifications currents flowing into the ic are negative. v cm = input common mode voltage
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 5/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de external components parameter symbol conditions min. typ. max. unit output capacitor c 1 1 5 nf compensation capacitor c 2 100 pf stabilization capacitor (optional) c 3 100 nf emv protection capacitor (optional) c 4 470 pf load resistor to gnd, to v cc r l vout = max. vout 20 k ? table 2 : external components of the calibration network recommended: ceramic capacitors description of functions am457 is an integrated precisi on circuit for the signa l amplification of high-impedance signal sources and for the signal conditioning of sensor bridges with low sensitivity, particularly for ceramic and dms sensing elemen ts or gmr measuring cells. the ic generates a rail-t o-rail output signal of 0.2 v to vcc-0.2 v. the amplification and offset can be set independently of one anot her using external resi stors. through the suit able dimensioning of external resistors in pa rticular an output voltage of 0.5...4.5 v can also be pr ovided. as these values are standard the following descrip tion refers to this output voltage. AM467 works on the principle of ratiometry with a supply voltage of 5 v 5%. the ic is distinguished by its low offset and extremely low thermal offset drift across a wide temperature range, enabling it to be classifi ed as a precision amplifier. the sink and source output stage is protec ted internally against short-circuiting. the AM467 design incorporates protective measur es against interference from emv and esd using suitable semiconductor structures. with the exception of the additional operational amplifier and the dia gnostic unit the ic is compatible with am457. AM467 thus has a smaller chip surface and a lower power consumption. AM467 can be used as an autonomous signal conditioning ic or as a preamplifier for an a/d converter for back-end digitization. example application am457 is suitable for the amplif ication of resistance networks connected up as a wheatstone bridge with four resistors and which have a low sensitivity, such as 1.0 to 3.5 mv/v with a 5 v supply, for example. by way of example a sensor application is described herein which is
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 6/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de based on a piezoceramic sensing element such as th e above and is to be calibrated to an output signal of 0.5...4.5 v (sensor system). the basic calibration procedure the sensor system is calibrated in two stages. using a mathematical algorithm (excel sheet: cali_AM467_rev1.xls) and taking AM467's measured output voltage values and the individual sensor bridge values as a basis, the values for the two resistors (calibration resistors) are calculated. in the calibration of the system a ll effective errors (parasitic eff ects and component tolerances) are taken into account. the sensing element is first measured and secondly th e electrical characteris tics of the sensor using predefined precision measuring resistors. this inform ation is then processed in an excel program to calculate the setpoint for two of the calibration resistors which are then swapped with the given measuring resistors and added to the circuit accordingly. if necessary, in a second stage the offset of the output voltage can be corrected using the information given in the excel sheet. the procedure in detail (for calibr ation with discrete resistors) in order to evaluate the sensor system signal four resistors (r 1 to r 4 ) are required (see figure 2 ). the values of the two resistors r 2 and r 4 are fixed for the network and do not have to be adjusted during calibration. like capacitors c 1 to c 4 they can be mounted on the ci rcuit board at the outset. resistors r 1 and r 3 act as measuring resistors and are asse mbled on the measuring apparatus for all sensor systems in one sens ing element category (see dimensioning ). their values must be as close as possible to those calculated by analog micr oelectronics for the relevant sensing element category. with the described components (r 1 to r 4 and c 1 to c 4 ) the sensor signal is at its operating point which then enables calibration. for this purpose a few of the electrical character istics of the sensing element are first determined without the evaluation circ uitry being connected (stage 1). usi ng the excel calibration software (kali_AM467.xls) the necessa ry calibration parameters are: th e sensing element resistance (rbr), average output voltage of the sensing element (vbr) and the system's supply voltage (vcc). once these three values have been determined the sensing element is connected up to the evaluation circuit. the output signal at the AM467 ic (out1 and out2) is meas ured at zero and full pressure (offset and full scale signal) and entered into the ex cel program with the bri dge values including the values for the given maximum pressure of the sensing element (pmax). taking these values and applying them to a calibration algorithm the two resistors r 1 and r 3 are calculated separately for each individual system. these then replace the measuring resistors and have to be soldered onto the circuit board.
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 7/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de resistors r 1 and r 3 are described as calibration resistors. these stipulate the final operating point. once these resistors have been mounted the calib ration procedure is complete. depending on the accuracy of the resistors used an offset value of 0.5 v and a full scale signal of 4.5 v should have been obtained. should the expected final accuracy re quire it, in an additional second stage (stage 2) the offset of the output voltage can be co rrected using resistor r 1 . to this end the AM467 output must again be measured at p = 0 bar. if at 0 bar the output is too high or too low by a few millivolts, for example, the excel program then calculates the necessary second correction of r 1 in ohms. the offset error which is then obtained if a diffe rent resistance is used in place of calculated resistance r 1 can also be determined by the excel sheet. the sensitivity in mv/ohm is instrumental here, providing information as to by how many millivo lts the offset shifts when the used resistor r 1 has a delta r (in ohms) to the calculated value. the given sensitivity value (mv/ohm) is multiplied by the delta r of resistor r 1 to obtain a change in offset which affects both the offset (0.5 v) and the full scale signal (4.5 v). figure 3: ceramic sensing element with AM467 and a calibration network with external resistors . dimensioning in the combination of ceramic sensing element a nd AM467 the operating point (the initial value of the four resistors r1 to r4) was determined for all sensing elements in a specific sensing element category. in the case of piezoceramic sensors the te rm "sensing element category" is used to denote sensing elements with the following characteristics: in+ in- AM467 3 2 amp vcc 8 out 1 gnd 4 vo u t gnd c1 c2 r2 r1 r4 r3 output stage with short circuit proofing c4 c3
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 8/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de bridge resistance: 11 kohm 20% sensitivity: 2.4 1 mv/v offset: 0 to 0.3 mv/v supply voltage: 5 v 10% the following values are obtained for th e above sensing element categories: fixed resistor r 2 = 33 k; accuracy of 1% fixed resistor r 4 = 12 k; accuracy of 1% measuring resistor r 1 = 11 k; accuracy of 0.1% measuring resistor r 3 = 120 k; accuracy of 0.1% c 1 = 100 nf (optional) c 2 = 100 pf (ceramic) c3 = 1 nf to 5 nf (ceramic) c4 = 470 pf optional (ceramic) notes for bridge circuits with characteristics other than the above (with other se nsing element categories, such as dms sensing elements, for example) analog microelectronics can adapt the dimensioning program on request. an excel program (cali_AM467_rev1.xls) is used fo r calibration (the calcula tion of the definitive operating point). this is av ailable on the www.analogmicro.de website. diagram and pinout figure 3: diagram of AM467 1 8 2 7 36 4 5 out in- in+ gnd vcc n.c. n.c. n.c. figure 4: AM467 pinout AM467 amp + - gnd 4 1 out 8 vcc in+ 3 in- 2 output stage with short circuit proofing
precision amplifier for bridge circuits AM467 analog microelectronics february 2007 analog microelectronics gmbh phone: +49 (0)6131/91 073 ? 0 9/9 an der fahrt 13, d ? 55124 mainz fax: +49 (0)6131/91 073 ? 30 rev. 1.0 internet: http://www.analogmicro.de email: info@analogmicro . de pin name explanation 1 out output 2 in- negative input 3 in+ positive input 4 gnd ic ground 5 n.c. no function 6 n.c. no function 7 n.c. no function 8 vcc 5v supply voltage table 3: pin configuration for AM467 for package dimensions see: http://www.anal ogmicro.de/products/analogmicro.de.en.package.pdf further reading as AM467 is identical in its speci fications to am457 ? with the exception of the diagnosis ? the calibration guidelines given for the latt er also apply to AM467, available at www.analogmicro.de for various different networks. an1009 application description an1011 calibration for dms sensing elements an1012 calibration for piezoceramic sensing elements delivery options AM467 is available as: ? an sop 8 ? dice in a 4" wafer on 5" blue foil (on request) notes analog microelectronics reserves the right to make amendments to dimensions, technical data and any other information without prior notice.
|
