4 20mA Interface-IC AM462 for industrial µ-processor applications

Transcription

1 Because of he grea number of indusrial buses now available he majoriy of indusrial measuremen echnology applicaions sill calls for he sandard analog curren nework. The reason for his lies in he fac ha wih an analog curren oupu of 0/4...20mA of a signal condiioning sysem a simple and relaively cheap inerfaces can be creaed which is immune o inerference and which allows a long disance communicaion.. Assuming ha he above scenario is applicable o ypical indusrial seups (Figure 1), where a measuremen signal is processed in a microprocessor (e.g. signal correcion), he digiized signal mus hen be adaped o sui indusrial analog requiremens. The following aricle presens a new indusrial curren inerface IC AM462 which has been specifically designed as an analog microprocessor inerface for he indusrial nework which can also be operaed as a sand-alone IC for indusrial signal amplificaion (/I Converer- IC). Proecion agains shor-circuiing and reverse polariy CREF = 3.3 REF = Measuremen signal µp D A AM462 0/4...20mA Figure 1: Typical indusrial applicaion of AM462 Simple analog circuis For indusrial applicaions which require curren oupus of 0/4 20mA, roughly speaking a muliple operaional amplifier (OP) can be used as an oupu circui and a discree circui assembled. A soluion such as his, however, is only very basic. Depending on he applicaion, wih regard o he overall sysem a sable oupu curren (which is as independen of he emperaure and supply volage as possible) and also oher funcions mus be realized which bring a direc influence o bear on he oupu sage. These funcions include proecive circuiry, volage adjusmen faciliies, reference sources e ceera, all of which go far beyond he limis of a simple OP circui and in order o mee he given requiremens call for a no inconsiderable knowledge of analog circui echnology. Alhough a number of inegraed componens permiing assembly wih he funcions menioned above are now available on he marke, here are several major disadvanages of working wih a more or less discree soluion: Analog Microelecronics GmbH Phone: +49 (0)6131/ /6 An der Fahr 13, D Mainz Fax: +49 (0)6131/ Inerne: hp:// info@analogmicro.de

2 Miniaurizaion: The more complex he applicaion, he greaer he space required which may have o be compensaed for using expensive miniaurized componens. Economic pressures: The discree componens currenly available ofen do no mee he requiremens of he applicaion on hand (e.g. regarding emperaure behavior), resuling in more demanding circuiry, all of which adds up o a more cosly soluion. Acquisiion difficulies: Problems of logisics arise when he soluion design requires specific componen models: "The device you require is unforunaely no available for he nex six monhs"! Design ime: The lack of applicaion suppor which ofen goes hand in hand wih sandard devices resuls in more hours of engineering and hus higher coss. Sysem proecion agains shor-circuiing may have o be aken ino consideraion, for example, or perhaps he emperaure effecs of he reference compensaed for. Inegraed ASSPs (applicaion-specific sandard producs) are an alernaive o he discree soluion, heir high degree of specializaion combined wih he advanage of being in a miniaurized form grealy simplifying a number of applicaions and hus making producs generally increasingly less expensive. AM462 Analog-IC wih an indusrial curren loop oupu In he inerface IC AM462 Analog Microelecronics GmbH [1] can now offer a monolihically inegraed circui which is par of a new series of indusrial amplifier ICs. This special chip has been developed wih he aim of providing engineers wih a universal /I converer device wih an adjusable volage inpu and curren oupu which can be used as an analog oupu sage for processor applicaions following D/A conversion, for example. AM462 (see Figure 3) is a mulisage amplifier IC wih a range of modular addiional and proecive funcions which can be used as required, providing a high degree of funcionaliy. The IC is modular hroughou. All funcion blocks can be individually accessed via he relevan pins and conneced up exernally or operaed separaely. AM462 wih differen funcion blocks: 1. An operaional amplifier This amplifier sage (OP1) is suiable for inpu signals of 0 o CC-5. The amplificaion is se using exernal resisors. The oupu of he operaional amplifier sage has been designed so ha i can be se righ down o 0 if he load requires. OP1 is volage limied, proecing follow-on sages even when overvolage occurs on he inpu side of he device. Figure 2: New series of amplifier and proecive ICs as indusrial oupu ICs for processor applicaions 2/6

3 CSET 2 INP 3 AM462 BG OP2 CREF SET REF OP1 olage Reference 16 SET I 11 RS+ 10 CC 9 RS- 8 IOUT 4 INN 5 OUTAD 6 INDAI GND 14 Figure 3: Block diagram of /I Converer IC AM A /I converer sage On he oupu side of he device he /I converer sage ranslaes he inpu volage ino a freely selecable oupu curren of beween 0 and 20mA. In doing so he converer oupu sage acivaes an exernal ransisor (Figure 4) which ensures ha power is dissipaed ouside he IC. The oupu curren range for he curren offse and final curren value can be easily se using wo exernal volage dividers. 3. A bandgap reference The reference volage source included in AM462 enables exernal componens, such as sensors and microprocessors ec., o be supplied wih volage. Reference volage REF can be se using pin SET. If pin SET is no conneced, REF = 5; if SET is conneced o ground, REF = 10. Using wo exernal resisors (insered beween pins REF and SET and pins SET and GND) inermediae values can also be se. 4. An operaional amplifier A second operaional amplifier (OP2) can be used as a curren or volage source o power exernal componens. OP2's posiive inpu is conneced inernally o volage BG, enabling he oupu curren or volage o be se wihin a wide range using one or wo exernal resisors. The oupu of his addiional operaional amplifier has a drive power of 10mA. The IC also includes inernal circuiry proecing amplifier sage OP1 agains overvolage, inegraed proecion agains reverse polariy wih regard o he oupu sage (/I converer) across he enire volage range and an oupu curren limier which proecs he IC agains desrucion. Using jus a minimum of exernal circuiry an oupu sage can hus be realized which is fully proeced. 3/6

4 CREF C 1 REF R 3 R 4 CSET INP 2 OP2 3 AM462 BG SET olage Reference OP1 I S R 0 T 1 D R 5 I OUT R 2 R 1 OUTAD INDAI Ground Figure 4: AM462 as ypical /I Converer-IC-circuiry Using AM462 as an indusrial processor oupu sage Many of oday's applicaions incorporae inexpensive RISC processors. These are suiable for signal correcion (compensaion, calibraion, linearizaion, ec.), for example, as driver unis wih he relevan algorihms and, if need be, also for idenificaion purposes. Should a curren (oupu) signal be required by he overall sysem for furher signal condiioning, afer passing hrough he processor he digial signal informaion mus be convered ino an analog signal. If he processor does no include an inegraed D/A converer, he digial signal hen has o be ranslaed by alernaive means, such as via a PWM (pulse-widh modulaion) signal wih a follow-on low-pass filer. The signal is hen convered ino he required curren signal by AM462 (see he schemaic diagrams in Figures 4 and 5). In he suggesed applicaion a sandard processor in Amel's AR series (an ATiny11) was used. ATiny11 is a fully-saic CMOS RISC processor in an 8-pin SO package wih 1kb of rewriable flash ROM, 32 byes of RAM-ype regisers and a 1MHz clock frequency. The processor does no require an exernal clock or quarz as i comes wih an inernal RC oscillaor. Depending on he processor ype he supply volage ranges from a maximum of a a power consumpion of ca. 4mA. For all ypes he operaing emperaure range is from -55 o +125 C. The flash memory can eiher be programmed via he expansion por on he Amel evaluaion board, using special programming unis from oher suppliers or by domesic assemblies by means of digial signal. As he seleced RISC processor has neiher is own A/D or D/A converer nor a frequency oupu nor PWM oupus realized by hardware, digial-o-analog conversion mus be realized by appropriae sofware using he given processor oupus. 4/6

5 PWM oupu as a subsiue DAC For he purpose of digial-o-analog conversion one of he ATiny11 I/O pins is programmed as a PWM oupu. I provides wo discree volages of 0 and 5 and is swiched on and off periodically wihin a defined ime span by sofware, wih he pulse widh maching he digial value. The resuling square-wave signal (PWM signal) is filered in a follow-on low-pass filer, producing an average DC oupu signal (U 2 ) which is proporional o he duy cycle. The following applies: p U2 = U () d U U T 2 = 1 T 0 where 1/T is equivalen o he basic frequency of he PWM signal, p he pulse widh and p/t he duy cycle. A mean volage is hus produced by he low-pass filer which is boh proporional o he duy cycle and provides he AM462 wih an inpu volage (Figures 5 and 6). The recified PWM signal is hus he volage U 2 which AM462 hen convers ino a sandard curren signal of 0/4 o 20mA, for example. As AM462 is a purely analog IC he achievable resoluion of he oupu signal is equivalen o he quanizaion of he lengh of he swich-on/swich-off ime. CREF = Tiny11 Low-pass filer AM 462 z.b. 0/4...20mA U 1 U 2 U 1 R C U 2 Reverse polariy proecion and Shor-circui proecion Figure 5: Typical indusrial applicaion of AM 462 wih a RISC processor As he processor's own power consumpion is well below 10mA his can be supplied by AM462's curren source. AM462 could also be used o power an exising signal condiioning circui (e.g. 3.3 or 5). The supply of addiional componens by he oupu IC has he advanage of hermal maching, hus reducing he emperaure error. Furhermore, all componens which are powered by AM462 are also guarded by he IC's proecive circuiry. 5/6

6 ariaions in pulse widh R ariaions in volage C Figure 6: The volage signal changes wih variaions in he pulse widh Conclusion In AM462 Analog Microelecronics has produced an IC which is suiable for a variey of applicaions. Is use as an indusrial oupu circui (20mA curren loop) In a processor circuiry has been illusraed in his aricle. Is many funcional modules and proecive unis enable a lo of differen indusrial circuis o be realized. Remarks The new AM461 [2] can be used in he same applicaion, differing from AM462 only in ha i generaes an adjusable oupu volage signal. The aricle described a 3 wires soluion because he curren consumpion of he RISC processor ogeher wih he IC consumpion is in he high emperaure range a he limi of 4mA. Wih a processor of lower curren consumpion or wih a lower conversion frequency i should be possible o realize also 2-wires applicaions. The conen of his aricle is available also for he AM422 [3], who has fewer funcions and is cheaper as he described AM462. Furher reading You will find deailed informaion on he producs deal wih in his aricle under he following links: [1] Analog Microelecronics : hp:// [2] Daa Shees: Daa Shee AM461 [3] Daa Shees: Daa Shee AM422 6/6