CHNG 6 GIAO TIP TNG T

LT3

6.1

GII THIU CHUNG GIAO TIP TN HIU TNG T

Trong k thut, hu ht cc tn hiu u di dng tn hiu tng t, trong khi cc h thng vi x l thc hin cc chng trnh, cho php iu khin t ng cc h thng k thut li l cc h thng s. Hnh 6.1 m t vic giao tip gia cc h thng vi x l v cc quy trnh i tng k thut, cc quy trnh cng ngh trong thc t. Trn hnh v, thu thp thng tin tng t t bn ngoi, h thng vi x l cn cc b giao tip chuyn i tng t sang s (ADC Analog to Digital Converter). V cung cp cc tn hiu iu khin di dng tng t h thng s dng cc giao tip bin i s sang tng t (DAC Digital to Analog Converter).

Hnh 6.1: M hnh giao tip cc tn hiu tng t trong cc h thng vi x l 6.2 6.2.1 MCH IN T TNG T NG DNG Mch ng dng khuch i thut ton

Khuch i thut ton l mt trong s nhng linh kin in t thng gp nht trong k thut tng t, v th trong k thut o lng v iu khin cng nghip, khuch i thut ton cng c mt trong rt nhiu thit b v h thng. Kh nng s dng ca cc b khuch i thut ton l rt vn nng, chng c p dng trong nhiu lnh vc nh cc b khuch i mt chiu, cc b khuch i xoay chiu, b lc tch cc, b dao ng, b bin i tr khng, b vi phn, b tch phn... lm ni bt tnh cht ca mt b khuch i thut ton, hy xt tnh nng ca mt b khuch i thut ton l tng: H s khuch i khi khng c phn hi m ln v cng. in tr li vo ln v cng. 179

rng di thng ln v cng. H s nn ng pha CMRR ln v cng. in tr li ra bng khng Thi gian p ng bng khng. Trn thc t, khng c b khuch i thut ton l tng m ch tn ti nhng khuch i thut ton thc c tnh cht gn vi nhng tiu chun nu. Cc tham s v cc mch ng dng ca b khuch i thut ton rt nhiu, khng th nu ht trong phn ny m y, ch nu ln nhng tham s c bn, cch tnh ton v cc mch c p dng trong h thng iu khin. Cc tham s c bn ca mch khuch i thut ton

Hnh 6.2: B khuch i thut ton. Ud in p vo hiu UP , IP in p vo v dng in vo ca thun. UN , IN in p vo v dng in vo ca o. Ur , Ir in p ra v dng in ra. B khuch i thut ton khuch i hiu in p U d = UP - UN vi h s khuch i K0 khc 0. Do in p ra: Ur = K0. Ud = K0(UP - UN) H s khuch i hiu K0 Khi khng ti c xc nh theo biu thc sau

K0 =

=

H s khuch i ng pha KCM Nu t vo ca thun v ca o ca b khuch i thut ton cc in p bng nhau, ngha l: UP = UN = UCM 0 th Ud = 0. Gi UCM l in p vo ng pha. Theo biu thc trn ta c Ur=0. Tuy nhin, thc t khng phi nh vy, gia in p ra v in p vo ng pha c quan h t l l h s khuch i ng pha KCM :

180

KCM = KCM ni chung ph thuc vo mc in p vo ng pha. H s nn ng pha CMRR Dng nh gi kh nng lm vic ca b khuch i thc so vi b khuch i l tng (KCM=0) CMRR = Dng vo tnh L tr trung bnh ca dng vo ca thun v dng vo ca o: It = vi UP = UN = 0 Dng vo lch khng l hiu cc dng vo tnh hai ca ca b khuch i thut ton I0 = IP - IN vi UP = UN = 0 Thng thng I0 = 0,1IP. Tr s ca dng vo lch khng thay i theo nhit . Hin tng ny gi l hin tng tri dng lch khng. Cc s c bn ca b khuch i thut ton S khuch i khng o

Hnh 6.3: S khuch i khng o H s khuch i ca mch:

K= Mch m y l trng hp c bit ca mch khuch i khng o

Hnh 6.4: S mch m 181

Mch c h s khuch i bng 1 v dng phi hp tr khng. Mch khuch i o

Hnh 6.5: S mch khuch i o H s khuch i ca mch: K= S bin i dng in - in p

Hnh 6.6: S bin i dng in - in p in p ra c tnh theo biu thc: UR = - R.IV Mch cng o

Hnh 6.7: Mch cng o

in p ng ra: 182

Mch tr:

Hnh 6.8: Mch tr

Mch cho php thay i cc tnh in p ra:

Hnh 6.9: Mch cho php thay i cc tnh in p ra Mch khuch i o lng:

Hnh 6.10: Mch khuch i o lng 183

6.2.2

Mch ghp knh v chuyn mch tng t

Khi c nhiu knh tng t ng vo ra cn giao tip vi h thng vi x l iu khin, trnh phi s dng nhiu b bin i A/D, D/A, ngi ta thng s dng cc chuyn mch tng t chn mt knh ti mt thi im.

Hnh 6.11: Cu to chuyn mch tng t Ging nh mt cng tc c kh cc chuyn mch tng t c nhim v cch ly hoc cho php tn hiu ng vo chuyn ti ng ra. Khc vi cc cng tc c kh s dng cc tip im kim loi, c iu khin ng ngt bng tay hoc bng in (nh relay), cc chuyn mch in t thc hin bng cc MOFET. Hnh 6.11 m t cu trc ca mt chuyn mch tng t, chuyn mch s dng 2 MOSFET knh N v P mc song song v c iu khin trc tip t bn ngoi. Cc cc cng N1 v P1 c kch lch pha 180o, khi N1 =15V th P=-15V v ngc li. Khi s dng cc chuyn mch tng t cn ch cc c tnh ca tn hiu cn chuyn mch nh: bin tn hiu cn chuyn mch, mo dng tn hiu . Sau y l mt s c tnh cn xem xt khi la chn cc chuyn mch tng t. V+: xc nh bin tn hiu tng t c th i qua chuyn mch tng t m khng b xn nh. Cc mch MOSFET bn trong cn c phn cc cho php ton di bin ca tn hiu ng vo. Mt s chuyn mch tng t c ngun phn cc i xng cho php chuyn mch cc tn hiu i xng. VIH/VIL: l tn hiu iu khin ng m chuyn mch tng t. Hu ht cc chuyn mch tng t u cho php ng m bng cc tn hiu iu khin s. Do cn xem xt mc tn hiu iu khin ny s dng cc cng s thch hp cho vic iu khin. Ron (On State Resistance): Ron s nh hng ti vic mt v suy gim tn hiu chuyn mch. Cc chuyn mch tng t c Ron nh cn s dng cc transistor c kch thc ln. Tuy nhin khi s dng cc transistor ln v bn dn c kch thc ln s lm tng in dung vo ra CI/O. Cc in dung ny s nh hng rt ln ti tn s p ng 184

ca chuyn mch tng t. Khi chuyn mch cc tn hiu vi tn s ln cn s dng cc chuyn mch c Ron v CI/O nh. Ron(flat) Di in tr ng: l khong t nh nht n ln nht ca in tr ng chuyn mch tng t. Thng s ny thng thay i theo nhit . CON/COFF in dung ON/OFF: thng s ny cn c xem xt v n nh hng ti p ng thi gian, thi gian n nh v kh nng ghp ni ng ra (fanout) ca chuyn mch tng t. Tn s p ng: hu ht cc chuyn mch CMOS u c gii hn trn cho tn s tn hiu chuyn mch kh thp. Ngi ta c th thc hin cc chuyn mch vi Ron v CI/O thp, tuy nhin vn c cc b lc thng thp k sinh cc ng ra lm gim tn s p ng ca cc chuyn mch tng t. THD - mo dng sng sin hoc tng hi mo dng: thng s ny xc nh tuyn tnh ca chuyn mch. S p ng khng tuyn tnh ca linh kin c th xy ra do nhiu nguyn nhn (nh do thit k, do tnh cht linh kin ). Nhng nh hng nhiu nht l do s thay i ca RON khi tn hiu vo thay i. Chuyn mch c RON nh l rt quan trng, nhng thng s RON(flat) cng quan trng khng km. Cc MOSFET knh N c c tnh RON(flat) kh tt khi 0 < VI/O < V+ - 2V), tuy nhin n s tng rt nhanh khi VI/O tin gn ti V+ hoc VGS gim. V vy nn chn cc chuyn mch tng t sao cho m bo iu kin 0 < VI/O < V+. Nhiu xuyn knh (Crosstalk): C hai loi xuyn knh trong chuyn mch tng t cn c xem xt l: Xuyn knh gia tn hiu iu khin v tn hiu ng ra: do c tnh t k sinh trong qu trnh thc hin cc MOSFET, nn s thay i ca tn hiu iu khin c th gy ra nhiu trn tn hiu ng ra. Xuyn knh gia cc chuyn mch ch to trn cng bn dn: cng do cc t k sinh khi ch to, tn hiu chuyn mch chuyn mch ny c th nh hng ti tn hiu chuyn mch mt knh khc.

-

Cch ly trng thi ng: tng ng vi tr khng trng thi cch ly tn hiu chuyn mch, n thng c o theo n v DB tn s p ng ca chuyn mch. Kh nng chn tn hiu tn s cao: Thng s ny lin quan n kh nng chn tn hiu khi tt ca chuyn mch. Ging nh nhiu xuyn knh, cc t k sinh c th ni cc tn hiu tn s cao t ng vo ti ng ra khi chuyn mch trng thi tt. chn bin i mt trong nhiu tn hiu tng t ng vo, ngi ta s s dng cc b dn knh tn hiu tng t. Cc b dn knh ny s c cu trc bao gm nhiu chuyn

185

mch tng t ni chung ng ra. Mt trong cc chuyn mch s c chn thng tn hiu ng vo bng mt b gii m a ch ging nh mt b dn knh s. 6.2.3 Mch to in p chun

in p chun nh hng trc tip ti chnh xc ca cc qu trnh bin i tng t s. Thng thng trong cc b bin i A/D, D/A tch hp sn cc khi to in p chun, cc vi mch khng tch hp sn c th s dng cc IC to p chun chuyn dng. Cc vi mch ny s to ra in p ng ra khng thay i theo thi gian, nhit hay theo in p cung cp cho n. Hnh 6.12 m t cu to v k hiu ca vi mch to p chun TL431A, tn hiu in p a vo chn REFERENCE s c so snh vi in p chun 2.5 V bn trong, khuc i qua OP AMP iu khin transitor dn gi mt in p khng i gia Anode v Kathode.

Hnh 6.12: Cu to vi mch to p chun TL431A Hnh 6.13 m t mt mch to p chun in hnh s dng TL431A, gi tr in p chun ng ra ca mch s thay i khi thay i gi tr bin tr R1.

Hnh 6.13: Mch to p chun s dng TL431A

186

6.2.4

Ly mu v gi mu (Sample & Hold)

Qu trnh ly mu (Sampling) l bc u tin trong chuyn i tng t s l ri rc ho theo thi gian tn hiu u vo ban u. Vic ly mu nh vy c b phn ly mu v gi Sample & Hold Circuit (S&H) thc hin.

Hnh 6.14: Ly mu tn hiu tng t Bc tip theo, b chuyn i tng t s A/D chuyn i cc mu nghim lin tc theo gi tr v c ri rc ho theo thi gian thnh tn hiu lng t ho theo gi tr v biu din di dng s nh phn. Phi chn tn s ly mu sao cho cc tr ly mu c th c trng cho tn hiu ban u (fsample 2 fmax), v c th phc hi li tn hiu ban u t cc mu ny.

Hnh 6.15: Mch ly mu v gi (Track & Hold) Mch ly mu v dng S&H: Trong k thut chuyn i tng t s, tn hiu tng t bin thin lin tc theo thi gian u(t) c ly mu nhng thi im nht nh, v gi tr tn hiu nhn c s c lu gi trong mt khong thi gian nh trc. 187

Trong nhiu b chuyn i A/D, tn hiu phi c gi khng i trong sut thi gian chuyn i. thc hin vic , dng mch Track-and-Hold T&H. Cu trc c bn ca mch T&H trn hnh 6.15. Sau mi ln m chuyn mch, t in np n gi tr in p u vo uvao(t) v lu gi gi tr tc thi ca in p u vo trc thi im m chuyn mch. C hai b khuch i thut ton lm b m buffer cho u vo cng nh u ra. Mt mch T&H khc c m t trn hnh 6.16.

Hnh 6.16: Mch ly mu v gi (Track & Hold) dng FET Mt transistor trng FET cc cng cch ly (Isolated Gate FET) c mc gia KTT o pha v KTT tch phn. Khi usamp = 0, transistor FET s dn v in p u u ra uA tng ng in p u vo, gi thit hng s thi gian tch phn ca KTT (c xc nh bi dng in u ra cc i ca KTT l nh). Vo thi im t=t0, transistor trng kha ,nh vy vic np t khng i, in p u ra U A c gi li mc gi tr UA(t)=UA(t0), cho n khi transistor trng c iu khin m mch. Khi FET ng, cc diode s dn v nh m trnh c hin tng KTT b qu iu khin.

Hnh 6.17: V d v DAC 4 bit

188

6.2.5

Cc b bin i tng t s, s - tng t (ADC/DAC)

6.2.5.1 Bin i s Tng t (DAC Digital to Analog Converter) Mc ch ca b bin i DA, l bin i tn hiu nh phn n bit thnh dng hay p tng ng. Hnh 6.17 m t mt b bin i DA 4 bit n gin. V nguyn tc b chuyn i s-tng t tip nhn mt m s n bit song song hoc ni tip li vo v bin i ra dng in hoc in p tng ng li ra. Dng in hay in p li ra l hm bin thin ph hp theo m s li vo. S khi mt b DAC hon chnh hnh 6.18 bao gm ba phn t c bn: in p quy chiu n nh bn ngoi (Vref) DAC c s Khuch i thut ton Nh vy in p u ra ca b bin i V0 s ph thuc vo m nh phn u vo theo cng thc sau: V0 = Vref (B020 + B121 + ... + Bn2n) Trong B0 l bit thp nht v Bn l bit cao nht ca m nh phn u vo, Vref l in p quy chiu. DAC c s cu to bng nhng chuyn mch tng t c iu khin bi m s u vo v cc in tr chnh xc. Cc chuyn mch tng t iu chnh dng in hay in p trch ra t in p quy chiu v to nn dng in hay in p u ra tng ng vi m s u vo. Mch khuch i thut ton dng y chuyn i dng thnh p ng thi c chc nng tng m. B bin i DAC c c im l i lng ra tng t khng lin tc, ri rc ca u ra ph thuc vo s bit ca b bin i, nhng DAC c s bit u vo ln th tng s nc in p ra cng ln v khong cch gia cc nc cng nh.

Hnh 6.18: S khi DAC

189

Cc tham s ca b chuyn i DA: phn gii (Solution): Lin quan n s bit ca mt DAC. Nu s bit l n th s trng thi ca tn hiu nh phn l 2n ngha l s c 2n mc in th (hoc dng in) khc nhau, do c phn gii l 1/2n. phn gii cng b th in th (hoc dng in u ra) cng c dng lin tc, cng gn vi thc t v ngc li. chnh xc (Accuracy): C th nh gi cht lng ca mt DAC bng sai s ca n. i lng biu din sai s l lch ti a gia i lng ra v mt ng thng ni im 0 vi im FS (Full Scale) trn c tuyn chuyn i DA. tuyn tnh (Linearity): tuyn tnh ca DAC cho bit lch in p so vi mt ng thng i qua nhng im nt ca c tuyn chuyn i. l c tnh thng gp nht vi DAC. ng cong c tuyn l n iu nu s thay i lch trn l khng i du. c mt DAC n iu, lch ny phi ln hn 0 cho mi nc thang. Ngoi ra mc tuyn tnh ca DAC phi nh hn hoc bng 1/2 LSB n tr nn n iu. Nh vy 1/2 LSB l c trng v gii hn n iu ca mt DAC. Phi tuyn vi sai: l i lng cho bit lch gia gi tr thc t v l tng cho mt nc in p ra ng vi mi thay i ca m s vo. i lng ny cho bit v nhn ca ng cong c tuyn i vi DAC. Thi gian thit lp: i vi mt DAC l thi gian cn thit in p ra t ti gi tr ti hn sai s xung quanh gi tr n nh. Gii hn ny thng l 1/2 LSB hoc biu din bng gi tr FS. Thi gian thit lp trc ht ph thuc vo kiu chuyn mch, kiu in tr v kiu khuch i dng xy dng b DAC. Thng thng n c nh ngha bng thi gian t khi in p bt u thay i cho ti khi t ti vng gii hn sai s cho trc. N khng bao gm thi gian tr tnh t khi c s thay i m s u vo cho ti khi in p ra bt u p ng. Cc mch DAC in hnh Cc DAC c th c xy dng theo mt trong nhng kiu mch sau: Chuyn i DA theo kiu in tr trng lng Chuyn i DA theo kiu mch R-2R Chuyn i DA theo phng php m ho Shannon-Rack B chuyn i DA theo kiu in tr trng lng (Weighted resistor DAC) Mch gm mt ngun in p chun U ch, cc chuyn mch, cc in tr c gi tr ln lt l R, R/2, R/4, ... , R/2n-1 v cc mch khuch i thut ton (hnh 6.19). Vi mch in ny, khi mt kho in no c ni vi ngun in th chun th s cung cp cho b KTT dng in. Dng in ny c lp vi cc kho cn li. Nh vy c th thy ngay rng bin in p ra ph thuc vo cc v tr c ng hay m kho ngha l c ni vi in p chun Uch hay ni cch khc ph thuc vo gi tr cc bit tng ng 190

trong tn hiu s a vo mch chuyn i. Mt cch tng qut, vi mt DAC c n bit th tn hiu ra c tnh theo cng thc: Ura = -Uch (2n-1.Bn-1 + 2n-2.Bn-2 + ... + 21.B1 + 20.B0) Trong B0 - Bn-1 c gi tr "0" hoc "1".

Hnh 6.19: DAC theo phng php in tr trng lng Mch c u im l n gin, nhng nhc im l chnh xc v tnh n nh ca kt qu ph thuc nhiu vo tr s tuyt i ca cc in tr v s n nh ca chng trong cc mi trng khc nhau. Ngoi ra cn ph thuc vo tnh n nh v chnh xc ca ngun in p chun. B chuyn i DA theo kiu thang in tr R-2R (R-2R ladder)

Hnh 6.20: DAC theo phng php mch R-2R DAC vi thang in tr R-2R (hnh 6.20) khc phc c mt s nhc im ca DAC in tr trng lng. Mch ch gm hai in tr R v 2R mc theo hnh thang vi nhiu kho in (mi kho in cho mt bit) v mt ngun in p chun Uch. i lng cn tm l dng Ith chy vo mch KTT khi c mt s kho in c ni vi Uch. Theo mch in ta c: 191

Ura = -Ith.Rf Xt ti chuyn mch tng ng vi bit th i, nt tng ng trn mch hnh thang l 2i. S dng nh l Thevenin, khi ng chuyn mch vo Uch th in th tng ng Thevenin ti nt 2i s l Uch/2 v ngun tng c ni tr l R, nh vy ti nt 2i+1 (tin v pha mch KTT) ta c ngun tng ng Thevenin c tr s l Uch/4 v ni tr l R. T nhng kt qu trn suy ra rng khi di chuyn v pha mch KTT th tr s in th Thevenin ti mi nt bng mt na tr s ti nt k cn bn tri n, v ti nt 2n-1 do c tnh ca b KTT in th c coi bng 0V. Mt cch tng qut, ta c cng thc tnh in p ra ca mt DAC n bit vi in tr hnh thang R-2R nh sau:

Trong B0 - Bn-1 c gi tr "0" hoc "1". Cc DAC theo thang in tr phi dng s in tr kh ln, v d nu mt DAC n bit th cn dng 2(n-1) in tr trong khi phng php in tr trng lng ch phi dng n thi. Nhng b li chnh xc v tnh n nh ca tn hiu ra c m bo tt hn. Ghp ni DAC vi h VXL Khi s dng mt DAC 8 bit trong mt h thng vi x l, mt cch n gin c th ni cc ng vo ca chng ti mt cng ng ra, hoc mt s b DAC cn cho php ni trc tip cc ng vo ca chng ti Bus d liu nh i vi mt cng vo ra thng thng. Cn i vi cc ng dng yu cu phn gii 12 bit, cn phi s dng cc DAC 12 bit. Nhng nu h thng s dng vi x l 8 bit, trc ht phi ni 8 bit thp ca DAC ti mt cng vo ra, cn 4 bit cao ca n phi ni ti mt cng vo ra khc. Trong chng trnh cn phi chuyn 8 bit thp ti bng mt lnh ghi, v 4 bit cao bng mt lnh ghi khc. Nhng khi s dng hai ln ghi nh vy, ng ra s b t bin, trc khi t c gi tr n nh nh mong mun. V d khi mun bin i gi tr ca 1 DAC 12 bit t 0000 1111 1111 thnh 0001 0000 0000. Khi ghi 8 bit thp ti DAC, ng ra ca n s thay i gi tr t 0000 1111 1111 thnh 0000 0000 0000, sau ghi ti 4 bit cao ng ra vi tr v gi tr mong mun l 0001 0000 0000. Nh vy trong khong thi gian gia hai ln ghi s l gi tr khng mong mun. Trong nhiu h thng vn trn s gy ra hot ng khng n nh. trnh c nhc im ny c th s dng cc b ci ti ng vo DAC. B ci s c np gi tr hai ln sau chuyn ng thi 12 bit ti DAC. Rt nhiu b DAC hin nay cho php thc hin cng vic trn mt cch d dng. Hnh 5.36a trnh by cu trc ng vo ca DAC1230 v DAC08 ca hng national, chng c b ci 4 bit thp v th c th ghi ti 12 bit ng vo bng hai lnh ghi t cc cng vo ra 8 bit, hoc trc tip t Bus d liu. Cng c th ni cc ng vo ca DAC ny ti cc ng d liu ca cc h thng 16 bit nh trn hnh 6.21. 192

Cc b DAC yu cu in p chun chnh xc, mch hnh 5.36b s dng in p chun 10,000V. Cc DAC cung cp dng ng ra, v vy cn ni thm mt mch Op Amp ng ra bin i thnh in p tng ng. Thng thng s dng cc b khuch i c ng vo FET, v dng phn cc ng vo ca cc b khuch i lng cc c th nh hng ti chnh xc ca ng ra. Ch trong mch s dng masse s v masse tng t ring nhau, trnh nhiu cho tn hiu s gy ra bi s mo dng ca tn hiu tng t.

16 bit Bus d liu

8 bit input latch

12 bit DAC register

Ti cc cng tc dng

Vcc

Cung cp t b gii m a ch

Byte1 /byte2 CS WR WR1 XFER WR2

(a)

Full - Scale Adjust 50 C2 +15V Vcc Rfb Iout1 DAC1208 Vref Vref Agnd Dgnd Iout2 20pF 3 2

+15V

7 1

U8 Vout 6 LF356 25K Vcc Zero Adjust

+ 4 5

Vout = - (Iout1 x Rfb) = -Vref (D)/4096 -15V

(b) Hnh 6.21: (a) Cu trc ci bn trong ca DAC1208. (b) Mch kt ni tng t.

193

6.2.5.2 Bin i tng t s (ADC Analog to Digital Converter) Trong phn trc chng ta bn n vic thu nhn cc tn hiu thng qua cc sensor nhit, p sut ... v cc mch khuch i thut ton dng khuch i v lc cc tn hiu in ny. Bc tip theo l b bin i AD bin i tn hiu tng t sang dng s c th lm vic c vi CPU. Trong mt b ADC, tn hiu tng t sau khi qua x l c a vo mch ly mu. Mch ly mu s Ly mu tn hiu tng t ti cc thi im khc nhau v cch u nhau, ni cch khc y l qu trnh ri rc ho tn hiu v mt thi gian. Sau khi ly mu, cc mu in p s c lng t ho v m ho. Qu trnh lng t ho v bn cht l qu trnh lm trn s c thc hin theo nguyn tc so snh, tn hiu cn chuyn i c so snh vi mt n v chun. Cn m ho l qu trnh xp xp li kt qu lng t theo mt quy lut nht nh tu thuc vo loi m yu cu u ra b bin i. Cc tham s c bn ca b chuyn i gm c: Di bin i ca in p tng t u vo l khong in p m ADC c th thc hin chuyn i c. Khong in p ny c th ly gi tr t 0 n mt gi tr m hay dng, hoc cng c th l in p hai cc tnh -UAM ti + UAM. chnh xc ca ADC: Tham s c trng cho chnh xc ca ADC l phn gii (Resolution). Tn hiu u ra ca mt ADC l cc gi tr s c xp xp theo mt quy lut no . S cc s hng m u ra (s bit trong t m nh phn) tng ng vi di bin i in p vo cho bit mc chnh xc ca php bin i. V d mt b bin i AD c s bit u ra l N=12 th s phn bit c 212=2096 mc trong di bin i in p u vo ca n - UAM chng hn. Nh vy mi mc ADC phn bit c u vo l UAM/2096. Trong thc t ngi ta dng s bit N c trng cho chnh xc ca mt ADC khi di bin i in p u vo l khng i. Lin quan n chnh xc ca mt ADC cn c cc tham s nh: mo phi tuyn, sai s khuch i, sai s lch khng, sai s n iu, sai s lng t ho ... Tc chuyn i cho bit s kt qu chuyn i trong mt giy, cn c gi l tn s chuyn i fc. Cng c th dng tham s thi gian chuyn i Tc c trng cho tc chuyn i (Tc = 1/fc). Tc chuyn i cng cao th chnh xc cng gim v ngc li. Cc phng php chuyn i AD C nhiu cch phn loi cc b bin i AD, hay dng hn c l phng php phn loi v mt thi gian. Phng php ny c u im l cho php phn on mt cch tng qut v tc chuyn i.

194

Chuyn i song song (Parallel Comparator ADC): Tn hiu cn chuyn i c so snh cng mt lc vi nhiu gi tr chun, v vy cc bit c xc nh ng thi v a n u ra. Chuyn i ni tip theo m m: Qu trnh so snh c thc hin tng bc theo quy lut m m. Kt qu chuyn i c xc nh bng cch m s lng gi tr tn hiu tng t cn chuyn i. Chuyn i ni tip theo m nh phn: Qu trnh so snh c thc hin tng bc theo quy lut ca m nh phn. Cc n v chun dng so snh ly cc gi tr gim dn theo m nh phn, do cc bit c xc nh ln lt t bit c ngha nht MSB (Most Significant Bit) n bit c t ngha nht LSB (Least Significant Bit). Chuyn i song song - ni tip kt hp: Trong phng php ny qua mi bc so snh c th xc nh c ti thiu l 2 bit ng thi. Chuyn i theo phng php song song

Hnh 6.22: S nguyn l b chuyn i AD theo phng php song song Nguyn tc ca b chuyn i: Tn hiu tng t UA c ng thi a n cc b so snh t S1 - Sm. in p chun Uch c a n u vo th hai ca b so snh qua thang in tr R. Do cc in p chun t vo b so snh ln cn khc nhau mt lng khng i v gim dn t S1 - Sm. u ra ca cc b so snh c in p chun ly trn thang in tr th c mc logic "1", cn ngc li c mc logic "0".

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Cc u ra ca mch c ni vi mt mch "AND", u cn li ca mch "AND" ni vi xung nhp. Ch khi xut hin xung nhp u vo mch "AND" th xung ra ca b so snh c vo mch nh Flip - Flop (FF). Nh vy xung nhp ng vai tr m bo cho qu trnh so snh v a tn hiu vo b nh, sau b m ho s bin i tn hiu vo di dng m m thnh m nh phn (c th l dng khc). Mch bin i song song c u im l tc chuyn i nhanh, nhng kt cu mch kh phc tp nu ta tng phn gii ca php bin i AD (resolution). Thc vy, thit lp mt b bin i c phn gii n bit th cn phi c (2n -1) b so snh, do vy phng php ny ch dng cho cc b ADC tc cao v phn gii thp. Chuyn i AD theo phng php m

Hnh 6.23: S khi mch ADC kiu m Mch bao gm cc phn t: B so snh, RS Flip-Flop, cng "AND" dng ng m xung nhp, b m n bit v mt DAC n bit. Cc phn t c mc thnh mch hi tip. Hot ng ca mch: Lc u mch Flip-Flop v mch m c Reset, li ra Q ca FF mc logic "0", b m cng xo v 0, do in p ra U DAC ca DAC l 0V. Cng AND m cho cc xung nhp vo mch m. Lc ny in p vo Ua ln hn UADC nn Uss tr thp. UDAC tng dn theo hnh bc thang v mch m lin tc i trng thi t thp ln cao, khi UDAC ln hn Ua th Uss chuyn ln cao lm cho li ra Q ca FF chuyn ln mc logic "0" lm cng AND ng li. Lc ny ni dung b m l tn hiu s n bit tng ng vi tn hiu tng t Ua cn chuyn i.

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Nh vy thi gian chuyn i ca ADC kiu m ph thuc vo ln ca tn hiu tng t Ua v tn s xung nhp. Nu Ua cng ln th thi gian chuyn i cng di, nu xung nhp cao th thi gian chuyn i ngn. Ngoi ra ADC kiu m cn c mt c im l thi gian chuyn i ca mch s hn ch tn s bin thin cao nht ca tn hiu tng t u vo Ua. Chuyn i AD theo phng php xp x lin tip

Hnh 6.24: S khi mch ADC xp x lin tip Chuyn i AD theo phng php xp x lin tip c u im ln l thi gian chuyn i t l thun vi s bit ca m s v thi gian thit lp ca mch ghi ch khng ph thuc vo ln ca tn hiu chuyn i. Khi u mch ghi trng thi Reset, sau mch iu khin kch thch bit ln nht xut hin. Khi s xy ra mt trong hai trng hp: Nu UDAC > Ua th bit ny c b qua (vn thit lp "0") Nu UADC < Ua th bit ny c duy tr (thit lp "1") Sau li kch thch bit k tip xut hin, v cng vic so snh c tip tc n khi UDAC = Ua th qu trnh chuyn i kt thc. Chuyn i AD theo phng php tch phn hai dc. Hnh 6.25 trnh by s cc khi chc nng ca mt b ADC hai dc. Loi ADC ny thng s dng trong cc volt k s v c s bit phn gii cao vi gi thnh h. Khi bt u bin i mch iu khin s xo b m v 0 v ni ng vo ca mch tch phn ti tn hiu in p cn bin i. Khi in p cn bin i dng, ng ra mch tch phn s l mt in p m. Khi ng ra mch so snh c in p m khong vi microvolt, ng ra b so snh vn mc cao, lc ny cng AND cho php xung 1 Mhz 197

a vo b m. Sau mt gi tr m xc nh no (thng l 1000), mch iu khin s chuyn ng vo ca mch tch phn ti in p chun m v xo b m. Vi in p ng vo m, in p ng ra ca mch tch phn s tng dn v 0. Khi ng ra mch tch phn t ti 0 volt, ng ra mch so snh s chuyn trng thi v mc thp ct xung clock cung cp ti b m. Gi tr m c trong b m cho ti khi ng ra mch tch phn t ti 0 volt s tng ng vi gi tr in p vo.IntegratorC1

1-Mhz Clock

3 2 R

4

Analog Vin

C

ComparatorU1A U3A TL022C 1 1 2 16 74AC11008 U2A8

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TL022C4

Vref

SW Control

MSB Reset Control Circuits

Latch

Hnh 6.25: B bin i AD hai dc. Ghp ni ADC vi h VXL Giao tip vi ADC so snh song song. Trong hu ht cc ng dng s dng cc b ADC bin i song song, cc b ADC ny s cung cp d liu ng ra thng thng rt nhanh so vi yu cu. V vy hon ton c th s dng mt mch thm nhp trc tip b nh np trc tip cc d liu ca ADC vo b nh m khng cn s iu khin ca vi x l. B vi x l sau c th x l cc d liu c ly vo b nh. Giao tip vi ADC hai dc v m lin tip. Vi cc b ADC hai dc v m lin tip, thi gian bin i l kh chm so vi tc hot ng ca CPU. c c d liu ng, cn kim tra trng thi ca tn hiu cht d liu cung cp t mch iu khin trong ADC. Khi tn hiu ny tc ng mc cao, vi x l c th c c mt m d liu ghi vo b nh. 198

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Binary or BCD counter Latchs Decoder/Drivers Display

Giao tip vi ADC xp x lin tip. c d liu trong cc b ADC xp x lin tip cn thc hin theo s iu khin ca chng trnh. Trc ht cn cung cp tn hiu khi ng qu trnh bin i SC (Start Converter) t vi x l ti ADC. Tip theo kim tra ng ra EOC ca ADC bit c thi im qu trnh bin i kt thc. Khi EOC tc ng, vi x l c th c c mt d liu hp l cung cp t ADC. Vi cc b ADC c nhiu knh vo, thng thng chng s c cc ng a ch v tn hiu ci a ch chn knh. Cng vic ny c thc hin d dng bng phn mm.

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