chuyen doi tuongtu-so.docx

BI 2.1: CHUYN I TNG T - S ADCTrang 11.1 S khi B chuyn i tng t sang s ADC (Analog to Digital Converter) ly mc in th vo tng t sau mt thi gian s sinh ra m u ra dng s biu din u vo tng t. Tin trnh bin i A/D thng phc tp v mt nhiu thi gian hn tin trnh chuyn i D/A. Do c nhiu phng php khc nhau chuyn i t tng t sang s. Hnh v 5.16 l s khi ca mt lp ADC n gin.

Hot ng c bn ca lp ADC thuc loi ny nh sau:Xung lnh START khi ng s hot ng ca h thng.Xung Clock quyt nh b iu khin lin tc chnh sa s nh phn lu trong thanh ghi.S nh phn trong thanh ghi c DAC chuyn i thnh mc in th tng t VAX.B so snh so snh VAX vi u vo trng t VA. Nu VAX < VA u ra ca b so snh ln mc cao. Nu VAX > VA t nht bng mt khong VT (in th ngng), u dra ca b so snh s xung mc thp v ngng tin trnh bin i s nh phn thanh ghi. Ti thi im ny VAX xp x VA. gi dtr nh phn thanh ghi l i lng s tng ng VAX v cng l i lng s tng ng VA, trong gii hn phn gii v chnh xc ca h thng.Logic iu khin kch hot tn hiu ECO khi chu k chuyn i kt thc.Tin trnh ny c th c nhiu thay di i vi mt s loi ADC khc, ch yu l s khc nhau cch thc b iu khin sa i s nh phn trong thanh ghi.1.2 Cc ch tiu k thut ch yu ca ADC phn gii phn gi ca mt ADC biu th bng s bit ca tn hiu s u ra. S lng bit nhiu sai s lng t cng nh, chnh xc cng cao.Di ng, in tr u vo.Mc logic ca tn hiu s u ra v kh nng chu ti (ni vo u vo). chnh xc tng iNu l tng ha th tt c cc im chuyn iphi nm trn mt ng thng. chnh xc tng i l sai ds ca cc im chuyn i thc t so vi c tuyn chuyn i l tng. Ngoi ra cn yu cu ADC khng b mt bit trong ton b phm vi cng tc.Tc chuyn iTc chuyn i c xc nh thi gian bi thi gian cn thit hon thnh mt ln chuyn i A/D. Thi gian ny tnh t khi xut hin tn hiu iu khin chuyn i n khi tn hiu s u ra n nh.H s nhit H s nhit l bin thin tng i tn hiu s u ra khi nhit bin i 10C trong phm vi nhit cng tc cho ph p vi iu kin mc tng t u vo khng i.T s ph thuc cng sutGi s in p tng t u vo khng i, nu ngun cung cp cho ADC bin thin m nh hng n tn hiu s u ra cng ln th t s ph thuc ngun cng ln.Cng sut tiu hao.1.3 Cc bc chuyn i ADQu trnh chuyn i A/D nhn chung c thc hin qua 4 bc c bn, l: ly mu; nh mu; lng t ha v m ha. Cc bc lun lun kt hp vi nhau trong mt qu trnh thng nht.1.3.1 nh l ly mui vi tn hiu tng t VI th tn hiu ly mu VS sau qu trnh ly mu c th khi phc tr li VI mt cch trung thc nu iu kin sau y tha mn:fS 2fImax (10) Trong fS : tn s ly mu fImax : l gii hn trn ca gii tn s tng tHnh 5.17 biu din cch ly mu tn hiu tng t u vo. Nu biu thc (10) c tha mn th ta c th dng b t lc thng thp khi phc VI t VS.V mi ln chuyn i in p ly mu thnh tn hiu s tng ng u cn c mt thi gian nht nh nn phi nh mu trong mt khong thi gian cn thit sau mi ln ly mu. in p tng t u vo c thc hin chuyn i A/D trn thc t l gi tr VI i din, gi tr ny l kt qu ca mi ln ly mu.

1.3.2 Lng t ha v m haTn hiu s khng nhng ri rc trong thi gian m cn khng lin tc trong bin i gi tr. Mt gi tr bt k ca tn hiu s u phi biu th bng bi s nguyn ln gi tr n v no , gi tr ny l nh nht c chn. Ngha l nu dng tn hiu s biu th in p ly mu th phi bt in p ly mu ha thnh bi s nguyn ln gi tr n v. Qu trnh ny gi l lng t ha. n v c chn theo qui nh ny gi l n v lng t, k hiu D. Nh vy gi tr bit 1 ca LSB tn hiu s bng D. Vic dng m nh phn biu th gi tr tn hiu s l m ha. M nh phn c c sau qu trnh trn chnh l tn hiu u ra ca chuyn i A/D.1.3.3 Mch ly mu v nh muKhi ni trc tip in th tng t vi u vo ca ADC, tin trnh bin i c th b tc ng ngc nu in th tng t thay i trong tin trnh bin i. Ta c th ci thin tnh n nh ca tin trnh chuyn i bng cch s dng mch ly mu v nh mu ghi nh in th tng t khng i trong khi chu k chuyn i din ra. Hnh 5.18 l mt s ca mch ly mu v nh mu.