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Bi Tho Lun Thit K H Thng Vi Vi iu KhinNi dung: thit k h thng v lp trnh cho vi iu khin c tn hiu vo t mch ADCADC chuyn i tng t s (analog digital converter)1.1S 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.

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 VAXvi u vo trng t VA. Nu VAX< VAu ra ca b so snh ln mc cao. Nu VAX> VAt 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 VAXxp x VA. gi dtr nh phn thanh ghi l i lng s tng ng VAXv 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.

1.2Cc 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.

1.3Cc 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 VIth tn hiu ly mu VSsau qu trnh ly mu c th khi phc tr li VImt cch trung thc nu iu kin sau y tha mn:fS 2fImax(10)Trong fS : tn s ly mufImax : l gii hn trn ca gii tn s tng tNu biu thc (10) c tha mn th ta c th dng b t lc thng thp khi phc VIt 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 llng 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 lm 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.Khi u vo iu khin = 1lc ny chuyn mch ngmch ch ly muKhi u vo iu khin = 0lc ny chuyn mch h mch ch gi muChuyn mch c ng mt thi gian di t Chnp n gi tr dng in ca tn hiu tng t. V d nu chuyn mch c ng ti thi im t0 th u ra A1s np nhanh t Chln n in th tng t V0. khi chuyn mch m th t Ch s duy tr in th ny u ra ca A2cung cp mc in th ny cho ADC. B khuch i m A2t tr khng cao ti u vo nhm khng x in th t mt cch ng k trong thi gian chuyn i ca ADC do ADC ch yu s nhn c in th DC vo, tc l V0.Trong thc t ngi ta s dng vi mch LF198 (hnh 5.19) l mch S/H tch hp c thi gian thu nhn d liu tiu biu l 4ms ng vi Ch = 1000pF, v 20ms ng vi Ch= 0.01mF. Tn hiu my tnh sau s m chuyn mch cho php Chduy tr gi tr ca n v cung cp mc in th tng t tng i n nh ti u ra A2.

2.1ADC dng sng bc thang 2.1.1 s khi Phin bn n gin nht ca lp ADC hnh 5.16 s dng b m nh phn lm thanh ghi v cho php xung nhp y b m tng mi mt bc, cho n khi VAX> VA. y gi l ADC sng dng bc thang, v dng sng ti VAXc tng bc i ln. Ngi ta cn gi l ADC loi b m.

Cc thnh phn ca DAC dng sng bc thang hnh 5.20 gm: mt b m, mt DAC, mt b so snh tng t, mt cng NAND 3 ng vo iu khin. u ra ca b so snh dng lm tn hiu (End OfConversion kt thc chuyn i).

2.1.2 Hot ng ca b ADC dng sng bc thangGi s VA, tc mc in th cn chuyn i l dng th tin trnh hot ng din ra nh sau:Xung Khi ng c a vo Reset b m v 0. Mc cao ca xung Khi ng cm khng cho xung nhp i qua cng AND vo b m.Nu u ca DAC ton bit 0 th u ra ca DAC s l VAX= 0V. VVA>VAXnn u ra b so snh s ln mc cao.Khi xung Khi ng v thp th cng AND cho php xung nhp i qua cng ny v vo b m.Khi gi tr b m tng ln th u ra DAC l VAXs tng mi ln mi bc, nh minh ha hnh 5.20.Tin trnh c tip tc cho n khi VAXln n bc vt qu VAmt khong VT. Ti thi im ny ng ra ca b so snh v thp v cm khng cho xung nhp i vo b m nn b m s ngng m.Tin trnh chuyn i hon tt khi tn hiu chuyn t trng thi cao xung thp v ni dung ca b m l biu th dng s ca in p tng t vo VA.B m s duy tr gi tr s cho n khi no xung Khi ng k tip vo bt u tin trnh chuyn i mi.

2.1.3 phn gii v chnh xc ca ADC dng sng bc thangTrong ADC dng sng bc thang c nhiu yu t nh hng n sai s ca qu trnh chuyn i nh: kch c bc thang, tc phn gii ca DAC ci trong n v nh nht. Nu gim kch c bc thang ta c th hn ch bt sai s nhng lun c khong cch chnh lch gia i lng thc t v v gi tr gn cho n. y gi l sai s lng t. Cng nh trong DAC, chnh xc khng nh hng n phn gii nhng li ty thuc vo chnh xc ca linh kin trong mch nh: b so snh, in tr chnh xc v chuyn mch dng ca DAC, ngun in quy chiu,Mc sai s = 0.01% gi tr cc i (y thang) cho bit kt qu ra t ADC c th sai bit mt khong nh th, do cc linh kin khng l tng.

2.1.3 Thi gian chuyn iThi gian chuyn i l khong thi gian gia im cui ca xung khi ng n thi im kch hot u ra ca . B m bt u m t 0 ln cho n khi VAXvt qu VA, ti thi im xung mc thp kt thc tin trnh chuyn i. Nh vy gi tr ca thi gian chuyn i tCph thuc vo VA. Thi gian chuyn i cc i xy ra khi VAnm ngay di bc thang cao nht. Sao cho VAXphi tin ln bc cui cng kch hot .Vi b chuyn i N bit, ta c: tC(max) = (2N 1) chu k xung nhpADC hnh 5.20 s c thi gian chuyn i cc i tC(max) = (210 1)x1ms = 1023msi khi thi gian chuyn i trung bnh c quy nh bng thi gian chuyn i cc i.Vi b chuyn i dng sng bc thang, ta c:Nhc im ca ADC dng sng bc thang l thi gian chuyn i tng gp i vi tng bit thm vo b m. Do vy ADC loi ny khng thch hp vi nhng ng dng i hi phi lin tc chuyn i mt tn hiu tng t thay i nhanh thnh tn hiu s. Tuy nhin vi cc ng dng tc chm th bn cht tng i n gin ca ADC dng sng bc thang l mt u im so vi cc loi ADC khc.

2.2ADC lin tip - xp xB chuyn i lin tip - xp x ( Successive Approximation Convetr-SAC) l mt trong nhng loi ADC thng dng nht. SAC c s phc tp hn nhiu so vi ADC dng sng bc thang. Ngoi ra SAC cn c gi tr tCc nh, khng ph thuc vo gi tr ca u vo tng t.Hnh 5.21 l mt cu hnh c bn ca SAC, tng t cu hnh ca ADC dng sng bc thang. Tuy nhin SAC khng s dng b m cung cp u vo cho DAC m thay vo l thanh ghi. Logic iu khin sa i ni dung lu trn thanh ghi theo tng bit mt cho n khi d liu thanh ghi bin thnh gi tr s tng ng vi u vo tng t VAtrong phm vi phn gii ca b chuyn i.

2.3ADC nhanhB chuyn i nhanh (flash converter) l ADC tc cao nht hin nay c mt trn th trng, nhng s mch phc tp hn cc loi khc. V d mt ADC nhanh 6 bit i hi 63 b so snh tng t, cn ADC nhanh 8 bit th con s ny ln n 255, 10 bit th ln n 1023. Nh vy s lng b so snh qu ln gii hn kch c ca ADC nhanh.

Hnh 5.23 l s ca mt ADC nhanh

ADC nhanh hnh 5.23 c phn gii 3 bit. Kch thc bc thang l 1V. B chia in th thit lp mc quy chiu cho tng b so snh c c 7 mc ng vi 1V ( trng s ca LSB ), 2V, 3V, 7V (y thang). u vo tng t VAc ni n u vo cn li ca tng b so snh.Vi VA< 1V th tt c u ra ca b so snh u ln mc cao. Vi VA> 1V th t mt u ra tr ln s xung mc thp. u ra ca b so snh c a vo b m ho u tin tch cc mc thp, sinh u ra ng vi u ra c s th t cao nht mc thpca b so snh.Bng s tht ca ADC nhanh 3 bit

ADC nhanh hnh 5.23 c phn gii 1V v u vo tng t phi thay i mi ln 1V mi c th a u ra s ln bc k tip. Mun c phn gii tinh hn th phi tng tng s mc in th vo (ngha l s dng nhiu in tr chia th hn) v tng s b so snh. Ni chung ADC nhanh N bit th cn 2N 1 b so snh, 2Nin tr, v logic m ho cn thit.Thi gian chuyn iB chuyn i nhanh khng cn thit tn hiu xung nhp v tin trnh ny xy ra lin tc. Khi gi tr u vo thay i th u ra ca b so snh s thay i lm cho ng ra ca b m ha thay i theo. Nh vy thi gian chuyn i l thi gian cn thit xut hin mt u ra s mi p li mt thay i VA. Thi gian chuyn i ch ph thuc vo khong tr do truyn ca b so snh v b m ha. V vy m ADC nhanh c thi gian chuyn i v cng gn.

IC chuyn i tng t s ADC0804

Chip ADC0804 l b chuyn i tng t s thuc h ADC800 ca hng National Semiconductor. Chip ny cng c nhiu hng khc sn xut. Chip c in p nui +5V v phn gii 8 bit. Ngoi phn gii th thi gian chuyn i cng l mt tham s quan trng khi nh gi b ADC. Thi gian chuyn i c nh ngha l thi gian m b ADC cn chuyn mt u vo tng t thnh mt s nh phn. i vi ADC0804 th thi gian chuyn i ph thuc vo tn s ng h c cp ti chn CLK v CLK IN v khng b hn 110s Cc chn khc ca ADC0804 c chc nng nh sau: CS (Chip select)Chn s 1, l chn chn Chip, u vo tch cc mc thp c s dng kch hot Chip ADC0804. truy cp ADC0804 th chn ny phi mc thp.RD (Read)Chn s 2, l mt tn hiu vo, tch cc mc thp. Cc b chuyn i u vo tng t thnh s nh phn v gi n mt thanh ghi trong. RD c s dng c d liu c chyn i ti u ra ca ADC0804.Khi CS = 0 nu c mt xung cao xung thp p n chn RD th d liu ra dng s 8 bit c a ti cc chn d liu (DB0 DB7).

WR (Write)Chn s 3, y l chn vo tch cc mc thp c dng bo cho ADC bit bt u qu trnh chuyn i. Nu CS = 0 khi WR to ra xung cao xung thp th b ADC0804 bt u qu trnh chuyn i gi tr u vo tng t Vin v s nh phn 8 bit. Khi vic chuyn i hon tt th chn INTR c ADC h xung thp.CLK IN v CLK RCLK IN (chn s 4), l chn vo ni ti ng h ngoi c s dng to thi gian.Tuy nhin ADC0804 cng c mt b to xung ng h ring. dng ng h ring th cc chn CLK IN v CLK R (chn s 19) c ni vi mt t in v mt in tr (nh hnh 2.3). Khi y tn s c xc nh bng biu thc:f =1/1.1RCVi R=10 kW, C=150pF v tn s f=606 kHz v thi gian chuyn i l 110 ms.

Ngt INTR (Interupt)Chn s 5, l chn ra tch cc mc thp. Bnh thng chn ny trng thi cao v khi vic chuyn i hon tt th n xung thp bo cho CPU bit l d liu chuyn i sn sng ly i. Sau khi INTR xung thp, cn t CS = 0 v gi mt xung cao xung thp ti chn RD a d liu ra.Vin (+) v Vin (-)Chn s 6 v chn s 7, y l 2 u vo tng t vi sai, trong Vin=Vin(+)Vin(-).Thng thng Vin (-) c ni ti t v Vin (+) c dng lm u vo tng t v s cchuyn i v dng s.VccChn s 20, l chn ngun nui +5V. Chn ny cn c dng lm in p tham chiu khi u vo Vref/2 h.

Vref/2Chn s 9, l chn in p u vo c dng lm in p tham chiu. Nu chn ny h th in p u vo tng t cho ADC0804 nm trong di 0 - +5V. Tuy nhin, c nhiu ng dng m u vo tng t p n Vin khc vi di 0 - +5V. Chn Vref/2 c dng thc hin cc in p u ra khc 0 - +5V.

Bng2.1 quan h in p Vref/2 vi Vin

D0 - D7D0 - D7, chn s 18 11, l cc chn ra d liu s (D7 l bit cao nht MSB v D0 l bit thp nht LSB). Cc chn ny c m ba trng thi v d liu c chuyn i ch c truy cp khi chn CS = 0 v chn RD a xung mc thp. tnh in p u ra ta tnh theo cng thc sau:

Mch ng dng chuyn i tng t s

Code m phngORG 0000H CLR C

TD: LCALL CDOI LCALL HEX_BCD LCALL BCD_7DOAN LCALL HIENTHI SJMP TDCDOI: CLR P3.6 SETB P3.6HERE: JNB P3.7,HERE CLR P3.5 MOV A,P1 RETHEX_BCD: MOV B,#10 DIV AB MOV 10H,B MOV 11H,A MOV A,#30

BCD_7DOAN: MOV DPTR,#900H MOV A,10H MOVC A,@A + DPTR MOV 22H,A MOV A,11H MOVC A,@A + DPTR MOV 23H,A MOV 21H,#09CH MOV 20H,#0C6H RETHIENTHI: MOV R0,#23H MOV A,#01H MOV R7,#04HT: MOV P0,@R0 MOV P2,A LCALL DELAY MOV P2,#00H DEC R0 RL A DJNZ R7,HT RETDELAY: MOV 7FH,#100 DJNZ 7FH,$ RETORG 900HDB 0C0H,0F9H,0A4H,0B0H,99H,92H,82H,0F8H,80H,90HEND