An Alternative DfT Methodology to Test High-Resolution ΣΔ Modulators

AN ALTERNATIVE DFT METHODOLOGY TO TEST HIGH-RESOLUTION 6' MODULATORS S. Escale a, J. M. Ga cía-González, O. Gue a, J. M. de la Rosa, F. Medei o, B. Pé ez-Ve dú and A. Rod íguez-Vázquez, (escale a, jmga cia, gue a, j osa, medei o, belen, [email p o ec ed]) Ins i u o de Mic oelec ónica de Se illa (IMSE-CNM-CSIC) A . Reina Me cedes, 41012-Se illa, SPAIN. Tl no.: 955056666 Fax:955056686 Abs ac In his pape , a no el D T me hodology o es high- esolu ion 6' Modula o s (6'M) is in oduced. The aim o he p oposal is o educe he es ime equi ed by con en ional me hodologies wi hou deg ading he accu acy o he esul s. A de ailed desc ip- ion o he addi ional ci cui y needed o pe o m hese es s is p esen ed as well as some ini ial simula ion esul s o show he u ili y o he app oach. 1.In oduc ion Al hough much e o has been paid in las yea s o ind a p ac- ical es me hodology o A/D and D/A con e e s, a he e y end, i has become a common p ac ice o use s anda d FFT ech- niques o he dynamic es and his og am-based app oaches o s a ic es o modula o s [1]. Un o una ely, bo h al e na i es e- qui e a huge amoun o da a when he esolu ion o he Ci cui s Unde Tes (CUTs) is beyond 15 bi s. This esul s in a ade-o among es ime and measu emen esolu ion ha imposes s ong limi a ions o he es se -up. To alle ia e his p oblem, a new me hodology, based on a di ide-and-conque app oach is in o- duced in his pape , paying special a en ion o he p ac ical ci - cui implemen a ion. Al hough a decomposi ion o he CUT in o analog and digi al subsys ems is, a leas a-p io y, easy o pe o m, u he decom- posi ion wi hin he analog pa is no so di ec . Howe e , a basic se o ules can be gi en: • Pe o m he pa i ioning aking in o accoun he speci ica- ions (i some speci ica ions can be p opaga ed o speci ic subsys ems, hei con ol can be pe o med by only check- ing he unc ionali y o hose subsys ems). • Pe o m he pa i ioning in subsys ems ha a e unc ionally independen o hei en i onmen . • Check ha subsys ems ha e non-o e lapping unc ionali y. • Decompose he sys em in o subsys ems ha can be es ed by well-known echniques. • Ca y ou he decomposi ion so ha sys em speci ica ions can be easily mapped in o subsys em speci ica ions. • Ca y ou he decomposi ion in a way ha es ing he sub- sys ems ake less ime han es ing he whole sys em. This se o ules jus i ies ha analog mac os based on a unc ion- al decomposi ion a e na u al candida es o es pa i ioning. Fo 6' A/D con e e s, an ob ious decomposi ion is spli ing he sys em in o he analog modula o and he digi al decima ion il e . Howe e , his le el o g anula i y is no enough when es applica ion ime is a c i ical issue. Tes ing modula o s is a ime- consuming ask and should be desi able a ine decomposi ion o he modula o . 2.Re e ence es ci cui and es me hods The me hodology p oposed in his pape is based on he idea ha a p ope decomposi ion o he modula o in o se e al blocks can be used o educe he ime in ol ed in he es o he whole s uc- u e. I is expec ed ha an adequa e es o he c i ical blocks in he modula o should educe d ama ically bo h he es ime and he es complexi y, o e ing also he possibili y o de ec he sou ces o e o s ha a ec he ci cui pe o mance, hus, p o- iding he ways o co ec hese aul s. Wo king ou di e en decomposi ion schemes will also equi e o combine wi h ano he ideas, o ins ance wi h he use o Sw- Opamps o any o he mul iplexing scheme. To de e mine he quali y o he p oposed es app oaches, a sen- so -in e ace 6'M will be used as he ci cui e e ence. The ci - cui has been implemen ed ollowing a cascade 2-1 a chi ec u e as shown in Fig. 1, unde he speci ica ions desc ibed below: • Resolu ion: DR = 17bi s, SNRpeak > 100dB • Digi al ou pu a e: 40kSample/s • Signal bandwid h= Bw = (0.1Hz-20kHz) • Tempe a u e ange: om -40oC o 175oC • Minimum powe consump ion Acco ding o he se o ules gi en abo e a numbe o c i ical blocks can be iden i ied: • Fi s , om all he in eg a o s, domina ing e o sou ces a ise mainly due o he i s one. The e o e, an exhaus i e es o con ol he i s in eg a o quali y will be o g ea in e es . • Second, al hough he quan ise non-ideali ies a e a enua ed by he gain o he in eg a o s, a es o check he pe o m- ance o his block will enable he de ec ion o aul s du ing he ab ica ion o he modula o . • Thi d, he las block o be es ed should be he DAC. This es is specially c i ical o mul i-bi quan ise s. Howe e , his is no he case he e, so his es should no be u he conside ed. Taking in o accoun hese esul s, h ee es p oposal should be Figu e 1. Block diag am o a 2-1 cascade 6' Modula o . i   DAC   DAC CANCELLATION LOGIC ou oi1 g 1 g ,1 g 2 g ,2 g 3 g ,,3 g ,3 ,;,((( ,6&$6 de ined and a e desc ibed now on. 2.1. Tes o he i s in eg a o A me hodology o de ec he sou ce o e o s ha deg ade he ci - cui pe o mance due o e o s in he i s in eg a o could be he ollowing: • Fi s , he i s in eg a o is con e ed in o an ampli ie . • Second, he wo emaining in eg a o s a e connec ed o o m a second o de 6'M. • Thi d, he inpu signal is connec ed o he inpu o he i s in eg a o (now ampli ie ) and, a e a delay, connec ed o he nega i e inpu o he second o de 6'M. Thus, he second o de 6'M will p ocess he e o signal gene a ed by he non- ideal pe o mance o he i s in eg a o . These s eps a e illus a ed in Fig. 2. Th ee di e en sou ces o e o s can be de ec ed using his a chi- ec u e: •DC gain e o s: using a pulse-coded inpu wi h e y low e- quency o elimina e he in luence o he se ling e o , he e o signal de ec ed a node oi1 will include in o ma ion abou he ini e gain o he ampli ie . I he In-Band-Noise (IBN) powe is measu ed o he 2nd o de modula o and he il e only, his IBN a he ou pu o he econ igu ed modula o can be compa ed o he one ob ained when he ampli ie is included in he ci cui . This compa ison enables he possibili y o gene a e a signa u e o decide i he DC gain e o o he i s ampli ie is o no wi hin he ole ance limi s. •Se ling e o : wi h he same s uc u e, now inc easing he equency o he inpu signal, i is possible o ca e o he e o in oduced by he se ling o he ampli ie . Again, a measu e o he IBN wi h and wi hou he inpu ampli ie allows o gene a e a signa u e wi h an ambigui y se o wo elemen s (DC gain e o and se ling o he i s ampli ie ). The in luence o he DC gain and se ling e o s has been simula - ed using ASIDES [2], and p elimina y esul s a e shown in Fig. 3(a). •The mal noise: i he inpu signal is a sine wa e wi h small ampli ude and low equency, by dec easing he equency sampling o he in eg a ion capaci o i is possible o inc ease he con ibu ion o he he mal noise signi ican ly, hus ena- bling i s di ec measu emen a he ou pu node. The ini ial esul s o his simula ion a e also shown in Fig. 3(b). 2.2. Tes o he quan ise In he case o a single bi , he quan ise block is educed o a com- pa a o . This kind o ci cui s has e o s domina ed by hei inpu o se and hys e esis. Howe e , due o he loca ion in o he modu- la o opology, hese e o sou ces a e la gely a enua ed by he DC gain o he in eg a o s. In spi e o his, o enable he isola ed es o he i s in eg a o i is impo an o check ha he quan isa- ion e o is accu a ely bounded. To his end, accessible nodes ha e o become a ailable in he ci cui , ollowing he app oach in Fig. 4. Then, i he modula o inpu is se o ze o and a es signal (sine-wa e s imulus) is applied, his will esul in an ou pu whe e he quan isa ion noise can be obse ed by pe o ming an FFT analysis wi h a educed se o samples. To make accessible his node, he applica ion o he Sw-Opamp concep desc ibed in he nex sec ion will be o g ea in e es . An example o he esul ing es is gi en in Fig. 5 o 16384 samples om he bi s eam. 2.3. Signal pa h deg ada ion An al e na i e o he me hodology desc ibed be o e, is ela ed o allow he obse abili y o some c i ical nodes o de ec he co ec ope a ion o he modula o . Fo he case unde s udy, he c i ical nodes a e loca ed a he ou pu o each in eg a o . I a bu e is added o hese nodes and an addi ional inpu is added o he ci cui o make i swi ch be ween es and no mal ope a ion mode, he signals a hose nodes can be obse ed. This al e na i e is illus a - ed in Fig. 6. 3.Building blocks o D T As in oduced in Sec ion 2, some ci cui y has o be added o he Figu e 2. Tes ing he pe o mance o he i s in eg a o . i   DAC  DAC oi1 Y1Y1  ou delay  SC-ampli ie con igu a ion 2nd-o de 6' Modula o Unused blocks T in = nT s i g1 g,1g2 g,2 g3 g,,3 g,3 0 2000 4000 6000 8000 10000 F equency (Hz) -175 -150 -125 -100 -75 -50 -25 0 PSD x eq. bin (dB) DCgain=100 DCgain=500 DCgain=1000 DCgain=2500 Figu e 3. DC gain e o s and noise o he i s in eg a o . 1e+02 1e+03 1e+04 1e+05 F equency (Hz) -175 -150 -125 -100 -75 -50 -25 PSD x eq. bin (dB) 0 1s -in eg a o ci cui noise measu emen (a) (b) Figu e 4. Tes signal injec ion in o he Quan ise . i   DAC oi1 + es signal ou pu g1g2 g,1g,2 , modula o i sel in o de o allow he di e en es me hodologies. The mos c i ical ci cui s a e hose ha a ec in some way o he signal pa h. Thus, he ou pu bu e s o con ol he p opaga ion and deg ada ion o he signal h ough he in eg a o s and he Sw- Opamps wi hin each in eg a o ha e been ca e ully designed o minimize he deg ada ion o he no mal ope a ion. 3.1. Analog ou pu bu e s The in e nal a chi ec u e o a bu e able o cope wi h he low-deg- ada ion equi emen s is shown in Fig. 7. The speci ica ions achie ed by his bu e a e: • DC Gain: -0.03 dB • -3dB: 30.9 MHz (wi h 10 pF ou pu load) • Powe : 0.68 mW I is impo an o poin ou ha his kind o blocks can be used as pa ame isable lib a y cells o be added o he ci cui which is de- si ed o es . 3.2. Sw-Opamps The idea he e [3],[4], is o sepa a e he link be ween he analogue blocks and he es s imuli by changing he ope a ional ampli ie s used o in eg a ion pu poses by Sw-Opamps, whe e wo di e en inpu s ages, con olled by a digi al signal, allow ha he ampli ie wo ks in no mal o in es mode. A way o ca y all his ask in an e icien , ye simple manne , is o use Sw-Opamps. They essen ially consis o ope a ional ampli i- e s in which he di e en ial inpu pai has been duplica ed, hus p o iding wo ope a ional modes, commanded by a digi al con ol signal: •No mal Mode: he ci cui ope a es as an opamp, wi h a di e en ial inpu and a single-ended (o di e en ial i equi ed) ou pu . •Bu e Mode: he ci cui ac s as a bu e , whe e he signal p esen a an ex a e minal is passed o he ou pu , isola ing in his mode he egula inpu signal. This is illus a ed in Fig. 8. To enable his modi ica ion, he di e en ial inpu pai o he am- pli ie has o be duplica ed and connec ed o he emaining ci cui - y by means o swi ches ha con ols he ope a ion mode. Then, a digi al con ol decides i he ope a ion o he ampli ie is es mode o no mal mode. Al hough his ope a ion can be pe o med by simply inse ing he swi ches as shown in Fig. 9, his al e na i e will esul in a signi - ican deg ada ion o he ci cui pe o mance due o he pa asi ic e- sis ance caused by S1,S2 in he signal pa h. The key poin he e is o design an Sw-Opamp ha minimizes he impac on he ci cui pe o mance, powe dissipa ion and a ea consump ion by inse ing he swi ches in app op ia e places. In Fig. 10 wo al e na i es a e shown, as in oduced in [5]. The i s one equi es eigh swi ches ex e nal o he ampli ie , while he second one equi es ou in e nal swi ches. In bo h cases he in- c ease o a ea is e y small. Also, he powe consump ion should no be inc eased since he wo ope a ion modes in ol e he same s uc u e. Th ee HSPICE simula ions ha e been done o compa e he o igi- nal beha iou (ampli ie wi hou a Sw-concep ) wi h he al e na- i es p oposed in Fig. 10. The esul s o his compa ison is summa ized in Table 1 o he olded-cascode opamp in Fig. 10, whe e he i s ow co esponds Figu e 5. Noise ans e unc ion e alua ion. Bu e MUX Bu e Bu e Figu e 6. Using bu e s o access o in e nal nodes. i   DAC   DAC CANCELLATION LOGIC ou g 1 g ,1 g 2 g ,2 g 3 g ,,3 g ,3 M 1 M 2 M 3 M 4 M 5 M 7 M 8 I b in o M 6 C c Figu e 7. Analog bu e p oposed o es objec i es. op on - + ip p in n Digi al con ol p, n: es da a inpu ip, in: no mal da a inpu Figu e 8. Basic in eg a o and SW-Opamp concep . op on - + ip in - + ip oi 1 p oi 1 n in es es es es S 1 S 2 S 3 S 4 Figu e 9. Inse ion o swi ches. , o he o iginal beha iou (Re ), he second o he al e na i e using in e nal swi ches (in-sw) and he hi d one o he al e na i e based on inse ing ex e nal swi ches (ex-sw). I can be seen om he simula ions how he o iginal beha iou is e y sligh ly a ied by he inclusion o he addi ional ci cui y. The inal implemen a ion o he i s ampli ie including he Sw- Opamp concep is shown in Fig. 11. Also, in Table 2, he simula ion esul s including in o ma ion abou he compa ison o he wo s -case beha iou in all he co - ne s es ed o he ope a ional ampli ie s, a e shown. Finally, he ope a ion o he ci cui in bu e mode has been es ed o ensu e ha he esolu ion is enough o p opaga e he s imulus signal wi hou a ec ing he es esul s. The ou come o his es is shown below: • Fi s Sw-ampli ie HD3 = -104.81 dB (17.47 bi s)1 • Second Sw-ampli ie HD3 = -101.67 dB (16.95 bi s) 4.Layou The layou o he p o o ype including he D T ci cui y is shown in Fig. 12. The D T ci cui y is highligh ed in he Figu e, showing ha he a ea o e head is small as compa ed wi h he o al a ea o he p o o ype. This p o o ype has been sen o ab ica ion and i is expec ed ha , in some weeks, p ac ical expe imen al esul s will be ob ained. 5.Conclusions In his pape , se e al al e na i es o he D T o high- esolu ion 6' modula o s ha depa om he usual echniques a e in oduced. Ci cui de ails a e gi en on how o implemen he di e en me h- odologies p oposed and p elimina y simula ion esul s a e shown while inal expe imen al esul s a e gene a ed. Re e ences [1] IEEE S anda d 1241-2000, IEEE S anda d o e minology and es me hods o analog- o-digi al con e e s, 2001. [2] F. Medei o, B. Pé ez-Ve dú, A. Rod íguez-Vázquez and J. L. Hue as, “A Ve ically In eg a ed Tool o Au oma ed Design o 6' Modula o s”, IEEE J. Solid-S a e Ci cui s, Vol. 30, No. 7, pp. 762-772, July 1995. [3] A. H. B a , R. J. Ha ey, A. P. Do ey and A. M. D. Rich- a dson, “Design- o -Tes s uc u e o acili a e es ec o applica ion wi h low pe o mance loss in non- es mode”, IEE Elec onics Le e s, Vol. 29, No. 16, pp. 1438-1440, Augus 1993. [4] D. Vázquez, A. Rueda and J. L. Hue as, “Fully Di e en ial Sw-Opamp o Tes ing Analog Embedded Modules”, P oc. IEEE In e na ional Mixed Signal Tes Wo kshop, IMSTW96, 15-18 May, Quebec (Canada), pp. 204-209, 1996. [5] G. N. S enbakken, T. M. Soude s and G. W. S ewa , “Am- bigui y g oups and es abili y”, IEEE T ans. on Ins umen- a ion and Measu emen , Vol. 38, No. 5, pp. 941-947, Oc o- be 1989. Table 1. Deg ada ion caused by he sw-concep . A0 (dB) GB (MHz) PM (deg) SR (V/Ps) A ea (Pm2) Powe (mW) Re 72.62 22.67 86.98 21.71 50.86 7.95 in-sw 72.61 22.59 84.49 21.71 51.6 7.96 ex-sw 72.54 22.52 86.43 21.71 51.6 7.94 Table 2. Wo s case ope a ions o i s and second ( hi d) ampli- ie s wi h and wi hou Sw-Opamp a chi ec u es. WC (1s ) WC (1s SW-OP) WC (2nd) WC (2nd SW-OP) a0 (dB) 71.09 69.93 65.13 64.51 gb (MHz) 13.99 13.73 21.08 20.55 PM (º) 85.50 83.33 82.61 76.45 SR (V/Ps) 21.10 20.21 30.23 29.90 os (V) 5.02 5.01 4.99 4.99 powe (mW) 7.24 7.78 2.48 3.06 M 1 M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M 10 M 11 I b - + o + o - M 12 M 13 M 15 M 14 M 16 M 17 AB Load I A =I I A =I/2 Load Ou pu Ou pu I B =I I B =I/2 Figu e 10. Folded-cascode opamp and possible sw- opamp al e na i es. Ou pu Load M 1 M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 9 M 10 M 11 I b - + o + o - M 12 M 13 M 15 M 14 M 16 M 17 AB M 1 M 2 M 5 + M 1 M 2 M 5 - Figu e 11. Sw-Opamp inal a chi ec u e. 1. HD3 is he mos dominan dis o ion e m due o he di e - en ial a chi ec u e chosen o he ampli ie s. Figu e 12. Layou o he 6'M including D T ci cui y. Bu e P og. D T Delay o Selec ion o Addi ional es mode Swi ches 1s ampli ie ,