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
,