1V CMOS Sub h eshold Log-Domain Pulse Du a ion
Modula o s
F ancisco Se a-G aells ([email p o ec ed])
Cen o Nacional de Mic oelec ´onica, Ins i u de Mic oelec `onica de Ba celona
Jos´e Luis Hue as ([email p o ec ed])
Cen o Nacional de Mic oelec ´onica, Ins i u o de Mic oelec ´onica de Se illa
Abs ac . A new CMOS ci cui s a egy o e y low- ol age Pulse-Du a ion
Modula o s (PDM) is p oposed. Op imiza ion o ol age supply scaling below he
sum o h eshold ol ages is based on Ins an aneous Log Companding p ocessing
h ough he MOSFET ope a ing in weak in e sion. A 1V VLSI PDM ci cui o
e y low- ol age audio applica ions such as Hea ing Aids is p esen ed, showing good
ag eemen be ween simula ed and expe imen al da a.
Keywo ds: Low-Vol age, CMOS, Sub h eshold, Log, PDM
1. In oduc ion
Pulse-Du a ion Modula ion (PDM) p ocessing is o special in e es
in ou pu powe s ages o Class-D d i ing o low-impedance loads.
Pa icula ly, po able low-powe VLSI sys em-on-a-chip applica ions
like Hea ing Aids equi e his ype o signal modula ion in o de o
ex end ba e y li e. Un o una ely, mos exis ing CMOS solu ions do
no allow good-enough low- ol age compa ibili y o eal single ba e y
cell ope a ion (down o 1.1V) [1, 2, 3], equi ing supply mul iplie s
which end o dec ease powe e iciency, and inc ease bo h ex e nal
componen s and Si a ea o e head as well. O he epo ed p oposals
make ex ensi e usage o esis o s o bipola de ices [4, 5]. A no el e y
low- ol age CMOS ci cui s a egy o PDM is p esen ed he e in he
ame o he gene al Ins an aneous Companding heo y[6, 7, 8, 9]. The
p oposed implemen a ion is based on he MOS ansis o ope a ing in
he weak in e sion egion o op imize bo h he ol age supply scaling
and he cu en consump ion.
2. P inciple o Ope a ion
In gene al, a PDM signal (you ) can be ob ained om a 1-bi compa i-
son be ween he base-band inpu (yin) and a highe equency iangle
c
°2002 Kluwe Academic Publishe s. P in ed in he Ne he lands.
alog990.02. e ised. ex; 6/06/2002; 11:22; p.2
(c) 2003 Kluwe Academic Publishe s h p://dx.doi.o g/10.1023/A:1022545414777
1V CMOS Sub h eshold Log Domain Pulse Du a ion Modula o s 3
wa e o m (y i) as depic ed in Figu e 1. The signal y i may be gene -
a ed by in eg a ing a cons an e e ence (ycons ), which is pe iodically
in e ed acco ding o an ou pu window (y hmin,y hmax).
(Place o Figu e 1)
Due o he e y low- ol age capabili ies wan ed o he ci cui , a
Log Companding app oach is selec ed he e o comp ess he in e nal
ol age dynamic ange. A his poin , he MOS ansis o ope a ing
in sub h eshold egion is p oposed as he main basic building block
o implemen such ype o signal p ocessing. In pa icula , he Ga e-
D i en (GD) law om he gene al s udy p esen ed by hese au ho s
in [10] will be used he e as he companding unc ion (F):
I=F(V) = ISe−
VT O+nVbias
nU eV
nU IS= 2nβU2
(1)
whe e he Th eshold Vol age (VT O), Cu en Fac o (β), Sub h esh-
old Slope (n) and Speci ic Cu en (IS) a e de ined acco ding o he
EKV de ice model [11]. Now, he ex e nally linea desc ip ion in he
y-domain (i.e. cu en I) o Figu e 1 mus be ansla ed in o he com-
p essed in e nal signal (i.e. ol age V). Since compa a o s a e easily
implemen ed in cu en mode, e o s a e ocused on he syn hesis in
he comp essed V-domain o he ollowing in eg a o equa ion:
dI i
d =±Icons
τI hmin < I i < I hmax (2)
whe e τs ands o a gene ic Time Cons an . A e applying he
Companding unc ion F om (1), he abo e s a e-space equa ion is
ansla ed o:
dV i
d =±nU
τe
Vcons −V i
nU (3)
In case o s o ing he comp essed signal V i ac oss a g ounded lin-
ea capaci o (C), exp ession (3) can be unde s ood as a non-linea
ansconduc ance d i ing C:
CdV i
d
| {z }
Icap
=±I une
Vcons −V i
nU I un .
=nU C
τ(4)
whe e I un s ands o he uning cu en , which con ols he pe iod
o he iangula oscilla o (T i) acco ding o:
T i = 2nU C
I un µI hmax −I hmin
Icons ¶(5)
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4F.Se a-G aells and J.L.Hue as
3. Low-Vol age CMOS Implemen a ion
The in eg a o equa ion in (4) can be implemen ed h ough he basic
building blocks de eloped by hese au ho s in [10], which a e a gene al-
iza ion o [12]. Howe e , in ou pa icula case o cons an inpu (Icons ),
a mo e compac ealiza ion is p oposed in Figu e 2, whe e all ma ked
MOS pai s a e assumed o ope a e in weak in e sion sa u a ion.
(Place o Figu e 2)
The iangle gene a o is displayed a he uppe pa o he igu e
and buil h ough ansis o s M1 o M18. This oscilla o consis s o : he
inpu comp esso M1 gene a ing he comp essed cons an inpu Vcons ,
he non-linea ansconduc ance pai M4-M5 d i ing he in eg a o
capaci o C, and he ou pu expande M2 ha gene a es he iangle
wa e o m I i. In o de o ob ain he desi ed I i, he expande equi es
he same e e ence le el V e o he comp esso (i.e. same sou ce ol -
ages be ween M1 and M2). Such low-impedance ol age copy is supplied
by M3 and M12. T ansis o M11 is used o ensu e p ope ope a ion o
g ounded de ices M12 and M13. Oscilla ion is o ced by he eedback
window compa a o M15-M16, which al e na i ely changes he sign
o he slope s o ed in a D- ype lip- lop (DFF). A sa e s a up signal
(Vs a ) should be equi ed a he lip- lop o a p ope ini ializa ion.
Such memo y elemen se s he cha ge and discha ge phases o he
capaci o Cby swi ching he mi o M9-M10, so Icap =±ID4. The
esul ing Log comp essed and expanded iangle wa e o m can be seen
in Figu e 3.
(Place o Figu e 3)
An ope a ional ans esis ance ampli ie , made o de ices M19 o
M24 in he lowe pa o Figu e 2, is also included o ensu e low-enough
inpu impedance. Such ea u e is o special in e es in case o equi ing
an addi ional V/Icon e sion o he incoming signal, which can be pe -
o med wi h jus a simple se ies esis o (and e en ually a decoupling
capaci o ) as indica ed in he same igu e in dashed lines. The ou pu
PDM signal (VP DM ) is compu ed om he 1-bi ol age quan iza ion
o (I i −Iin). A p ope quiescen bias Ibias = (I hmax +I hmin)/2 mus
be designed o ob ain a 50% PDM ou pu in silence. Also, a ypical
s uc u e o audio applica ions is depic ed in Figu e 2, whe e a digi al
bu e s age is ed by he VP DM signal o ope a e he ou pu ecei e
in Class D.
alog990.02. e ised. ex; 6/06/2002; 11:22; p.4
1V CMOS Sub h eshold Log Domain Pulse Du a ion Modula o s 5
4. Design Example
Based on he ci cui s a egy p oposed in p e ious sec ion, a 100KHz
Class-D ou pu s age o 1.1V VLSI Hea ing-Aids-on-a-chip has been
implemen ed. In his case, a single slope (i.e. saw oo h) oscilla o has
been buil wi h design pa ame e s Ibias = 1µA, I un = 100nA, C= 5pF
and I h = 4µA. The comple e PDM ci cui has been in eg a ed in a
1.2µm VLSI CMOS double-me al double-polySi p ocess as depic ed
in Figu e 4, whe e de ice a ea is abou 0.10mm2wi hou conside -
ing digi al powe bu e s. Bo h BSIM3 simula ed and expe imen al
pe o mances a e epo ed in Table I unde ac ual ba e y supply op-
e a ion (1.1V o 1.5V). All boxed de ices in Figu e 2 we e sized a
10 ×40µm
3µm o ensu e weak in e sion ope a ion o he wan ed ange
o cu en s. On he o he hand, he emaining MOS ansis o s we e
designed wi h aspec a ios as long as allowed by he supply ol age o
imp o e D ain-cu en ma ching be ween de ices, so ou pu o se s. In
his sense, bo h Mon eca lo simula ions and expe imen al da a e u n
du y-cycle de ia ions (2σ) o less han 1%. Due o he a ge applica ion
in he ield o Hea ing Aids (i.e. low empe a u e- ange, low- ol age
supplies and ecei e s impedances >50Ω), he esul ing s a ic-powe
consump ion is qui e accep able (i.e. <100µA). In case o equi ing
na owe du y-cycle de ia ions, some eedback can be applied om
he ou pu bu e s o he inpu o he p oposed modula o in o de o
p o ide a sel -biasing mechanism, like [13]. Finally, an elec ical PDM
s imulus sample and he co esponding acous ic ou pu a he ecei e
a e displayed in Figu e 5. The ha monic dis o ion measu ed is qui e
sa is ac o y o in eg a ed audio applica ions.
(Place o Figu e 4)
(Place o Table I)
(Place o Figu e 5)
5. Conclusions
The new CMOS design s a egy p oposed o implemen PDM modu-
la o s allows s ong ol age supply scaling-down below VT ON +|VT OP |.
A compac ci cui s uc u e has been epo ed wo king a e y low
alog990.02. e ised. ex; 6/06/2002; 11:22; p.5
6F.Se a-G aells and J.L.Hue as
cu en . The ci cui was ully in eg a ed in s anda d CMOS echnology
and i s expe imen al da a was ound in good ag eemen wi h simula-
ion. As a esul , his modula o is sui able o e y low- ol age VLSI
audio applica ions like Hea ing-Aids-on-a-chip (1.1V o 1.5V).
Acknowledgemen s
Resea ch pa ially suppo ed by Spanish CICYT-TIC97-1159, CICYT-
TIC99-1084 and Eu opean ESPRIT-FUSE-23068 p ojec s.
Re e ences
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[5] H.A.Gu can: 1993, ‘Class D BiCMOS Hea ing Aid Ou pu Ampli ie ’. U.S.
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[7] D.R.F ey: 1993, ‘Log-Domain Fil e ing: an App oach o Cu en -Mode
Fil e ing’. IEE P oceedings 140(6), 406–415.
[8] C.Toumazou, J.Nga mnil, and T.S.Lande: 1994, ‘Mic opowe Log-Domain
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[9] J.Mulde , A. de Woe d, W.A.Se dijn, and A. Roe mund: 1997, ‘Gene al
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[10] F.Se a-G aells: 2000, ‘VLSI CMOS Low-Vol age Log Companding Fil e s’.
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[11] C.C.Enz, F.K ummenache , and E.A.Vi oz: 1995, ‘An Analy ical MOS
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1V CMOS Sub h eshold Log Domain Pulse Du a ion Modula o s 7
[12] M.Punzenbe ge and C.Enz: 1996, ‘A New 1.2V BiCMOS Log-Domain In-
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8F.Se a-G aells and J.L.Hue as
Lis o Tables
I Typical esul s o he modula o a oom empe a u e. 9
alog990.02. e ised. ex; 6/06/2002; 11:22; p.8
1V CMOS Sub h eshold Log Domain Pulse Du a ion Modula o s 9
Table I. Typical esul s o he modula o a oom empe a u e.
Pa ame e Simula ed Expe imen al Uni s
Min. Supply Vol age 1.1 1.1 V
Max. VT ON +|VT OP |1.3 1.2 V
Powe Consump ion 45 <50 µW
PDM Fac o 2.6 2.7 µs/µA
Inpu Full Scale 4 3.9 µApp
Dynamic Range >70 >63 dB
THD @90% Full Scale <2.8 <3 %
Pulse F equency 102 104 KHz
Du y-cycle De ia ions (2σ)<1<1 %
alog990.02. e ised. ex; 6/06/2002; 11:22; p.9
10 F.Se a-G aells and J.L.Hue as
Lis o Figu es
1 Gene al design app oach o PDM gene a ion. 11
2 Simpli ied schema ic o he PDM modula o . 12
3 Simula ed ol age ac oss in eg a o capaci o (uppe )
and ou pu cu en (lowe ) ope a ing a 1.0V supply. 13
4 Mic oscope pho og aph o he PDM modula o . 14
5 Expe imen al di e en ial PDM and acous ic audio signal
a ecei e o Iin=2µApp@4KHz. 15
alog990.02. e ised. ex; 6/06/2002; 11:22; p.10
1V CMOS Sub h eshold Log Domain Pulse Du a ion Modula o s 17
Add ess o O p in s:
D .F ancisco Se a-G aells
mail o:[email p o ec ed]
h p://www.cnm.es/~pse a
Tel: +34 93 594 77 00
Fax: +34 93 580 14 96
Cen o Nacional de Mic oelec ´onica
Ins i u de Mic oelec `onica de Ba celona
Campus UAB
08193 Bella e a
Spain
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