PRINCIPLES
OFOPERATION
The
K-I002-Z
I Bit Aud'io
System
consists
of three dist'inct sections. The
8
bjt digital-to-analog
converter
(DAC)
acceptsan 8 b'it b'inaryinput from the user
port on
the PET
and
produces
a DCvoltage
d'irectly
proportional
to the unsigned
binary valueof the input. The
6-po1elowpassfilter blocks
all sanpling
di
stortion frequencjes
above
approx'imate'ly
3.5kHz. Thi
s f il ter j
s necessary
f
or
cleansounding
musicfrom the DAC. Theaudio
power
anp'lifier boosts
the filter
output signa'l
to the level required
for drjv'inga speaker. The
CB2
pin on
the PET
user
port is also tied'into the anplifier circuitry. A volume
control determines
the gain
of the amplifjer and
thus
the volumeof the soundreproduced'inthe
speaker. In order for the boardto operatesolely from
a s'ing1e5 volt power
supply,
several
jnnovative
circujt techn'iques
have
beenincorporated.
Lookingat the DAC
sectjon
first (1eft portjon
of the schematic
draw'ing),
it
is seen
that the weighted
resjstor methodof conversionis used. The
Clv1OS
buffers
in Ul and
U2make
very
good
analog
switches
whichswitchthejr outputsbetween
exactly
lfound
and
exactly
the supp'lyvoltage
(+5
volts) jn responseto the input
s'igna1.
The
only error in thjs switchingact'ion
is a f inite output impedanceof
approximatel
y 200ohms. Ctvl0Sbuff
ers, rather
than
i nverters, are usedbecause
the
two stagesof "gain" internally assurescompleteswitchingof the output
even
jf
the input swingsless than 5 volts. TheDACnetwork
produces
an output vo'ltage
d'irectly wi
th a source
impedance
of approximately
6.25K. tl'ith
al
I zeroes
input,
the output is zero
volts; with all ones
input, the output
is 5 volts. Loading
the
output
hasno
effect on
ljnearity but it will reducethe s'igna'lswing.
In order to insure
accurate,monotonic
performance
of the DAC,the most
si
gn'if
jcant bit js actually
f
our ClvlOS
gates
and
four 51Kres'istors in parallel
whjle
the next mostsign'ificant b'it is two in paralle'I. Theremain'ing
b'its are
single gates
sjnce
the ratio of the wejghtingresjstors to the gate
output
'impedance
is large
enough
to ignore. By
using
para'l'lel
andseries combinations
of
51K
resistors for the most
sjgnificant 5 bits, jt'is possible
to use
relatively
jnaccurate
res'istors
in the DAC
andstill achieve
1/4 LSBlinearity wh'ich
js about
.4. This'is dueto statistjcal averag'ing
among
the resistors, particularly the
critical most
significant b'it. Even
so, factory assembledunits have
hadthe 51K
resjstors matchedto w'ithin 1%. Althoughthe
morecommon
R-ZR
res'istor
ladder
networkcould have
been
used,moreresjstors wouldhavebeen
requiredto get the
sane
degnee
of statist'ical match'ing.
An integrated
circuit DAC
was
not practical
sjnce
all that are currently ava'ilablerequire a negative
supplyvoltage
for
either the DAC
itself or for a current-to-voltage
converter
operational
anplifier.
Note
that the 5 volt power
supp'ly
is fjltered andusedas a reference
for the DAC.
Wh'ilesmall anounts
of noise
are
fi'ltered out, 60Hzripp'le on
the 5 volt supply
is
I
jkely to result in hum
from the speaker.
Thefilter c'ircu'it is wherethings start getting
unconventional.
Thef ilter
actually consists of three.two-po1e
stagesconnected
'in
cascade.
Eachsection
js
a resonantlowpassfilter, 'i.e., the response
curve
may
peak
somewhat
just before
cutoff. t^lith
proper
selection of sectioncutoff frequencies and
Q
f
actors
(peaking),
a very nearly
flat passband
andsharp
cutoff is obtained. Passband
ri pple
'is less than .5dBand
the cutoff slope
is such
that 30dBattenuat
jon 'is
obta'ined
at just 1.35tjmes the cutoff frequencyof 3.5kHz. Note
that the 5
volt
sw'ingof the
DAC
'is
reduced
to about
2,5 volts through
the filter by virtue of the
22W,'input
resistor to the f irst fjlter stage.
Eachfilter sectjon'is implsnentedas a biquadratic
fjlter which
consjsts
of
an invertjng surnmingamplif
jern d leaky inteE"fEEb7i-ffi?an ideal integrator all
connected'ina 1oop. Althoughthree operationalanplifiers are
required
for the
ci
rcu'it, 'its advantagesare
many. In parti
cular, hi
gh
Q
f
actors are
possi
bl
e wi
th
modest
anpf
ifier gain. In addjtion, sensjtivity of the responsecurve
to
componenttolerance'is very
1ol. These
characteristicsallol the useof linearly
bjased
Cl{S
gates
as
jnverting
operationalanpf
if iers. Perf
ormance
of the f ilter
using
the CtflS
gates
'is
indist'inguishablefrom the performance
using
true op-anps
such
as
the 741. See
the NationalSemiconductor
CISS
data
bookfor more
jnformation
on
linear CIOSapplicat'ions. 3
