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ranye of IREf is 60 (at the intersection ot the load
line, RREF, and the MAX. curve) to
8U~A
(at the intersec-
tion of RREF and MIN.) when RREF is 5Ukn and VDD is 5V.
Under
'
this condition, the ranye of IAVU is l.Y to 2.YmA
because IVAO is 32 to 36 times that of IREF. Also, if the
impedance is hiyh, fluctuations in the output level of
the synthesized spee~h are about 4dH, which is not so high.
Therefore, a highly precise fixed resistor can be used as
RR'
E F instead of the variable resistor except for occa-
sions when the synthesized speed output level must be
constant. In contrast, if the impedance of the amplifier
connected to the AVO pin is low, determine RREF by
referring to Fig. 3-4. In this fiyure, the ranye of
IAVO at a
VOD
of 5V and a RREF of 1kn is 12 to 3UmA. If
the impedance is
low, fluctuations
in'
output levels of
the synthesized speech are increased to about 8dB.
Therefore, using a variable resistor such as RREF is
recommended.
VDD
VDD
RREF
pPD7755,pPD7756
REF
IIREF
I
----;-\
IVA
AVO
- - lAva
~
conver-
ter
Fi~.
3-2
O/A Converter Reference Current
3-3
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