2001 Apr 17
65
Philips Semiconductors
Product specification
Digital telephone answering machine chip
PCD6001
14.2
Analog Voltage Source (AVS)
The analog voltage source generates the following
voltages:
A precise reference voltage V
REF
. The value in register
VREFR determines the V
REF
. In the application this
voltage should be tuned to 2000 mV, since it will
determine the absolute accuracy of the auxiliary
analog-to-digital and digital-to-analog conversion. V
REF
is the direct output of an opamp which can source an
output current, and not sink. An external capacitor
should be connected between V
REF
and V
SSA
for
stability and noise performance. The reference voltage
can also directly supply an external electret microphone
via pin V
MIC
. The switch between V
REF
and V
MIC
is
controlled via bit 4 in the SYMOD special function
register.
An analog output voltage DAOUT. This voltage can be
set between approximately 8 mV (1 LSB = V
REF
/256)
and V
REF
(= 2000 mV) by changing the contents of
register GPDAR. This large range is possible when no
opamp is used.
This causes a relatively high output resistance with a
settling time of about 10 ms. The dynamic switching of
DAOUT causes the output resistance to be dependent
of the actual load on DAOUT. This effect can be
cancelled if an external capacitor larger than 500 pF
between DAOUT and V
SSA
is applied. This will however
result in a slower settling time of the output voltage, to
about 30
μ
s.
The internal analog common mode voltage V
acm
, used
in the CODEC.
The internal voltage V
adc
is used only when an
analog-to-digital conversion is executed.
As mentioned above, for highest analog performance the
reference voltage V
REF
has to be adjusted in the
application to 2000 mV. For this purpose the VREFR SFR
has been defined. The reset state should ensure that the
reference voltage is about 2000 mV on a typical device.
Exact adjustment has to be done under software control
using the VREFR register, where increasing the V
REFR
value will decrease the reference voltage.
14.2.1
V
OLTAGE
R
EFERENCE
R
EGISTER
(VREFR)
Table 57
Voltage Reference Register (SFR address BAH); reset state A0H
7
6
5
4
3
2
1
0
VREF.7
VREF.6
VREF.5
VREF.4
VREF.3
VREF.2
VREF.1
VREF.0