PCD6003 datasheet(65/96 Pages) PHILIPS

65 / 96 page

2001 Apr 17

Philips Semiconductors

Product speciﬁcation

Digital telephone answering machine chip

PCD6003

14.2

Analog Voltage Source (AVS)

The analog voltage source generates the following

voltages:

• A precise reference voltage VREF. The value in register

VREFR determines the VREF. 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. VREF

is the direct output of an opamp which can source an

output current, and not sink. An external capacitor

should be connected between VREF and VSSA for

stability and noise performance. The reference voltage

can also directly supply an external electret microphone

via pin VMIC. The switch between VREF and VMIC is

controlled via bit 4 in the SYMOD special function

• An analog output voltage DAOUT. This voltage can be

set between approximately 8 mV (1 LSB = VREF/256)

and VREF (= 2000 mV) by changing the contents of

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 VSSA is applied. This will however

result in a slower settling time of the output voltage, to

about 30

µs.

• The internal analog common mode voltage Vacm, used

in the CODEC.

• The internal voltage Vadc is used only when an

analog-to-digital conversion is executed.

As mentioned above, for highest analog performance the

reference voltage VREF 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 VREFR

value will decrease the reference voltage.

14.2.1

VOLTAGE REFERENCE REGISTER (VREFR)

Table 57 Voltage Reference Register (SFR address BAH); reset state A0H

76543210

VREF.7

VREF.6

VREF.5

VREF.4

VREF.3

VREF.2

VREF.1

VREF.0

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