ucb1300 NXP Semiconductors, ucb1300 Datasheet - Page 26

• Manufacturer Part Number: ucb1300
• Description: Advanced Modem/audio Analog Front-end
• Manufacturer: NXP Semiconductors

Philips Semiconductors
ON-CHIP REFERENCE CIRCUIT
The UCB1300 contains an on-chip reference voltage source, which generates the reference voltages for the 10-bit ADC
and the virtual analog ground. Alternatively the UCB1300 can be driven from an external reference voltage source.
The internal reference voltage is connected to the VREFBYP pin, where an external capacitor could be connected to filter
this reference voltage, if VREF_CON (register 10) is set. THIS IS NOT RECOMMENDED since the internal impedance
of the reference (several 100 M ) will be sensitive to board leakage and the turn on time constant will be very long. When
choosing a capacitor, the internal impedance (around 50 k ) should be taken into account.
An external voltage reference connected to the VREFBYP pin is used as voltage reference by the UCB1300 circuit, if the
EXT_REF_ENA bit (register 10) is set. Two bits in the ADC control register determine the mode of operation of this
reference voltage circuit. VREFBYP_CON connects the internal reference voltage to the VREFBYP pin, while
EXT_VREF_ENA disables the internal reference voltage and switches the UCB1300 into the external voltage reference
mode.
2000 Jun 26
Advanced modem/audio analog front-end
tsc_bias_ena
aud_out_ena
vrefbyp_con
aud_in_ena
tel_out_ena
ext_vref_ena
tel_in_ena
adc_en
internal
bandgap
reference
voltage
circuitry
ena
Vbg
Fig.20 Block diagram of the reference circuit.
&
26
&
internal
analog
ground
internal
ADC
reference
vrefbyp
Product specification
UCB1300