U2731B Datasheet, PDF (10/25 Pages) ATMEL Corporation – DAB ONE- CHIP FRONT END

Switching Outputs
D/A Converters
A high-gain amplifier (output pin: VD), which is implemented in order to construct a loop
filter, as shown in the application circuit, can be switched off by means of the two-wire
bus bit OS.
An internal lock detector checks if the phase difference of the input signals of the phase
detector is smaller than approximately 250 ns in seven subsequent comparisons. If
phase lock is detected, the open collector output pin PLCK is set to H (logical value). It
should be noted that the output current of this pin must be limited by external circuitry as
it is not limited internally. If the two-wire bus bit TRI is set to H, the lock detector function
is deactivated and the logical value of the PLCK output is undefined.
Three switching outputs controlled by the two-wire bus bits SWA, SWB, SWC can be
used for any switching task on the front-end board. The currents of these outputs are not
limited internally. They have to be limited by an external circuit.
Three D/A converters, A, B and C, offer the possibility of generating three output volt-
ages at a resolution of 8 bits. These voltages appear at the output pins CAO, CBO and
CCO. The converters are controlled via the two-wire bus interface by means of the con-
trol bits CA0, ..., CA7, CB0, ..., CB7 and CC0, ..., CC7 respectively as described in the
section “Two-wire Bus Instruction Codes”. The output voltages are defined as
7
∑ ^VCAO
=
-^V-----^M---
128
×
CAj × 2^j
j=0
7
∑ ^VCBO
=
-^V-----^M---
128
×
CBj × 2^j
j=0
7
∑ ^VCCO
=
-^V-----^M---
128
×
CCj × 2^j
j=0
where VM = 4.25 V nominally. Due to the rail-to-rail outputs of these converters, almost
the full voltage range from 0 to 8.5 V can be used. A common application of these
converters is the digital synthesis of control signals for the tuning of preselectors. The
output pins CAU, CBO and CCO must be blocked externally with capacitors (100 nF) as
shown in the application circuit (see Figure 8 on page 20).
10 U2731B
4671C–DAB–06/04