AD723 Analog Devices, AD723 Datasheet - Page 14

• Manufacturer Part Number: AD723
• Description: 2.7 V to 5.5 V RGB-to-NTSC/PAL Encoder with Load Detect and Input Termination Switch
• Manufacturer: Analog Devices

AD723
Basic Connections
Some simple applications will not require use of all of the fea-
tures of the AD723. In such a case, some of the pins must be
connected to appropriate levels such that the rest of the device
can operate. Figure 7 is a schematic of a very basic connection
of the AD723.
The following pins do not require any connection and can be
left open circuited if their function is not needed:
Inputs to a CMOS device should never be left floating, even if
their function is ignored. The following inputs should be dealt
with accordingly:
Pin 1, STND—can be hard-wired either high or low, if only
either NTSC or PAL output is desired.
Pin 2, SA—For most systems, this pin should be tied low (ground).
Some of the video standards used in South America can be
enabled by a high logic level on this pin.
Pin 3, CE—If continuous enabled operation is desired, this pin
can be hard-wired to a high logic level.
Pin 4, TERM—This signal should be tied low (ground) if the
on-chip termination switches are not used.
75
14.31818MHz-NTSC
17.734475MHz-PAL
Pin 9, RT
Pin 10, GT
Pin 11, BT
Pin 18, TVDET
Pin 21, YTRAP
HSYNC AND VSYNC OR
CSYNC AND POLARITY
SOURCE
R, G, B
FROM
HIGH FOR NTSC
LOW FOR PAL
(SEE TEXT)
NC = NO CONNECT
3V
75
75
75
10 F
0.1 F
0.1 F
0.1 F
NC
NC
NC
+
0.1 F
STND
SA
CE
TERM
RIN
GIN
BIN
RT
GT
BT
HSYNC
VSYNC
4FSC
DVDD
AGND
3V
TGND
AD723
AVDD
3V
DGND
0.1 F
CVSET
YTRAP
AVDD1
TVDET
AGND
CSET
YSET
3V
CV
C
Y
0.1 F
NC
NC
+
10 F
374
301
374
301
374
301
TO 75
TEMINATION
TO 75
TEMINATION
TO 75
TEMINATION
Most systems will use only one output type at a time—either
composite video or S-video. In such a case, it is desirable that
unused outputs go to their power-down state. The only compo-
nent necessary for these outputs is a resistor of 300 Ω from the
appropriate XSET pin to ground. If no load is detected on the
output pin, the corresponding output stage will be powered
down to minimum current.
PC Graphics Interface
The AD723 has an extended feature set that simplifies the task
of generating composite TV output signals from a PC from the
conventional RGB and sync outputs. In order for this to function,
however, the RGB output scanning must be interlaced and at the
proper scanning frequencies for either NTSC or PAL operation.
Figure 8 shows the connections for interfacing to a PC graphics
chipset. The RGB signals now must serve two different destina-
tions and two different termination conditions.
There is a direct path from the RGB signals to the RGB moni-
tor. This is the conventional path, and the presence of the AD723
should not interfere with it. The RGB signals are doubly shunt-
terminated by the 75 Ω resistors near the graphics chip and the
75 Ω terminations in the monitor. This situation does not require
any additional termination, so the TERM pin of the AD723
should be low so that the termination switches are turned off.
If the TV output is desired, there are two possibilities: either the
RGB monitor will be plugged in or, since it is not necessary, it
can be removed. The case where it is plugged in has the same
termination scheme as above, so the TERM signal should be
low to prevent switching in any additional termination.
However, if the RGB monitor is unplugged, there is only one set
of shunt terminations on the RGB signals. In this case, TERM
should be switched high (3 V). This will provide the second termi-
nation by switching the three 75 Ω resistors to ground.
General-purpose outputs (GPO) are used from the I/O control-
ler device to control the logic inputs to the AD723: TERM, CE,
SA, and STND. Any of these can be hardwired in the desired
state if it is not going to be changed in normal operation. A
general-purpose input (GPI) can be used to monitor TVDET if
this feature is used.
The RGB signals are ESD-protected by the diodes to the sup-
plies. The Pi networks on these signal lines prevent EMI from
radiating from the monitor cable.
Low Cost Crystal Oscillator
A low cost oscillator can be made that provides a CW clock that
can be used to drive both the AD723 4FSC and other devices in
the system that require a clock at this frequency. Figure 9 shows
a circuit that uses one inverter of a 74HC04 package to create a
crystal oscillator and another inverter to buffer the oscillator and
drive other loads. The logic family must be a CMOS type that
can support the frequency of operation, and it must NOT be a
Schmitt trigger type of inverter. Resistor R1 from input to out-
put of U1A linearizes the inverter’s gain such that it provides
useful gain and a 180 degree phase shift to drive the oscillator.