ADV7128
Reference Input
An external 1.23 V voltage reference is required to drive the
ADV7128. The AD589 from Analog Devices is an ideal choice
of reference. It is a two-terminal, low cost, temperature com-
pensated bandgap voltage reference which provides a fixed
1.23 V output voltage for input currents between 50
µA
and
5 mA. Figure 4 shows a typical reference circuit connection dia-
gram. The voltage reference gets its current drive from the
ADV7128’s V
AA
through an on-board 1 kΩ resistor to the V
REF
pin. A 0.1
µF
ceramic capacitor is required between the COMP
pin and V
AA
. This is necessary so as to provide compensation for
the internal reference amplifier.
A resistance R
SET
connected between R
SET
and GND deter-
mines the amplitude of the output video level according to the
following equation:
I
OUT
(mA) =
7,969
×
V
REF
(V)/R
SET
(Ω)
(1)
Using a variable value of R
SET
, as shown in Figure 4, allows for
accurate adjustment of the analog output video levels. Use of a
fixed 560
Ω
R
SET
resistor yields the analog output levels as
quoted in the specification page. These values typically corre-
spond to the RS-343A video waveform values as shown in
Figure 3.
ANALOG POWER PLANE
0.01µF
COMP
V
AA
+5V
will develop RS-343A video output voltage levels across a 75
Ω
monitor.
I
OUT
DAC
(CABLE)
Z
S
= 75Ω
(SOURCE
TERMINATION)
Z
L
= 75Ω
(MONITOR)
Z
O
= 75Ω
Figure 5a. Analog Output Termination for RS-343A
A suggested method of driving RS-170 video levels into a 75
Ω
monitor is shown in Figure 5b. The output current level of the
DAC remains unchanged, but the source termination resistance,
Z
S
, on the DAC is increased from 75
Ω
to 150
Ω.
I
OUT
DAC
Z
O
= 75Ω
(CABLE)
Z
S
= 150Ω
(SOURCE
TERMINATION)
Z
L
= 75Ω
(MONITOR)
Figure 5b. Analog Output Termination for RS-170
I
REF
~ 5mA
1kΩ
V
REF
TO DAC
RSET
More detailed information regarding load terminations for vari-
ous output configurations, including RS-343A and RS-170, is
available in an Application Note entitled “Video Formats & Re-
quired Load Terminations” available from Analog Devices,
publication no. E1228-15-1/89.
AD589
(1.235V
VOLTAGE
REFERENCE)
500Ω
R
SET
560Ω
GND
100Ω
Figure 3 shows the video waveforms associated with the current
output driving the doubly terminated 75
Ω
load of Figure 5a.
Gray Scale Operation
ADV7128
The ADV7128 can be used for stand-alone, gray scale (mono-
chrome) or composite video applications (i.e., only one channel
used for video information).
Video Output Buffer
*ADDITIONAL CIRCUITRY, INCLUDING DECOUPLING COMPONENTS,
EXCLUDED FOR CLARITY
Figure 4. Reference Circuit
D/A Converter
The ADV7128 contains a 10-bit D/A converter. The DAC is
designed using an advanced, high speed, segmented architec-
ture. The bit currents corresponding to each digital input are
routed to either the analog output (bit = “1”) or GND (bit =
“0”) by a sophisticated decoding scheme. The use of identical
current sources in a monolithic design guarantees monotonicity
and low glitch. The on-board operational amplifier stabilizes the
full-scale output current against temperature and power supply
variations.
Analog Output
The ADV7128 is specified to drive transmission line loads,
which is what most monitors are rated as. The analog output
configurations to drive such loads are described in the Analog
Interface section and illustrated in Figure 5. However, in some
applications it may be required to drive long “transmission line”
cable lengths. Cable lengths greater than 10 meters can attenu-
ate and distort high frequency analog output pulses. The inclu-
sion of output buffers will compensate for some cable distortion.
Buffers with large full power bandwidths and gains between 2
and 4 will be required. These buffers will also need to be able to
supply sufficient current over the complete output voltage
swing. Analog Devices produces a range of suitable op amps for
such applications. These include the AD84x series of monolithic
op amps. In very high frequency applications (80 MHz), the
AD9617 is recommended. More information on line driver buff-
ering circuits is given in the relevant op amp data sheets.
Use of buffer amplifiers also allows implementation of other
video standards besides RS-343A and RS-170. Altering the gain
components of the buffer circuit will result in any desired
video level.
–6–
REV. 0
The analog output of the ADV7128 is a high impedance current
source. The current output is capable of directly driving a
37.5
Ω
load, such as a doubly terminated 75
Ω
coaxial cable.
Figure 5a shows the required configuration for the output con-
nected into a doubly terminated 75
Ω
load. This arrangement
AD [ ANALOG DEVICES ]
