Table II - Video Control Operation (Output values for set-up = 10 IRE and 75 ohm standard load)
Sync
1
0
Blank
X
1
Ref
White
X
X
Bright
X
X
Data
Input
X
X
Out - (mA)
28.57
20.83
Out - (V)
-1.071
-0.781
Out - (IRE)
-40
0
Description
Sync Level
Blank Level
0
0
0
0
1
1
1
0
X
X
0.00
1.95
0.000
-0.073
110
100
Enhanced High Level
Normal High Level
0
0
0
0
0
0
0
0
000...
111...
19.40
1.95
-0.728
-0.073
7.5
100
Normal Low Level
Normal High Level
0
0
0
0
0
0
1
1
000...
111...
17.44
0.00
-0.654
0.000
17.5
110
Enhanced Low Level
Enhanced High Level
The SPT1018 is usually configured in the synchronous
mode. In this mode, the controls and data are synchronized
to prevent pixel dropout. This reduces screen-edge distor-
tions and provides the lowest output noise while maintaining
the highest conversion rate. By leaving the Feedthrough (FT)
control open (low), each rising edge of the convert (CONV)
clock latches decoded data and control values into a D-type
internal register. The registered data is then converted into
the appropriate analog output by the switched current sinks.
When FT is tied high, the control inputs and data are not
registered. The analog output asynchronously tracks the
input data and video controls. Feedthrough itself is asynchro-
nous and usually used as a DC control.
The controls and data have to be present at the input pins for
a set-up time of t
s
before, and a hold time of t
h
after, the rising
edge of the clock (CONV) in order to be synchronously
registered. The set-up and hold times are not important in the
asynchronous mode. The minimum pulse widths high (t
PWH
)
and low (t
PWL
) as well as settling time become the limiting
factors. (See figure 3.)
The video controls produce the output levels needed for
horizontal blanking, frame synchronization, etc., to be com-
patible with video system standards as described in
RS-343-A. Table II shows the video control effects on the
analog output. Internal logic governs Blank, Sync, and Force
High so that they override the data inputs as needed in video
applications. Sync overrides both the data and other controls
to produce full negative video output (figure 9).
Reference White video level output is provided by Force
High, which drives the internal digital data to full scale output
or 100 IRE units. Bright gives an additional 10% of full scale
value to the output level. This function can be used in graphic
displays for highlighting menus, cursors or warning mes-
sages. Again, if the devices are used in non-video applica-
tions, the video controls can be left open.
CONVERT CLOCK
For best performance, the clock should be ECL driven,
differentially, by utilizing CONV and
CONV
(figure 4). By
driving the clock this way, clock noise and power supply/
output intermodulation will be minimized. The rising edge of
the clock synchronizes the data and control inputs to the
SPT1018. Since the actual switching threshold of
CONV
is
determined by CONV, the clock can be driven single-ended
by connecting a bias voltage to
CONV
. The switching thresh-
old of CONV is set by this bias voltage.
ANALOG OUTPUTS
The SPT1018 has two analog outputs that are high imped-
ance, complementary current sinks. The outputs vary in
proportion to the input data, controls and reference current
values so that the full scale output can be changed by setting
I
Set
as mentioned earlier.
In video applications, the outputs can drive a doubly termi-
nated 50 or 75 ohm load to standard video levels. In the
standard configuration of figure 5, the output voltage is the
product of the output current and load impedance and is
between 0 and -1.07 V. The Out- output (figure 9) will provide
a video output waveform with the Sync pulse bottom at the
-1.07 V level. The Out+ is inverted with Sync up.
SPT1018
7
5/14/97
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
