SPT
7
3/28/00
SPT5140
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 asynchronous 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 tS before, and a hold time of tH after,
the rising edge of the clock (CONV) in order to be syn-
chronously registered. The set-up and hold times are not
important in the asynchronous mode. The minimum
pulse widths high (tPWH) and low (tPWL) 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
compatible with video system standards as described in
RS-343-A. Table I 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 4).
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 messages. Again, if the devices are used in
non-video applications, the video controls can be left
open.
Table I – Video Control Operation (Output values for setup = 10 IRE and 75 ohm standard load)
Ref
Data
Sync
Blank
White
Bright
Input
Out – (mA) Out – (V) Out – (IRE)
Description
1
XXXX
28.57
–1.071
–40
Sync Level
0
1
X
X
X
20.83
–0.781
0
Blank Level
0011
X
0.00
0.000
110
Enhanced High Level
0010
X
1.95
–0.073
100
Normal High Level
0000
000...
19.40
–0.728
7.5
Normal Low Level
0000
111...
1.95
–0.073
100
Normal High Level
0001
000...
17.44
–0.654
17.5
Enhanced Low Level
0001
111...
0.00
0.000
110
Enhanced High Level
Figure 4 – Video Output Waveform for Standard Load
256 Gray Levels
Normal Low (Black)
Sync
Video
Blank
Bright
Normal High (White)
110
100
7.5
0
–40
0 mV
–73 mV
–728 mV
–781 mV
–1071 mV
IRE
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