THS8135
TRIPLE 10-BIT, 80/240 MSPS VIDEO DAC WITH TRI-LEVEL SYNC AND VIDEO (ITU-R.BT601)
– COMPLIANT FULL SCALE RANGE
SLVSXXXX—10/02/00 3:44 PM
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Copyright 2000 Texas Instruments Incorporated
PRODUCT PREVIEW information concerns products in the
formative or design phase of development. Characteristic data
and other specifications are design goals. Texas Instruments
reserves the right to change or discontinue these products
Functional Description
THS8135 is a fast well-matched triple DAC with current outputs, optimized for video applications but without
sacrificing its usefulness as a general-purpose triple DAC thanks to a ‘generic’ DAC mode. For video applications,
the device can embed an analog output (composite) sync on only the green/luma channel or on all three DAC
output channels; offers compatibility with several popular video bus formats and provides standard analog output
compliance levels for video digitized according to the ITU-R.BT601 sampling standard. The DAC full-scale range is
also adjustable.
Sync generation
Control inputs /SYNC and SYNC_T[rilevel] enable the superposition of an additional current onto the AGY channel
or on all three channels, depending on the setting of INS3_INT(see later). Using a combination of the /SYNC and
SYNC_T control inputs, either bilevel negative going pulses or trilevel pulses can be generated. Depending on the
timing controls for these signals, both horizontal and vertical sync’s can be generated in a video system. Assertion
of /SYNC (active low) will identify the sync period, while assertion of SYNC_T (active high) within this period will
identify the positive excursion of a trilevel sync.
Blanking generation
A control input /BLANK is provided that will fix the output amplitude on all channels to the blanking level,
irrespective of the value on the data input ports. The position of the blanking level on each channel and its relation
to active video is determined by the operation mode of the device (see later).
Generic mode vs. Video Mode
In video mode, the device provides a dc bias on all channels in RGB mode and on the Y channel in YPbPr mode,
according to Figure 7, in order to allow for a negative sync excursion below the blanking level.
Since this introduces a dc bias for the DAC output codes on at least one of the three DAC outputs, this might be
undesirable in applications where no sync embedding is needed. Therefore THS8135 includes a ‘generic DAC’
mode that disables the dc bias on all outputs.
Selection between ‘generic’ vs. ‘video’ mode is controlled through a combination of /SYNC, SYNC_T and /BLANK
settings. Since in video mode, SYNC_T only determines the sync polarity, this signal has don’t care status when no
sync insertion takes place i.e. when /SYNC is high. Therefore one of both states can be used for configuring the
device in generic mode.
THS8135 enters generic mode when SYNC_T is high outside the sync insertion interval, in other words when both
SYNC_T and /SYNC are high. Note that all other features described below (input formatter setting, blank override,
color space selection determining blank level position) are still available in generic DAC mode. Generic mode only
disables the dc bias, such that the analog DAC output amplitudes corresponding to codes 0 becomes 0V on all
channels.
Table 3 shows all combinations of these control signals. Note that when /SYNC is low, it takes precedence over
/BLANK.
Table 3: Video vs. generic mode selection
/SYNC
/BLANK
SYNC_T
Operation mode & DAC output