THS8135 Datasheet, PDF (9/23 Pages) Texas Instruments – TRIPLE 10-BIT, 240 MSPS VIDEO DAC WITH TRI-LEVEL SYNC AND VIDEO (ITU-R.BT601) COMPLIANT FULL SCALE RANGE

THS8135
TRIPLE 10-BIT, 240 MSPS VIDEO DAC WITH TRI-LEVEL SYNC AND VIDEO
(ITU-R.BT601)—COMPLIANT FULL SCALE RANGE
SLAS343A – MAY 2001 – REVISED JUNE 2002
selection of full or reduced-scale ITU.BT601 modes (available in video DAC mode only) (continued)
When using reduced-scale range, the output sync:video amplitude ratio is still 7:3, but now takes into account
the reduced code range, not the full-scale range, to determine this ratio. Therefore, proper sync amplitudes are
preserved in either mode, when the full-scale current is modified as explained higher. When changing DAC
full-scale current using R_FS, the sync amplitude level always scales proportionally with the video output
compliance.
Note that even when using reduced-scale range, the midscale blanking level on ABPb and ARPr channels still
corresponds to code 512 = [64+(960-64)/2] when using YCbCr color space configuration. Table 6 shows the
valid reduced input code ranges for RGB and YCbCr operation on each of the input data buses. While the
THS8135 allows reduced-scale code range with RGB data, video systems normally use it only with YCbCr type
data.
Table 6. Input Code Ranges for ITU.BT601 Modes
OPERATING MODE
RGB
YCbCr
GY[ ]
64–940
64–940
RPr[ ]
64–940
64–960
BPb[ ]
64–940
64–960
DAC operation
The analog output drivers generate a current of which the drive level can be user-modified by choosing an
appropriate resistor value R_FS, connected to the FSADJ pin.
All current source amplitudes (video, blanking, and sync) are derived from an internal voltage reference such
that the relative amplitudes of sync, blank, and video are always equal to their nominal relationships.
Figure 1 through Figure 4 show the nominal output voltage levels for full- or reduced-range input code range
configurations on R, G, B, and Y versus Cb and Cr channels.
Note that in full-scale input code range configuration, the blanking level is at 350 mV on all outputs; while in
reduced-scale operation it is at 400 mV on all outputs.
In reduced scale modes, after proper adjustment of R_FS, the nominal 700-mV output compliance is reached from
an input code range of only 876 (=940-64) codes on G/Y, and of only 896 (=960-64) codes on R/Pr and B/Pr
output channels. The maximum excursions are ~817mV (= 1023/876 x 700 mV) on G/Y and ~800 mV (=
876/896 x 817 mV) on B/Pb and R/Pr channels. Figure 4 shows that when using reduced-scale input code
range, the blanking level needs to be at 400 mV to accommodate the maximum negative excursion on B/Pr and
R/Pr channels.
The figures also show the excursions for the sync level positions in either full-scale or reduced-scale
configurations. These levels are internally adjusted and assure 300-mV sync excursions when using nominal
termination loads and properly adjusting R_FS, as explained before.
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