Datasheet

THS8135
www.ti.com SLAS343B –MAY 2001–REVISED APRIL 2013
DETAILED DESCRIPTION
The THS8135 is a fast well-matched triple DAC with current outputs optimized for video applications 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) bi-level or tri-level sync on only the green/luma
channel or on all three DAC output channels. The THS8135 offers compatibility with several popular video data
formats and provides standard analog output compliance levels for component video digitized according to the
ITU-R.BT601 sampling standard. The DAC full-scale range is also adjustable.
Sync Generation
The SYNC and SYNC_T control inputs enable the superposition of an additional current onto the AGY channel or
onto all three channels, depending on the setting of INS3_INT. Using a combination of the SYNC and SYNC_T
control inputs, either bi-level negative going pulses or tri-level pulses can be generated. By driving these
terminals with the correct timing inputs, the user can insert onto the analog output(s) any composite sync format
consisting of horizontal sync, vertical sync, pre- and post-serration, and equalization pulses. Assertion of SYNC
(active low) identifies the sync period, while assertion of SYNC_T (active high) within this period identifies the
positive excursion of a tri-level sync.
Blanking Generation
The BLANK control input fixes 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
different depending on the RGB versus YCbCr color space configuration: bottom-range blanking level for R, G,
B, and Y outputs versus mid-range blanking level for Pb and Pr outputs. This also depends on the full-scale
versus reduced-scale (ITU-R.BT601) input code range configuration: bottom-range blanking levels correspond to
input code 0 when in full-scale, or to code 64 when in reduced-scale input code range configuration; mid-range
blanking levels remain at 512 in all cases.
Generic DAC Mode vs Video DAC Mode
In video DAC mode, the device provides additional dc bias on R, G, B, and Y channels to provide headroom for
negative sync insertion, as shown on Figure 1 and Figure 3.
Such bias might be undesirable in applications where no sync embedding is needed, because it causes
additional power consumption and might prevent dc coupling of the DAC outputs. In such cases, only a triple
DAC operation without dc bias (that is, DAC input code 0 corresponding to 0-V output) might be preferred as
there is no need for sync insertion. Therefore, the THS8135 includes a generic DAC mode that does not add any
dc bias. Sync insertion is not supported in generic DAC mode. Also, in this mode, only full-scale operation is
available. Other features are still available in generic DAC mode: different input data formats, RGB versus YCbCr
color space selection, and blanking level override option.
Because of the eliminated dc bias, the DAC output compliance for full-range video input can be higher in generic
DAC mode: up to 1.286 Vpp at nominal double 75-Ω termination load. This is high enough for the D/A conversion
of composite video (NTSC/PAL/SECAM), where the signal fed to the device contains the complete digital
composite waveform, including sync and color-burst.
Selection between generic DAC versus video DAC mode is controlled through a combination of SYNC and
SYNC_T settings. Because SYNC_T only determines the sync polarity in video DAC mode, this signal has don’t
care status when no sync insertion takes place that is, when SYNC is high. The THS8135 uses the logic level on
the SYNC_T input when SYNC is high to enter generic DAC mode: SYNC_T and SYNC are high → generic DAC
mode. Therefore, the user must make sure to keep SYNC_T low outside the sync insertion period (when SYNC
is high) to prevent entering generic DAC mode, when he intends to use the device in video DAC mode.
Table 3 shows how to select between video DAC and generic DAC mode.
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