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

5

POST OFFICE BOX 655303

Terminal Functions (Continued)

TERMINAL

I/O

DESCRIPTION

NAME

NO.

I/O

DESCRIPTION

SYNC_T

25

I

Sync tri-level control, active high. A rising edge on CLK latches SYNC_T. When asserted (high), a positive

sync (higher than blanking level) is generated when SYNC is low. When disabled (low), a negative sync (lower

than blanking level) is generated when SYNC is low. When generating a tri-level (negative-to-positive) sync,

an L->H transition on this signal positions the start of the positive transition. See Figure 10 for timing control.

SYNC_T is also used to put the device in generic DAC mode: SYNC=H AND SYNC_T = H -> generic DAC

mode. Therefore, the user should always drive SYNC_T low outside the sync period when video DAC mode

operation is intended.

VREF

37

O

Voltage reference for DACs. An internal voltage reference of nominally 1.2 V is provided, which requires an

external 0.1-

µF ceramic capacitor between VREF and AVSS.

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 versus 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, since 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 (i.e., 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.