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

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

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

7

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