TLC5733A

20 MSPS 3-CHANNEL ANALOG-TO-DIGITAL CONVERTER

WITH HIGH-PRECISION CLAMP

SLAS104A – JULY 1995 – REVISED NOVEMBER 1996

8

POST OFFICE BOX 655303

detailed description

clamp function

The clamp function is optimized for a YUV video signal and has two clamp modes. The first mode uses the

COMPOSITE SYNC signal as the input to the EXTCLP terminal to generate an internal clamp pulse and the

second mode uses an externally generated clamp pulse as the input to the EXTCLP terminal.

In the first mode, the device detects false pulses in the COMPOSITE SYNC signal by monitoring the rising and

falling edges of the COMPOSITE SYNC signal pulses. This monitoring prevents faulty operation caused by

disturbances and missing pulses of the COMPOSITE SYNC signal input on EXTCLP and external spike noise.

When fault pulses are detected, the device internally generates a train of clamp pulses at the proper positions

(1H) by an internal 910-counter for NTSC and a 1136-counter for PAL. The device checks clamp pulses for 1H

time and generates clamp pulses at correct positions when COMPOSITE SYNC pulses are in error in time.

The internal counter continually produces a horizontal sync period (1H) that is NTSC or PAL compatible as

selected by the condition of the NT/PAL terminal.

clamp voltages and selection

Table 1 shows the clamping level during the clamp interval. Table 2 shows the selection of the internal or external

clamp pulse. With either NTSC or PAL, the internal clamp pulse is always used.

Table 1. Clamp Level (Internal Connection Level)

CHANNEL OF ADC

OUTPUT CODE

APPLICATION

CHANNEL OF ADC

OUTPUT CODE

APPLICATION

ADC A

• VI(A)

00010000

Y

ADC B

• VI(B)

10000000

(U, V)

ADC C

• VI(C)

10000000

(U, V)

Table 2. Clamp Level (Internal Connection Level)

CONDITION

FUNCTION (EACH ADC)

CLPEN

EXTCLP

NT/PAL

INTERNAL CLAMP

CLAMP PULSE

L

Don’t Care

Inactive

External clamp pulse

L

L

Don’t Care

Inactive

No clamping

H

COMPOSITE SYNC input

L

Active

Synchronous with NTSC

H

COMPOSITE SYNC input

H

Active

Synchronous with PAL

The clamp circuit is shown in Figure 6. The clamp voltage is stored on capacitor C2 during the back porch of

the horizontal blanking period.

During the clamp pulse the input to channel A is clamped to:

COMPOSITE SYNC time monitoring

When CLPEN is high, COMPOSITE SYNC generates an internal clamp pulse on the horizontal blanking interval

back porch. The TLC5733A has a timing window into which the horizontal sync tip must occur. There is a noise

time window for the falling edge and one for the rising edge (see Figure 1, Figure 2, and Table 3).