TLV5734

TRIPLE 8BIT 30MSPS ADC WITH HIGH

PRECISION CLAMP FOR YUV/RGB VIDEO

SLES026A – APRIL 2002 – REVISED JANUARY 2003

7

POST OFFICE BOX 655303

detailed description (continued)

clamp function

The TLV5734 employs a clamp feedback circuit on each channel to correct the clamp level mismatch

contributed by the offset of each channel. The clamp levels are used for either the YUV or RGB video signal.

Figure 2 shows the clamp circuit and the external R and C required for the clamp operation. The clamp circuit

also requires an externally generated active-high clamp pulse to be applied to input EXTCLP. The clamp pulse

defines the timing window during which the clamp circuit is enabled. For video applications, the clamp pulse

must be applied during the back porch of the sync portion of the horizontal blanking interval. For an embedded

sync video input (G/Y), the external clamp high must start after the gap A between sync and clamp start (see

Figure 3 and Table 3).

The clamp is enabled by applying a logic high on the EXTCLP input. This closes switch SW1 and enables the

digital comparator. The output of the ADC is then compared digitally with the preset clamp level (as defined in

Table 1), then the voltage on clamp capacitor C2 is charged/discharged through the 3-state buffer and resistor

R to make the ADC output equal to the desired clamp level. Once the desired clamp level is attained, the 3-state

buffer is put in the high-impedance mode and the clamp voltage is stored on input capacitor C1 until the next

clamp pulse.

R

15 k

Ω

Video Input

Digital

Comparator

C2

0.1

µF

C1

0.1

µF

TLV5734

ADC_OUT

ADC

CLPV

CLP OUT

EXTCLP

SW1

CLP_OE

C_OUT

Preset Clamp Level

(See Table 1)

3-State

Buffer

Figure 2. Clamp Feedback Circuit

Table 1. Preset Clamp Level

ADC CHANNEL

YUV (G/Y = HIGH)

RGB (G/Y = LOW)

ADC CHANNEL

OUTPUT CODE

APPLICATION

OUTPUT CODE

APPLICATION

CHANNEL A

00010000b

Y

00010000b

R, G, B

CHANNEL B

10000000b

U, V

00010000b

R, G, B

CHANNEL C

10000000b

U, V

00010000b

R, G, B