CVBS

R

S-Video Y’

R

R

PU

R

PU

R

PU

S-Video C’

R

Y'/G'

R

1

2

3

4

5

6

7

8

9

10

20

19

18

17

16

15

14

13

12

11

SD1OUT

SD2OUT

SD3OUT

DisableSD

GND

DisableHD

SF1OUT

SF2OUT

SF3OUT

Bypass SF

SD1IN

SD2IN

SD3IN

NC

V

S+

NC

SF1IN

SF2IN

SF3IN

BypassSD

THS7360

Bypass

SDLPF

Bypass

LPF

SF

P' /B'

B

R

P' /R'

R

R

+2.7Vto

+5V

Disable SF

DisableSD

75 W

75 W

330 F

m

(2)

75 W

Y'Out

75 W

330 F

m

(2)

75 W

P’ Out

B

75 W

330 F

m

(2)

75 W

Y'Out

75 W

330 F

m

(2)

75 W

P' Out

B

75 W

330 F

m

(2)

75 W

P' Out

R

75 W

330 F

m

(2)

ToGPIOor

GND/V

S+

ToGPIOor

GND/V

S+

+V

+V

+V

+V

+V

+V

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

0.1 F

m

(1)

+2.7Vto+5V

THS7360

SLOS674 – JUNE 2010

www.ti.com

(1) AC-coupled input is shown in this example. DC-coupling is also allowed as long as the DAC output voltage is within the allowable linear

with connecting the DAC termination resistors (R) to ground.

(2) This example shows an ac-coupled output. DC-coupling is also allowed by simply removing these capacitors.

(3) SF indicates selectable filter.

Figure 48. Typical AC Input System Driving AC-Coupled Video Lines

The use of other type of filters, such as elliptic or

LOW-PASS FILTER

chebyshev,

are

not

recommended

for

video

applications because of the very large group delay

Each

channel

of

the

THS7360

incorporates

a

variations near the corner frequency resulting in

sixth-order,

low-pass

filter.

These

video

significant overshoot and ringing. While these filters

reconstruction filters minimize DAC images from

may help meet the video standard specifications with

being passed onto the video receiver. Depending on

respect to amplitude attenuation, the group delay is

the receiver design, failure to eliminate these DAC

well beyond the standard specifications. Considering

images can cause picture quality problems because

this delay with the fact that video can go from a white

of aliasing of the ADC in the receiver. Another benefit

pixel to a black pixel over and over again, it is easy to

of the filter is to smooth out aberrations in the signal

see that ringing can occur. Ringing typically causes a

that some DACs can have if the internal filtering is

display to have ghosting or fuzziness appear on the

not very good. This benefit helps with picture quality

edges of a sharp transition. On the other hand, a

and ensures that the signal meets video bandwidth

Bessel filter has ideal group delay response, but the

requirements.

rate of attenuation is typically too low for acceptable

Each

filter

has

an

associated

Butterworth

image rejection. Thus, the Butterworth filter is a

characteristic.

The

benefit

of

the

Butterworth

respectable compromise for both attenuation and

response is that the frequency response is flat, with a

group delay.

relatively steep initial attenuation at the corner

The THS7360 SD filters have a nominal corner

frequency. The problem with this characteristic is that

(–3-dB) frequency at 9.5 MHz and a –1-dB passband

the group delay rises near the corner frequency.

typically at 8.2 MHz. This 9.5-MHz filter is ideal for

Group delay is defined as the change in phase

SD NTSC, PAL, and SECAM composite video

(radians/second) divided by a change in frequency.

(CVBS) signals. It is also useful for S-Video signals

An increase in group delay corresponds to a time

domain pulse response that has overshoot and some

signals, and computer R'G'B' video signals. The

possible ringing associated with the overshoot.

30

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