THS7313 datasheet(28/48 Pages) TI

28 / 48 page

www.ti.com

LOW PASS FILTER MODE

THS7313

SLOS483 – NOVEMBER 2005

APPLICATION INFORMATION (continued)

Each channel of the THS7313 incorporates a 5th-Order Low Pass Filter. These video reconstruction filters

minimize DAC images from being passed onto the video receiver. Depending on the receiver design, failure to

eliminate these DAC images can cause picture quality problems due to aliasing of the ADC. Another benefit of

the filter is to smooth out aberrations in the signal which some DACs can have if their own internal filtering is not

good. This helps with picture quality and helps insure the signal meets video bandwidth requirements.

Each filter has a Butterworth characteristic associated with it. They have been modified with a slightly lower Q

than the traditional Q associated with the Butterworth response. The benefit of the Butterworth response is the

frequency response is flat with a relatively steep initial attenuation at the corner frequency. The problem is that

the group delay rises near the corner frequency. Group delay is defined as the change in phase (radians/second)

divided by a change in frequency. An increase in group delay corresponds to a time domain pulse response that

has overshoot and some possible ringing associated with the overshoot.

The use of other types of filters such as elliptic or chebyshev is not recommended for video applications due to

their large group delay variations near the corner frequency which results in overshoot and ringing. While these

elliptic or chebyshev filters may help meet the video standard specifications with respect to amplitude

attenuation, their group delay is beyond the standards specifications. Couple this with the fact that video can go

from a white pixel to a black pixel over and over again, ringing occurs. Ringing typically causes a display to have

ghosting or fuzziness appear on the edges of a sharp transition. However, a Bessel filter has an ideal group

delay response, but the rate of attenuation is typically too low for acceptable image rejection. Thus, the

Butterworth filter is a respectable compromise for both attenuation and group delay.

The THS7313 filter has a slightly lower group delay variation near the corner frequency compared to an ideal

Butterworth filter. This results in a time domain pulse response which still has some overshoot, but not as much

as a true Butterworth filter. Additionally, the initial rate of attenuation in the frequency response is not as fast as

an ideal Butterworth response, but it is an acceptable initial rate of attenuation considering the pulse and group

delay characteristic benefits.

One concern about an active filter in an integrated circuit is the variation of the filter characteristics when the

ambient temperature and the subsequent die temperature changes. To minimize temperature effects, the

THS7313 uses thin-film metal resistors and high quality - low temperature coefficient capacitors found in the

BiCom-3 process. The filters have been specified by design to account for process variations and temperature

variations to maintain proper filter characteristics. This maintains a low channel-to-channel time delay which is

required for proper video signal performance.

The THS7313 filters have a nominal corner (-3 dB) frequency selectable at 8 MHz. The 8-MHz filter is ideal for

standard definition (SD) NTSC, PAL, and SECAM composite video (CVBS) signals. It is also useful for S-Video

signals (Y’/C’), 480i / 576i Y’P’BP’R , and Y’U’V’ video signals. The -3-dB corner frequency was designed to be 8

MHz to allow a maximally flat video signal while achieving 48-dB of attenuation at 27 MHz – a common

frequency between the ADC 2nd and 3rd Nyquist zones found in many video receivers. This is important because

any signal appearing around this frequency can appear in the baseband due to aliasing effects of an analog to

digital converter found in a receiver.

The 8-MHz filter frequency was chosen to account for process variations in the THS7313. To ensure the required

video frequencies are the least affected, the filter corner frequency must be high enough to allow for component

variations. The other consideration is the attenuation must be large enough to ensure the anti-aliasing /

reconstruction filtering meets the system demands. Thus, the selection of the filter frequencies was not chosen

arbitrarily.

The I2C interface of the THS7313 allows each channel to be configured totally independent of the other

channels. One of the benefits is that a multiple output encoder (or DAC) can be routed through one THS7313

with the proper input configuration regardless of the signal. This is useful for a portable system or in a low cost

system where only one set (or 2 sets in parallel) is desired on the output of the system. An update of the I2C

commands changes the THS7313 channels. An example is shown in Figure 56 where the input MUX allows for

one set of component video signals to be put into the THS7313, and then through an I2C update, a

CVBS/S-Video set of signals is sent through the THS7313 with the proper input mode.

THS7313 Datasheet(PDF) 28 Page - Texas Instruments

THS7313 Datasheet(HTML) 28 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL