THS7375IPWR datasheet(24/36 Pages) TI

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LOW-PASS FILTER

Benefits Over Passive Filtering

THS7375

SBOS449 – SEPTEMBER 2008......................................................................................................................................................................................... www.ti.com

Keep in mind that images do not stop at the DAC

sampling frequency, fS (for example, 27 MHz for

Each

channel

the

THS7375

incorporates

traditional SD DACs); they continue around the

sixth-order low-pass filter. These video reconstruction

sampling frequencies of 2X fS, 3X fS, 4X fS, etc. (54

filters minimize the passing of DAC images to the

MHz, 81 MHz, 108 MHz). Because of these multiple

video receiver. Depending on the receiver design,

images that an ADC can fold down into the baseband

failure to eliminate these DAC images can cause

signal, the low-pass filter must also eliminate these

picture quality problems as a result of ADC aliasing.

higher

order

images.

The

THS7375

filters

are

Another benefit of the filter is to smooth out

Butterworth filters and as such, do not bounce at

aberrations in the signal that some DACs can have if

higher frequencies and maintain good attenuation

the internal device filtering is not very good. This

performance.

technique helps with picture quality and helps ensure

that the signal meets video bandwidth requirements.

The 9.5-MHz filter frequency was chosen to account

for process variations in the THS7375. To ensure that

Each

filter

has

associated

Butterworth

the required video frequencies are effectively passed,

characteristic.

The

benefit

the

Butterworth

the filter corner frequency must be high enough to

response is that the frequency response is flat, with a

allow component variations. The other consideration

relatively steep initial attenuation at the corner

is that the attenuation must be large enough to

frequency. The problem with this characteristic is that

ensure the

anti-aliasing/reconstruction

filtering

the group delay rises near the corner frequency.

adequate to meet the system demands. Thus, the

Group delay is defined as the change in phase

filter frequencies were not arbitrarily selected and are

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

a good compromise that should meet the demands of

An increase in group delay corresponds to a time

most systems.

domain pulse response that has overshoot and some

possible ringing associated with the overshoot.

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

Two key benefits of using an integrated filter system

chebyshev,

are

not

recommended

for

video

such as the THS7375 over a passive system are

applications because of the very large group delay

PCB area and filter variations. The small TSSOP-14

variations near the corner frequency that results in

package for four video channels is much smaller over

significant overshoot and ringing. While these elliptic

a passive RLC network, especially a six-pole passive

or chebyshev filters may help meet the video

network. Additionally, consider that inductors have at

standard specifications with respect to amplitude

best ±10% tolerances (normally ±15% to ±20% are

attenuation, the group delay is well beyond the

common)

and

capacitors

typically

have

±10%

standard specifications. When considering these filter

tolerances. Using a Monte Carlo analysis shows that

types, keep in mind that video can go from a white

the filter corner frequency (–3 dB), flatness (–1 dB), Q

pixel to a black pixel over and over again, and ringing

factor (or peaking), and channel-to-channel delay

can easily occur. Ringing typically causes a display to

have wide variations. This approach can lead to

have ghosting or fuzziness appear on the edges of a

potential

performance

and

quality

issues

sharp transition. On the other hand, a Bessel filter

mass-production environments. The THS7375 solves

has ideal group delay response, but the rate of

most of these problems with the corner frequency

attenuation is typically too slow for acceptable image

being essentially the only variable.

rejection. Thus, the Butterworth filter is a respectable

compromise for both attenuation and group delay.

Another concern about passive filters is the use of

inductors. Inductors are magnetic components and

The THS7375 filters have a nominal corner (–3 dB)

are

therefore

susceptible

electromagnetic

frequency at 9.5 MHz and a –1-dB passband typically

coupling/interference

(EMC/EMI).

Some

common

at 8 MHz. This 9.5-MHz filter is ideal for standard

coupling can occur because of other nearby video

definition (SD) NTSC, PAL, and SECAM composite

channels that use inductors for filtering, or it can

video (CVBS) signals. It is also useful for s-video

come from nearby switched-mode power supplies.

signals (Y'C'), 480i/576i Y'P'BP'R, Y'U'V', broadcast

Some other forms of coupling could be from outside

G'B'R' (R'G'B') signals, and computer video signals.

sources with strong EMI radiation that can cause

The 9.5-MHz, –3-dB corner frequency was designed

failure in EMC testing such as required for CE

to achieve 54 dB of attenuation at 27 MHz—a

compliance.

common sampling frequency between the DAC/ADC

second and third Nyquist zones found in many video

One concern about an active filter in an integrated

systems. This consideration is important because any

circuit is the variation of the filter characteristics when

signal that appears around this frequency can also

the ambient temperature and the subsequent die

appear in the baseband as the result of aliasing

temperature

change.

minimize

temperature

effects of an ADC found in a receiver.

effects,

the

THS7375

uses

low

temperature

coefficient resistors and high-quality/low-temperature

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