THS7364IPWR datasheet(36/47 Pages) TI

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THS7364

SBOS530 – AUGUST 2010

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BENEFITS OVER PASSIVE FILTERING

54 dB of attenuation at 27 MHz—a common sampling

Two key benefits of using an integrated filter system,

frequency between the DAC/ADC second and third

such as the THS7364, over a passive system are

Nyquist zones found in many video systems. This

PCB area and filter variations. The small TSSOP-20

consideration is important because any signal that

package for six video channels is much smaller over

appears around this frequency can also appear in the

a passive RLC network, especially a six-pole passive

baseband as a result of aliasing effects of an ADC

network. Additionally, consider that inductors have at

found in a receiver.

best ±10% tolerances (normally, ±15% to ±20% is

The THS7364 FHD filters have a nominal corner

common)

and

capacitors

typically

have

±10%

(–3 dB) frequency at 72 MHz and a –1-dB passband

tolerances. Using a Monte Carlo analysis shows that

typically at 60 MHz. This 72-MHz filter is ideal for

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

1080p50 or 1080p60 component video. It is also ideal

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

for oversampling systems where the video DAC

have wide variations. These variances can lead to

upsamples the video signal such as 720p or 1080i

potential

performance

and

quality

issues

upsampled to 148.5 MHz. The benefit is an extremely

mass-production environments. The THS7364 solves

flat passband response along with almost no group

most of these problems with the corner frequency

delay within the HD video passband. In bypass mode,

being essentially the only variable.

these filters can also be used for some computer

Another concern about passive filters is the use of

R’G’B’ video signals including VGA, SVGA, XGA,

inductors. Inductors are magnetic components, and

SXGA, and QXGA.

are

therefore

susceptible

electromagnetic

Keep in mind that images do not stop at the DAC

coupling/interference

(EMC/EMI).

Some

common

sampling frequency, fS (for example, 27 MHz for

coupling can occur because of other video channels

traditional SD DACs); they continue around the

nearby using inductors for filtering, or it can come

sampling frequencies of 2x fS, 3x fS, 4x fS, and so on

from nearby switched-mode power supplies. Some

(that is, 54 MHz, 81 MHz, 108 MHz, etc.). Because of

other forms of coupling could be from outside sources

these multiple images, an ADC can fold down into the

with strong EMI radiation and can cause failure in

baseband signal, meaning that the low-pass filter

EMC testing such as required for CE compliance.

must also eliminate these higher-order images. The

One concern about an active filter in an integrated

THS7364 filters are Butterworth filters and, as such,

circuit is the variation of the filter characteristics when

do not bounce at higher frequencies, thus maintaining

the ambient temperature and the subsequent die

good attenuation performance.

temperature

changes.

minimize

temperature

The filter frequencies were chosen to account for

effects,

the

THS7364

uses

low-temperature

process variations in the THS7364. To ensure the

coefficient resistors and high-quality, low-temperature

required video frequencies are effectively passed, the

coefficient capacitors found in the BiCom3X process.

filter corner frequency must be high enough to allow

These filters have been specified by design to

component variations. The other consideration is that

account for process variations and temperature

the attenuation must be large enough to ensure the

variations to maintain proper filter characteristics.

anti-aliasing/reconstruction filtering is sufficient to

This approach maintains a low channel-to-channel

meet the system demands. Thus, the selection of the

time delay that is required for proper video signal

filter frequencies was not arbitrarily selected and is a

performance.

good compromise that should meet the demands of

Another benefit of the THS7364 over a passive RLC

most systems.

filter is the input and output impedance. The input

One of the features of the THS7364 is that these

impedance presented to the DAC varies significantly,

filters can be bypassed. Bypassing the SD filters

from 35

Ω to over 1.5 kΩ with a passive network, and

results in an amplifier with 150-MHz bandwidth and

may cause voltage variations over frequency. The

100-V/ms slew rate. This configuration can be helpful

THS7364 input impedance is 800 k

Ω, and only the

when diagnosing potential system issues or when

2-pF

input

capacitance

plus

the

PCB

trace

simply wishing to pass higher frequency signals

capacitance impact the input impedance. As such,

through the system.

the voltage variation appearing at the DAC output is

better controlled with a fixed termination resistor and

Bypassing the FHD filters results in a amplifier

the high input impedance buffer of the THS7364.

supporting 350-MHz bandwidth and 500-V/ms slew

rate. This configuration supports computer R'G'B'

On the output side of the filter, a passive filter again

signals up to UWXGA resolution.

has a large impedance variation over frequency. The

EIA/CEA-770 specifications require the return loss to

be at least 25 dB over the video frequency range of

usage. For a video system, this requirement implies

the source impedance (which includes the source,

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