ML6420CS-4 datasheet(9/20 Pages) MICRO-LINEAR

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ML6420

ALIASING: THE PROBLEM

Aliasing is a signal distorting process that occurs when an

analog signal is sampled. If the analog signal contains

frequencies greater than half of the sampling rate, those

frequencies will be altered and “folded back” in the

frequency domain. These frequencies represent a distortion

of the original signal as represented in the sampled domain,

and cannot be corrected after sampling.

THE RESULT OF ALIASING IN A TV PICTURE

Aliasing causes several disturbing distortions to a picture.

Since the folded spectrum adds to the original spectrum, it

will sometimes be in phase, and sometimes out of phase

causing ripples in response that depend on the position of

the picture element relative to the clock. The net effect is

that picture elements, edges, highlights, and details will

“wink” in amplitude as they move across a picture if they

have high frequency content above the Nyquist frequency

of the sampler.

ANTI-ALIASING

Anti-aliasing reduces the bandwidth of the signal to a

value appropriate for the sample processing system. Some

detail information is lost, but only the information that

cannot be unambiguously displayed is removed. Assuming

that the passband contains the “real” picture information,

the only distortion that occurs is due to amplitude and

phase variations of the anti-aliasing filter in the passband.

The following section shows approaches using digital and

analog filters in an oversampled system, and a monolithic

analog filter as a lower cost alternative.

OVERSAMPLING

Aliasing cannot be removed once it occurs, it must be

prevented at the signal sampler. Many current systems are

choosing to prevent aliasing by increasing the clock rate of

the sampler. This is known as “oversampling”. Doubling

the clock rate greatly reduces the burden on the analog

anti-alias filter, but the increased data rate greatly

increases the size, complexity and cost of the Digital

Signal Processing (DSP) circuitry. Since the higher clock

rate generates more samples than are necessary to

represent the desired passband content, a digital filter may

be used to decimate the signal back to a lower sample

rate, saving size, complexity and power in the

downstream circuitry. Since this digital filter itself is a

complex digital block, this method cannot be considered

the lowest cost approach to solving the anti-alias problem.

NYQUIST SAMPLING

In traditional systems, before the advent of higher speed

ADCs, anti-aliasing filters were designed in the analog

domain. The movement toward higher sampling rates was

an attempt to circumvent the difficult challenge of

designing a sharp roll-off, linear phase, non-distorting

analog filter. The ML6420 series of filters solve this

problem where it is best solved, in the analog domain.

Since they are monolithic, their application is simple.

Since they have flat amplitude and linear phase, they are

low distortion. And since the aliasing is removed at the

analog input to the ADC, the clock rates are minimized,

an expensive DSP half band filter is eliminated, and

significant power is conserved.

Oversampling vs Nyquist sampling

Clearly the purely analog monolithic solution versus the

analog/digital solution using DSP filtering are different

ways of solving the same problem. Other than costs

(purely analog is many times less expensive) there are no

real differences in performance for applications that

require flatness specs of ±0.5db to 4.5MHz for consumer

and pro-sumer video applications. The ML6420/ML6422

are also phase corrected for flat group delay, a feature not

found in typical low cost analog filters, and a

characteristic often associated with digital filters alone.

The following section highlights the importance of linear

phase response in video applications.

TIME DOMAIN RESPONSE:

TRANSIENTS AND RINGING

The phase response of filters is often ignored in

applications where time domain waveforms are not

relevant. But in video applications the time domain

waveform is the signal that is finally presented on the

screen to the viewer, and so time domain characteristics

such as pulse response symmetry, pre-shoot, over-shoot

and ringing are very important. Video applications are

very demanding in that they require both sharp cutoff

characteristics and linear phase. The application of DSP to

the problem is based on the linear phase characteristic of

a particular class of digital filters known as symmetrical

FIR filters. Use of these filters guarantees the best possible

time domain characteristics for a given amplitude

characteristic. In the analog domain phase linearity is not

automatic (except for special phase linear filters such as

Bessel or Thomson filters, both of which have inadequate

amplitude characteristics for most video anti-alias

applications) and it is often assumed that linear phase is

unachievable. This is not true. Similarly, in the digital

domain it is often assumed that sharp cutoff amplitude

characteristics can be achieved without overshoot and

ringing. This is also not true. Phase linear filters whether

digital or analog have symmetrical response to

symmetrical inputs. High roll-off rate uncompensated

filters (whether analog or digital) have ringing and

overshoot. In the example below, the traditional 2T test

pulse is applied to a traditional, non-phase linear analog

filter, the ML6420 pure analog anti-alias filter (5.5MHz)

and the combined analog/digital filters (9.3MHz analog

filter and half-band digital filter.)

As seen in Figure 19c, the ML6420 filters provide a time

domain response that is comparable to more complex and

expensive filters.

Typical Passive Filter

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