LMF100 datasheet(25/28 Pages) NSC

25 / 28 page

3.0 Applications Information (Continued)

3.5 SAMPLED DATA SYSTEM CONSIDERATIONS

The LMF100 is a sampled data filter, and as such, differs in

many ways from conventional continuous-time filters. An im-

portant characteristic of sampled-data systems is their effect

on signals at frequencies greater than one-half the sampling

frequency. (The LMF100’s sampling frequency is the same

as its clock frequency.) If a signal with a frequency greater

than one-half the sampling frequency is applied to the input

of a sampled data system, it will be “reflected” to a frequency

less than one-half the sampling frequency. Thus, an input

signal whose frequency is f

s/2 + 100 Hz will cause the sys-

tem to respond as though the input frequency was

s/2 − 100 Hz. This phenomenon is known as “aliasing”, and

can be reduced or eliminated by limiting the input signal

spectrum to less than f

s/2. This may in some cases require

the use of a bandwidth-limiting filter ahead of the LMF100 to

limit the input spectrum. However, since the clock frequency

is much higher than the center frequency, this will often not

be necessary.

Another characteristic of sampled-data circuits is that the

output signal changes amplitude once every sampling pe-

riod, resulting in “steps” in the output voltage which occur at

the clock rate (

Figure 25). If necessary, these can be

“smoothed” with a simple R-C low-pass filter at the LMF100

output.

The ratio of f

CLK to fc (normally either 50:1 or 100:1) will also

affect performance. A ratio of 100:1 will reduce any aliasing

problems and is usually recommended for wide-band input

DS005645-39

= 100 k

Ω

R2 = 1k

Ω

R3 = 100 k

Ω

R4 = 100 k

Ω

Rg = 10 k

Ω

Rl = 10 k

Ω

Rh = 10 k

Ω

FIGURE 23. Second-Order Notch Filter

DS005645-13

FIGURE 24. Method for Trimming V

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