ALIASING

Another important characteristic of sampled-data systems is

their effect on signals at frequencies greater than one-half

the sampling frequency (The LMF90’s sampling frequency

is the same as the filter’s clock frequency This is the fre-

quency at the CLK pin) 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 frequen-

cy less than one-half the sampling frequency Thus an input

10 Hz This phenomenon is known as ‘‘aliasing’’ Aliasing

can be reduced or eliminated by limiting the input signal

In some cases it may be necessary to use a bandwidth

limiting filter (often a simple passive RC low-pass) ahead of

the bandpass input Although the summing amplifier uses

switched-capacitor techniques it does not exhibit aliasing

behavior and the anti-aliasing filter need not be in its input

signal path The filter can be placed ahead of pin 12 as

shown in

Figure 4 with the non-band limited input signal

applied to pin 11 The output spectrum will therefore be

wideband although limited by the bandwidth of the sum-

ming amplifier’s output buffer amplifier (typically 1 MHz)

ing filter will affect the accuracy of the notch transfer func-

tion however so it is best to use the highest available

clock-to-center-frequency ratio (1001) and set the RC filter

cutoff frequency to about 15 to 20 times the notch frequen-

cy This will provide reasonable attenuation of high-frequen-

cy input signals while avoiding degradation of the overall

notch response If the anti-aliasing filter’s cutoff frequency is

too low it will introduce phase shift and gain errors large

enough to shift the frequency of the notch and reduce its

depth A cutoff frequency that is too high may not provide

sufficient attenuation of unwanted high-frequency signals

TLH10354 – 7

FIGURE 3 Output waveform of a switched-capacitor

filter Note the voltage steps caused by sampling

at the clock frequency

TLH10354 – 8

FIGURE 4 Using a simple passive low-pass filter to prevent aliasing in the presence of high-frequency input signals

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