10

LTC1063

S

APPLICATI

I FOR ATIO

Aliasing

Aliasing is an inherent phenomenon of sampled data filters

and it primarily occurs when the frequency of an input

signal approaches the sampling frequency. For the

LTC1063, an input signal whose frequency is in the range

passband and stopband. Table 4 shows details.

Example:

An input RC can be used to attenuate incoming signals

close to the filter clock frequency (Figure 10). A Butterworth

passband response will be maintained if the value of the

input resistor follows Table 1.

Table 4. Aliasing Data

INPUT FREQUENCY

OUTPUT FREQUENCY

0dB

0dB

0.99

0.01

–3

dB

– 10.2 dB

– 17.7 dB

– 24.3 dB

0.98

0.02

–30

dB

–40

dB

0.97

0.03

–48

dB

– 54.5 dB

0.96

0.04

– 60.4 dB

– 65.5 dB

0.95

0.05

– 70.16 dB

0.94

0.06

– 78.25 dB

0.93

0.07

– 85.3 dB

0.9

0.1

– 100.3 dB

Figure 10. Adding an Input Anti-Aliasing RC

noise ratio at a given distortion level. The wideband noise

(

frequency and excludes the clock feedthrough. The

LTC1063’s typical wideband noise is 95

shows the same scope photo as Figure 8 but with a more

sensitive vertical scale: The clock feedthrough is imbed-

ded in the filter’s wideband noise. The peak-to-peak

wideband noise of the filter can be clearly seen; it is

approximately 500

95

factor or 5.25.

OUTPUT AMPLITUDE

REFERENCED TO

INPUT SIGNAL

2

µs/DIV

1063 F08

2

µs/DIV

1063 F09

Figure 8. LTC1063 Output Clock Feedthrough + Noise

Figure 9. LTC1063 Output Clock Feedthrough + Noise

1063 F10

C

R

0.1

µF

1

2

3

4

8

7

6

5

LTC1063

0.1

µF

20

10

2

πRC

≤≤

1