鈥?/div>
Passband Frequency
Passband Ripple
Stopband Frequency
Stopband Attenuation
f
A
f
S
鈥?f
A
f
S
f
S
+ f
A
Signal Frequency
Sampling Spectrum
f
FIGURE 2. Sampling Spectrum.
SUPPRESSION OF THE SAMPLING SPECTRA
BY DIGITAL FILTERING
A sampling spectra level is given by the stopband attenua-
tion response of the D/A converter combined with the
oversampling signal filter. Figure 3 shows this relation.
When signals f
A1
and f
A2
are input into a digital filter with
stopband attenuation like Figure 3, the sampling spectrum
distributes on f
S
鹵f
A1
, f
S
鹵f
A2
(only the lower frequencies of
spectrum). If attenuation at f
S
鈥?f
A1
and f
S
鈥?f
A2
frequencies
are P
1
and P
2
respectively, then each spectra level should be
P
1
and P
2
. Notice that this attenuation level depends upon
the input signal frequency f
A
.
The passband frequency and passband ripple only affect
audio band characteristics, while the stopband frequency
and stopband attenuation do not influence THD+N versus
frequency characteristics directly because these are out of
the audio band. But depending on the test condition, the
appearance of THD+N versus frequency characteristics is
decreased so that the out of audio band spectrum remains in
the measurement band.
Figure 1 shows the digital filter frequency response. This is
an example of 8x oversampling, but stopband attenuation
against the f
S
/2 of the passband depends upon the signal
frequency.
Passband
ATT
0dB
Stop Band
ATT (dB)
Stop Band Attenuation
0
鈥?0
f
f
S
/2
f
S
2f
S
4f
S
6f
S
8f
S
P
1
鈥?0
鈥?0
鈥?0
鈥?00
f
S
/2
P
2
F
FIGURE 1. Digital Filter Attenuation.
SIGNAL FREQUENCY AND SAMPLING SPECTRA
Given a signal frequency (f
A
), the sampling spectra of digital-
to-analog conversion is then f
S
鹵f
A
. This is not caused by the
DAC architecture, it is a result of sampling theory. This is
shown in Figure 2.
漏
f
S
鈥?f
A1
f
S
鈥?f
A2
f
S
FIGURE 3. Digital Filter Attenuation with Spectrum.
1997 Burr-Brown Corporation
AB-124
1
Printed in U.S.A. November, 1997
1
2
3
