MT-017 Datasheet, PDF (1/7 Pages) Analog Devices – Oversampling Interpolating DACs

MT-017
TUTORIAL
Oversampling Interpolating DACs
INTRODUCTION
by Walt Kester
Oversampling and digital filtering eases the requirements on the antialiasing filter which
precedes an ADC. The concept of oversampling and interpolation can be used in a similar
manner with a reconstruction DAC. For instance, oversampling is common in digital audio CD
players, where the basic update rate of the data from the CD is 44.1 kSPS. Early CD players used
traditional binary DACs and inserted "zeros" into the parallel data, thereby increasing the
effective update rate to 4-times, 8-times, or 16-times the fundamental throughput rate. The 4×,
8×, or 16× data stream is passed through a digital interpolation filter which generates the extra
data points. The high oversampling rate moves the image frequencies higher, thereby allowing a
less complex lower cost filter with a wider transition band. In addition, there is an increase in the
SNR within the signal bandwidth because of the process gain. The sigma-delta DAC architecture
uses a much higher oversampling rate and represents the ultimate extension of this concept and
has become popular in modern CD players.
The same concept of oversampling and interpolation is also utilized in high speed DACs used in
communications applications, relaxing the requirements on the output filter as well as increasing
the SNR due to process gain.
OUTPUT SPECTRUM OF A RECONSTRUCTION DAC
The output of a reconstruction DAC can be represented as a series of rectangular pulses whose
width is equal to the reciprocal of the clock rate as shown in Figure 1.
SAMPLED
SIGNAL
t
RECONSTRUCTED
SIGNAL
t
1
1
A
–3.92dB
f_c
πf
sin
A=
f_c
πf
IMAGES
IMAGES
f_c
IMAGES
0
f
0.5f_c
f_c
1.5f_c
2f_c
2.5f_c
3f_c
Figure 1: Unfiltered DAC Output Showing Images and sin (x)/x Roll Off
Rev.A, 10/08, WK
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