SM5816AF_06 Datasheet, PDF (18/27 Pages) Nippon Precision Circuits Inc – 6-channel DSD-PCM Converter

SM5816AF
Decimation Filter Selection
The decimation filter coefficient set can be selected from 5 sets of fixed coefficients (8fs: 4 sets, 2fs: 1 set) and
1 set of programmable coefficients (8fs or 2fs). The coefficient set is selected using SELRAM and SELFIR1,2.
Fixed coefficient selection
The fixed coefficients are selected when SELRAM = “L”. The filter characteristic is 240th-order for 8fs, and
480th-order for 2fs. When 8fs PCM output is selected, 4 sets of characteristic filter coefficients are available for
selection using SELFIR1, 2.
SELFIR2
L
L
H
H
SELFIR1
L
H
L
H
coeff1
coeff2
coeff3
coeff4
Filter characteristic
NPC standard filter
40kHz cutoff filter
50kHz cutoff filter
70kHz cutoff filter
RAM coefficient selection
A RAM coefficient set is selected when SELRAM = “H”.
Initially, the data in RAM is indeterminate, so coefficient data must first be written to the RAM. While data is
being written to RAM, the result of any RAM coefficient operation is not guaranteed, hence a fixed coefficient
set should be used before switching to RAM coefficients or the PCM outputs should be muted.
The RAM supports 29-bit × 240 words for combined 8fs and 2fs output. The filter characteristic is 240th-order
for 8fs, and 480th-order for 2fs. The filter structure uses an even number order of symmetrical coefficients,
reducing by half the number of coefficients to write to RAM. The coefficient data for 8fs and 2fs is in 29-bit 2s
complement format.
Details about writing coefficient data are described in “RAM coefficient write mode”.
DSD Gain Switching
The DSD signal 50% level can be set to 0dB, in 8fs/2fs output modes, using DSGAIN.
DSGAIN = “L” : 100% modulation = 0dB (PCM)
DSGAIN = “H” : 50% modulation = 0dB (PCM) *+ 6dB internal amplification
+1.0
+0.5
0dB when
DSGAIN="H"
[7FFFFFFFh]
0dB when
DSGAIN="L"
[7FFFFFFFh]
0
[00000000h]
[00000000h]
-0.5
[80000000h]
-1.0
Figure 4. DSD modulation level
Note. With DSGAIN = “H” and 50% DSD modulation or greater, a limiter is needed to prevent output signal clipping.
[80000000h]
SEIKO NPC CORPORATION —18