ISL5216_07 Datasheet, PDF (8/65 Pages) Intersil Corporation – Four-Channel Programmable Digital Downconverter

ISL5216
point modes. The floating point modes and the mapping of the
parallel 17-bit input format is discussed below.
Floating Point Input Mode Bit Mapping
The input bit weighting for fixed point inputs on busses A, B,
C, and D is:
bit 15 (MSB): 2⁰, bit 14: 2^-1, bit 13: 2^-2, ..., bit 0: 2^-15.
For floating point modes, the least significant two or three
bits are used as exponent bits (See Floating Point Input
Mode Bit Mapping Tables).
The first three floating point modes shown below are included
for backward compatibility with the HSP50216 and their
functionality remains unchanged. The 14-bit mantissa/2-bit
exponent mode present in the HSP50216 has been extended
from a 12dB range to 18dB in the ISL5216. This mode as well
as those which follow it in the tables below use the CIC’s
barrel shifter to provide the gain. This places a limit on the
CIC’s largest available decimation. As an example, assume
the CIC is set for 5th order and the decimation needs to be
300. The CIC’s gain, 300⁵, is compensated for in the barrel
shifter with a shift factor of 45 - ceil(log₂(300⁵)) = 3 where
shifts are from LSB towards MSB and a shift of 45
corresponds to no attenuation. If the shift factor is set as 0 in
this example, there is room for 3 * 6 = 18dB of gain. Raising
the CIC decimation lowers the shift factor (to further attenuate
the CIC input signal) and limits the available gain range. This
CIC decimation/floating point gain range trade off is handled
automatically by the evaluation board software. Additional
information on the CIC can be found in the CIC Filter section
of this data sheet.
Floating Point Input Mode Bit Mapping Tables
((
11-BIT MODE: 11 TO 13-BIT MANTISSA (15:3), 3-BIT EXPONENT (2:0), 30dB EXPONENT RANGE (Note 3)
EXPONENT GAIN (dB)
PIN BIT WEIGHTING TO 16-BIT INPUT MAPPING
X(2:0) = 000 0
X(2:0) = 001 6
X15 X15 X15 X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5
X15 X15 X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4
X(2:0) = 010 12
X15 X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3
X(2:0) = 011 18
X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0
X(2:0) = 100 24
X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0 0
X(2:0) = 101 30
(Note 1)
X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0 0 0
NOTES:
1. Or 110 or 111, the exponent input saturates at 101.
2. “Xnn” = input A, B, C, or D bit nn.
3. To select this mode, set IWA *000H/GWA F804H bits 17, 16, 8 and 7 to 0.
12-BIT MODE: 12 TO 13-BIT MANTISSA (15:3), 3-BIT EXPONENT (2:0), 24dB EXPONENT RANGE (Note 5)
EXPONENT GAIN (dB)
PIN BIT WEIGHTING TO 16-BIT INPUT MAPPING
X(2:0) = 000
0
X15 X15 X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4
X(2:0) = 001
6
X15 X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3
X(2:0) = 010
12
X15 X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0
X(2:0) = 011
18
X15 X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0 0
X(2:0) = 100
24
(Note 4)
X15 X14 X13 X12 X11 X10 X9 X8 X7 X6 X5 X4 X3 0 0 0
NOTES:
4. Or 101, 110, or 111, the exponent input saturates at 100.
5. To select this mode, set IWA *000H/GWA F804H bits 17, 16, 8 and 7 to 0, 0, 0 and 1 respectively.
8
FN6013.3
July 13, 2007