ISL5216_05 INTERSIL [Intersil Corporation], ISL5216_05 Datasheet - Page 12

• Manufacturer Part Number: ISL5216_05
• Description: Four-Channel Programmable Digital DownConverter
• Manufacturer: INTERSIL [Intersil Corporation]

ISL5216
integrators are slightly oversized to reduce the quantization
noise at each stage.
N
A CIC filter has a gain of R
, where R is the decimation factor
and N is the number of stages. Because the CIC filter gain
can become very large with decimation, an attenuator is
provided ahead of the CIC to prevent overflow. The 24 bits of
sample data are placed on the low 24 bits of a 69 bit bus
-45
(width of the first CIC integrator) for a gain of 2
. A 48 bit
0
47
barrel shifter then provides a gain of 2
to 2
inclusive
before passing the data to the CIC. The overall gain in the
pre-CIC attenuator can therefore be programmed to be any
-45
one of 48 values from 2
to 4, inclusive (see IWA=*004,
bits 19:14). This shift factor is adjusted to keep the total
barrel shifter and CIC filter gain between 0.5 and 1.0. The
equation which should be used to compute the necessary
shift factor is:
N
Shift Factor = 45 - Ceiling(log
(R
)).
2
CIC barrel shifts of greater than 45 will cause MSB bits to be
lost. Most of the floating point modes on the ISL5216 make
use of the CIC barrel shifter for gain. This limits the
maximum usable decimation. In particular, shift factor minus
maximum exponent must be greater than or equal to zero.
Maximum exponent ranges from 0 to 1, 3, or 7 for 1, 2 and 3
exponent bits, representing up to 6, 18, or 42dB of gain,
respectively. See Floating Point Input Mode section for
details.
12
July 8, 2005