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SBAS288D − JUNE 2003 − REVISED AUGUST 2004
Table 11 shows the averaging and corresponding data rate
for each of the 16 valid DRATE register settings when
fCLKIN = 7.68MHz. Note that the data rate scales directly
with the CLKIN frequency. For example, reducing fCLKIN
from 7.68MHz to 3.84MHz reduces the data rate for
DR[7:0] = 11110000 from 30,000SPS to 15,000SPS.
DIGITAL FILTER
The programmable low-pass digital filter receives the
modulator output and produces a high-resolution digital
output. By adjusting the amount of filtering, tradeoffs can
be made between resolution and data rate: filter more for
higher resolution, filter less for higher data rate. The filter
is comprised of two sections, a fixed filter followed by a
programmable filter. Figure 13 shows the block diagram of
the analog modulator and digital filter. Data is supplied to
the filter from the analog modulator at a rate of fCLKIN/4.
The fixed filter is a 5th-order sinc filter with a decimation
value of 64 that outputs data at a rate of fCLKIN/256. The
second stage of the filter is a programmable averager
(1st-order sinc filter) with the number of averages set by
the DRATE register. The data rate is a function of the
number of averages (Num_Ave) and is given by
Equation 1.
Table 11. Number of Averages and Data Rate for
Each Valid DRATE Register Setting
NUMBER OF AVERAGES FOR
(1)
DRATE
DR[7:0]
DATA RATE
(SPS)
PROGRAMMABLE FILTER
(Num_Ave)
11110000
11100000
11010000
11000000
10110000
10100001
10010010
10000010
01110010
01100011
01010011
01000011
00110011
00100011
00010011
00000011
1 (averager bypassed)
30,000
15,000
7500
3750
2000
1000
500
100
60
2
4
8
15
fCLKIN
Data Rate + ǒ Ǔǒ
256
1
Ǔ
30
60
Num_Ave
(1)
300
500
Modulator Rate =
/4
f
600
50
f
CLKIN
1
CLKIN
256
ǒ
Ǔ
+ ǒ Ǔ
f
DataRate
CLKIN
DataRate +
256
Num_Ave
1000
30
1200
25
5
Analog
Modulator
sinc
Programmable
Averager
2000
15
Filter
3000
10
6000
5
Num_Ave
12,000
= 7.68MHz.
2.5
(set by DRATE)
(1)
for f
CLKIN
Digital Filter
Figure 13. Block Diagram of the Analog
Modulator and Digital Filter
18
BB [ BURR-BROWN CORPORATION ]
