ADS1255IDB Burr-Brown Corporation, ADS1255IDB Datasheet - Page 18

• Manufacturer Part Number: ADS1255IDB
• Description: Very Low Noise/ 24-Bit Analog-to-Digital Converter
• Manufacturer: Burr-Brown Corporation

ADS1255
ADS1256
SBAS288D − JUNE 2003 − REVISED AUGUST 2004
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 f
The fixed filter is a 5th-order sinc filter with a decimation
value of 64 that outputs data at a rate of f
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.
18
Modulator Rate =
Modulator
f CLKIN /4
Analog
Figure 13. Block Diagram of the Analog
Data Rate +
Modulator and Digital Filter
DataRate +
sinc 5
Filter
f
CLKIN
256
f
256
CLKIN
Digital Filter
(set by DRATE)
Programmable
DataRate +
Num_Ave
Averager
Num_Ave
1
f
CLKIN
256
CLKIN
Num_Ave
1
/256. The
CLKIN
(1)
/4.
Table 11 shows the averaging and corresponding data rate
for each of the 16 valid DRATE register settings when
f
with the CLKIN frequency. For example, reducing f
from 7.68MHz to 3.84MHz reduces the data rate for
DR[7:0] = 11110000 from 30,000SPS to 15,000SPS.
CLKIN
(1) for f CLKIN = 7.68MHz.
Table 11. Number of Averages and Data Rate for
10100001
10010010
10000010
11110000
11100000
11010000
11000000
10110000
01110010
01100011
01010011
01000011
00110011
00100011
00010011
00000011
DRATE
DR[7:0]
= 7.68MHz. Note that the data rate scales directly
Each Valid DRATE Register Setting
NUMBER OF AVERAGES FOR
PROGRAMMABLE FILTER
1 (averager bypassed)
(Num_Ave)
12,000
1000
1200
2000
3000
6000
300
500
600
15
30
60
2
4
8
DATA RATE (1)
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30,000
15,000
(SPS)
7500
3750
2000
1000
500
100
2.5
60
50
30
25
15
10
5
CLKIN