Datasheet

ADS1255
ADS1256
SBAS288K − JUNE 2003 − REVISED SEPTEMBER 2013
www.ti.com
18
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
CLKIN
/4.
The fixed filter is a 5th-order sinc filter with a decimation
value of 64 that outputs data at a rate of f
CLKIN
/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.
Data Rate +
ǒ
f
CLKIN
256
Ǔǒ
1
Num_Ave
Ǔ
Num_Ave
(set by DRATE)
Modulator Rate =
f
CLKIN
/4
Analog
Modulator
sinc
5
Filter
Programmable
Averager
Digital Filter
DataRate +
f
CLKIN
256
DataRate +
ǒ
f
CLKIN
256
Ǔ
ǒ
1
Num_Ave
Ǔ
Figure 13. Block Diagram of the Analog
Modulator and Digital Filter
Table 11 shows the averaging and corresponding data rate
for each of the 16 valid DRATE register settings when
f
CLKIN
= 7.68MHz. Note that the data rate scales directly
with the CLKIN frequency. For example, reducing f
CLKIN
from 7.68MHz to 3.84MHz reduces the data rate for
DR[7:0] = 11110000 from 30,000SPS to 15,000SPS.
Table 11. Number of Averages and Data Rate for
Each Valid DRATE Register Setting
DRATE
DR[7:0]
NUMBER OF AVERAGES FOR
PROGRAMMABLE FILTER
(Num_Ave)
DATA RATE
(1)
(SPS)
11110000 1 (averager bypassed) 30,000
11100000 2 15,000
11010000 4 7500
11000000 8 3750
10110000 15 2000
10100001 30 1000
10010010 60 500
10000010 300 100
01110010 500 60
01100011 600 50
01010011 1000 30
01000011 1200 25
00110011 2000 15
00100011 3000 10
00010011 6000 5
00000011 12,000 2.5
(1)
for f
CLKIN
= 7.68MHz.
FREQUENCY RESPONSE
The low-pass digital filter sets the overall frequency
response for the ADS1255/6. The filter response is the
product of the responses of the fixed and programmable
filter sections and is given by Equation 2.
|
H
(
f
)|
+
Ť
H
sinc
5
(
f
)
Ť
·
Ť
H
Averager
(
f
)
Ť
+
ȧ
ȧ
ȧ
ȧ
sin
ǒ
256p · f
f
CLKIN
Ǔ
64 · sin
ǒ
4p · f
f
CLKIN
Ǔ
ȧ
ȧ
ȧ
ȧ
5
·
ȧ
ȧ
ȧ
ȧ
sin
ǒ
256p · Num_Ave f
f
CLKIN
Ǔ
Num_Ave · sin
ǒ
256p · f
f
CLKIN
Ǔ
ȧ
ȧ
ȧ
ȧ
The digital filter attenuates noise on the modulator output,
including noise from within the ADS1255/6 and external
noise present on the ADS1255/6 input signal. Adjusting
the filtering by changing the number of averages used in
the programmable filter changes the filter bandwidth. With
a higher number of averages, bandwidth is reduced and
more noise is attenuated.
The low-pass filter has notches (or zeros) at the data
output rate and multiples thereof. At these frequencies, the
filter has zero gain. This feature can be useful when trying
to eliminate a particular interference signal. For example,
to eliminate 60Hz (and the harmonics) pickup, set the data
rate equal to 2.5SPS, 5SPS, 10SPS, 15SPS, 30SPS, or
60SPS. To help illustrate the filter characteristics,
(1)
(2)