Datasheet

Ť
H
ǒ
f
Ǔ
Ť
+
Ť
H
sinc
5
ǒ
f
Ǔ
Ť
Ť
H
Averager
ǒ
f
Ǔ
Ť
+
ȧ
ȧ
ȧ
ȧ
sin
ǒ
128p f
f
CLK
Ǔ
64 sin
ǒ
2p f
f
CLK
Ǔ
ȧ
ȧ
ȧ
ȧ
5
ȧ
ȧ
ȧ
ȧ
sin
ǒ
128p Num_Ave f
f
CLK
Ǔ
Num_Ave sin
ǒ
128p f
f
CLK
Ǔ
ȧ
ȧ
ȧ
ȧ
0
−
20
−
40
−
60
−
80
−
100
−
120
−
140
Frequency (kHz)
Gain (dB)
125 2500 375 500 625
Data Rate
Auto−Scan Mode
(23.739kSPS)
Data Rate
Fixed−Channel Mode
(125kSPS)
0
−
20
−
40
−
60
−
80
−
100
−
120
−
140
Frequency (kHz)
Gain (dB)
125 2500 375 500 625
Data Rate
Auto−Scan Mode
(15.123kSPS)
Data Rate
Fixed−Channel Mode
(31.25kSPS)
ADS1258-EP
SBAS445D –MARCH 2009– REVISED MARCH 2011 www.ti.com
Table 3 shows a listing of the averaging and data Figure 44 shows the response with averaging set to 4
rates for each of the four DRATE[1:0] register (DRATE[1:0] = 10). 4-reading, post-averaging
settings for the Auto-Scan and Fixed-Channel modes, produces three equally-spaced notches between
with CHOP, DLY = 0. Note that the data rate scales each main notch of the sinc
5
filter. The frequency
directly with f
CLK
. For example, reducing f
CLK
by 2x response of DRATE[1:0] = 01 and 00 follows a similar
reduces the maximum data rate by 2x. pattern, but with 15 and 63 equally-spaced notches
between the main sinc
5
notches, respectively.
FREQUENCY RESPONSE
The low-pass digital filter sets the overall frequency
response for the ADS1258. The filter response is the
product of the responses of the fixed and
programmable filter sections and is given by
Equation 4:
(4)
The digital filter attenuates noise on the modulator
output including noise from within the ADS1258 and
external noise present within the ADS1258 input
signal. Adjusting the filtering by changing the number
Figure 43. Frequency Response, DRATE[1:0] = 11
of averages used in the programmable filter changes
the filter bandwidth. With a higher number of
averages, the bandwidth is reduced and more noise
is attenuated.
The low-pass filter has notches (or zeros) at the data
output rate and multiples thereof. The sinc
5
part of
the filter produces wide notches at f
CLK
/128 and
multiples thereof. At these frequencies, the filter has
zero gain. Figure 43 shows the response with no post
averaging. Note that in Auto-Scan mode, the data
rate is reduced while retaining the same frequency
response as in Fixed-Channel mode.
With programmable averaging, the wide notches
produced by the sinc
5
filter remain, but a number of
narrow notches are superimposed in the response.
The number of the superimposed notches is
determined by the number of readings
averaged (minus one).
Figure 44. Frequency Response, DRATE[1:0] = 10
Table 3. Data Rates
(1)
DATA RATE AUTO-SCAN DATA RATE FIXED-CHANNEL –3dB BANDWIDTH
DRATE[1:0] Num_Ave
(2)
MODE (SPS)
(3)
MODE (SPS) (Hz)
11 1 23739 125000 25390
10 4 15123 31250 12402
01 16 6168 7813 3418
00 64 1831 1953 869
(1) f
CLK
= 16 MHz, Chop = 0, and Delay = 0.
(2) Num_Ave is the number of averages performed by the digital filter second stage.
(3) In Auto-Scan mode, the data rate listed is for a single channel; the effective data rate for multiple channels (on a per-channel basis) is
the value shown in Figure 43 and Figure 44 divided by the number of active channels in a scan loop.
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