Datasheet

0 0.05
NormalizedInputFrequency(f /f )
IN DATA
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
-
-
-
-
Magnitude(mdB)
0.10 0.15 0.20 0.400.25 0.30 0.35
0.35 0.40 0.45
NormalizedInputFrequency(f /f )
IN DATA
0
-20
-40
-60
-80
-100
-140
Magnitude(dB)
0.65
-120
0.50 0.55 0.60
0 5 10
TimeIndex(1/f )
DATA
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
-0.2
StepSize
15 20 25 30 35 40 45 50 55 60 65
LinearPhaseFilter
MinimumPhaseFilter
ADS1281
SBAS378D –AUGUST 2007–REVISED JUNE 2010 www.ti.com
As shown in Figure 29, the FIR frequency response back (or alias) into the passband and cause errors.
provides a flat passband to 0.375 of the data rate Placing an anti-alias, low-pass filter in front of the
(±0.003dB passband ripple). Figure 30 shows the ADS1281 inputs is recommended to limit possible
transition from passband to stop band. out-of-band input signals. Often, a single RC filter is
sufficient.
GROUP DELAY AND STEP RESPONSE
The FIR block is implemented as a multi-stage FIR
structure with selectable linear or minimum phase
response. The passband, transition band, and stop
band responses of the filters are nearly identical but
differ in the respective phase responses.
Linear Phase Response
Linear phase filters exhibit constant delay time versus
input frequency (that is, constant group delay). Linear
phase filters have the property that the time delay
from any instant of the input signal to the same
instant of the output data is constant and is
independent of the signal nature. This filter behavior
Figure 29. FIR Passband Response (f
DATA
=
results in essentially zero phase error when analyzing
500Hz)
multi-tone signals. However, the group delay and
settling time of the linear phase filter are somewhat
larger than the minimum phase filter, as shown in
Figure 31.
Figure 30. FIR Transition Band Response
Although not shown in Figure 30, the passband
Figure 31. FIR Step Response
response repeats at multiples of the modulator
frequency (Nf
MOD
– f
0
and Nf
MOD
+ f
0
, where N = 1, 2,
etc. and f
0
= passband). These image frequencies, if
present in the signal and not externally filtered, fold
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