ꢋ ꢑꢉ ꢀꢁ ꢗꢇ
ꢋ ꢑꢉ ꢀꢁ ꢗꢁ
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SBAS274E − MARCH 2003 − REVISED JUNE 2004
FREQUENCY RESPONSE
0.0025
0.0020
0.0015
0.0010
0.0005
0
The linear phase FIR digital filter sets the overall frequency
response. The decimation rate is set to 8 (2XMODE = low)
for all the figures shown in this section. Figure 19 shows
the frequency response from dc to 20MHz for
fCLK = 40MHz
f
CLK = 40MHz. The frequency response of the ADS1605/6
filter scales directly with CLK frequency. For example, if
the CLK frequency is decreased by half (to 20MHz), the
values on the X-axis in Figure 19 would need to be scaled
by half, with the span becoming dc to 10MHz.
−
−
−
−
0.0005
0.0010
0.0015
0.0020
Figure 20 shows the passband ripple from dc to 2.2MHz
(fCLK = 40MHz). Figure 21 shows a closer view of the
passband transition by plotting the response from 2.0MHz
to 2.5MHz (fCLK = 40MHz).
0
0.5
1.0
1.5
2.0
2.5
Frequency (MHz)
The overall frequency response repeats at multiples of the
CLK frequency. To help illustrate this, Figure 22 shows the
response out to 120MHz (fCLK = 40MHz). Notice how the
passband response repeats at 40MHz, 80MHz and
120MHz; it is important to consider this when there is
high-frequency noise present with the signal. The
modulator bandwidth extends to 100MHz. High-frequency
noise around 40MHz and 80MHz will not be attenuated by
either the modulator or the digital filter. This noise will alias
back in-band and reduce the overall SNR performance
unless it is filtered out prior to the ADS1605/6. To prevent
this, place an anti-alias filter in front of the ADS1605/6 that
rolls off before 37MHz.
Figure 20. Passband Ripple
1
0
−
−
−
−
−
−
−
1
2
3
4
5
6
7
fCLK = 40MHz
20
fCLK = 40MHz
0
2.0 2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5
Frequency (MHz)
−
−
−
−
20
40
60
80
Figure 21. Passband Transition
−
−
100
120
20
0
fCLK = 40MHz
0
2
4
6
8
10
12 14
16 18
20
Frequency (MHz)
−
−
−
−
20
40
60
80
Figure 19. Frequency Response.
−
100
0
20
40
60
80
100
120
Frequency (MHz)
Figure 22. Frequency Response Out to 120MHz
23
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