Datasheet

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SBAS274H − MARCH 2003 − REVISED MAY 2007
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23
FREQUENCY RESPONSE
The linear phase FIR digital filter sets the overall fre-
quency 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 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.
Figure 20 shows the passband ripple from dc to 2.2MHz
(f
CLK
= 40MHz). Figure 21 shows a closer view of the
passband transition by plotting the response from
2.0MHz to 2.5MHz (f
CLK
= 40MHz).
The overall frequency response repeats at multiples of
the CLK frequency. To help illustrate this, Figure 22
shows the response out to 120MHz (f
CLK
= 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 sig-
nal. 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.
20
0
−
20
−
40
−
60
−
80
−
100
−
120
042 6 10 14 188
Frequency (MHz)
12 16 20
Magnitude (dB)
f
CLK
=40MHz
Figure 19. Frequency Response
0.0025
0.0020
0.0015
0.0010
0.0005
0
−
0.0005
−
0.0010
−
0.0015
−
0.0020
00.51.0
Frequency (MHz)
1.5 2.0 2.5
Magnitude (dB)
f
CLK
=40MHz
Figure 20. Passband Ripple
1
0
−
1
−
2
−
3
−
4
−
5
−
6
−
7
2.0 2.12.05 2.15 2.25 2.35 2.452.2
Frequency (MHz)
2.3 2.4 2.5
Magnitude (dB)
f
CLK
=40MHz
Figure 21. Passband Transition
20
0
−
20
−
40
−
60
−
80
−
100
02040
Frequency (MHz)
60 80 120100
Magnitude (dB)
f
CLK
= 40MHz
Figure 22. Frequency Response Out to 120MHz