ADS54J20 Datasheet, PDF (28/81 Pages) Texas Instruments – Dual-Channel, 12-Bit, 1.0-GSPS, Analog-to-Digital Converter

ADS54J20
SBAS766A – MAY 2016 – REVISED MAY 2016
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8.3.2.2 Decimate-by-4 Filter Using a Digital Mixer
This band-pass decimation filter consists of a digital mixer and three concatenated FIR filters with a combined
latency of approximately 28 output clock cycles. The alias-band attenuation is approximately 55 dB and the pass-
band flatness is ±0.1 dB. By default after reset, the band-pass filter is centered at f_S / 16. Using the SPI, the
center frequency can be programmed at N × f_S / 16 (where N = 1, 3, 5, or 7). Table 2 shows corner frequencies
for two extreme options.
Table 2. Corner frequencies for the Decimate-by-4 Filter
CORNERS (dB)
–0.1
–0.5
–1
–3
CORNER FREQUENCY AT LOWER SIDE
(Center Frequency f_S / 16)
0.011 × f_S
0.010 × f_S
0.008 × f_S
0.006 × f_S
CORNER FREQUENCY AT HIGHER SIDE
(Center Frequency f_S / 16)
0.114 × f_S
0.116 × f_S
0.117 × f_S
0.120 × f_S
Figure 62 and Figure 63 show the frequency response of the decimate-by-4 filter for center frequencies f_S / 16
and 3 × f_S / 16 (N = 1 and N = 3, respectively).
10
0
-10
-20
-30
-40
-50
-60
-70
-80
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Frequency Response
D015
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1
0
0.05
0.1
0.15
0.2
Frequency Response
0.25
D016
Figure 62. Decimate-by-4 Filter Response
Figure 63. Decimate-by-4 Filter Response (Zoomed)
8.3.2.3 Decimate-by-4 Filter with IQ Outputs
In this configuration, the DDC block includes a fixed digital f_S / 4 mixer. Thus, the IQ pass band is approximately
±0.11 f_S, centered at f_S / 4. This decimation filter has 41 taps with a latency of approximately ten output clock
cycles. The stop-band attenuation is approximately 90 dB and the pass-band flatness is ±0.05 dB. Table 3 shows
the corner frequencies for a low-pass, decimate-by-4 IQ filter.
Table 3. Corner Frequencies for a Decimate-by-4 IQ Output Filter
CORNERS (dB)
–0.1
–0.5
–1
–3
LOW PASS
0.107 × f_S
0.112 × f_S
0.115 × f_S
0.120 × f_S
28
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