AD9684 Datasheet, PDF (33/64 Pages) Analog Devices – Dual, 14-Bit, 1.25 GSPS, 1.2 V/2.5 V, Analog-to-Digital Converter

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ADC
REAL INPUT—SAMPLED AT f_S
REAL
BANDWIDTH OF
INTEREST IMAGE
ADC
SAMPLING
REAL
AT f_S
BANDWIDTH OF
INTEREST
Sample
& Buy
AD9684
–f_S/2
–f_S/32 f_S/32
–f_S/3
–f_S/4
–f_S/8 –f_S/16 DC
f_S/16 f_S/8
f_S/4
FREQUENCY TRANSLATION STAGE (OPTIONAL)
DIGITAL MIXER + NCO FOR f_S/3 TUNING, THE FREQUENCY
TUNING WORD = ROUND ((f_S/3)/f_S × 4096) = +1365 (0x555)
REAL
12-BIT
NCO
I
cos(wt)
90°
0°
–sin(wt)
Q
DIGITAL FILTER
RESPONSE
BANDWIDTH OF INTEREST
(–6dB LOSS DUE TO
NCO + MIXER)
f_S/3
f_S/2
NCO TUNES CENTER OF
BANDWIDTH OF INTEREST
TO BASEBAND
BANDWIDTH OF
INTEREST IMAGE
(–6dB LOSS DUE TO
NCO + MIXER)
–f_S/32 f_S/32
–f_S/2
–f_S/3
–f_S/4
–f_S/8 –f_S/16
DC
f_S/16 f_S/8
f_S/4
f_S/3
f_S/2
FILTERING STAGE
4 DIGITAL HALF-BAND FILTERS
(HB4 + HB3 + HB2 + HB1)
HB4 FIR
HALF-
I
BAND
FILTER
2
HB3 FIR
HALF-
BAND
FILTER 2
HB2 FIR
HALF-
BAND
FILTER 2
HB1 FIR
HALF-
BAND
FILTER
I
HB4 FIR
HALF-
Q
BAND
FILTER
2
HB3 FIR
HALF-
BAND
FILTER 2
DIGITAL FILTER
RESPONSE
HB2 FIR
HALF-
BAND
FILTER 2
HB1 FIR
HALF-
BAND
FILTER
Q
6dB GAIN TO
COMPENSATE FOR
NCO + MIXER LOSS
GAIN STAGE (OPTIONAL)
0dB OR 6dB GAIN
I
2
I
+6dB
COMPLEX (I/Q) OUTPUTS
DECIMATE BY 16
GAIN STAGE (OPTIONAL)
0dB OR 6dB GAIN
COMPLEX TO REAL
CONVERSION STAGE (OPTIONAL)
f_S/4 MIXING + COMPLEX FILTER TO REMOVE Q
REAL (I) OUTPUTS
DECIMATE BY 8
–f_S/32 f_S/32
–f_S/8 –f_S/16
DC
f_S/16 f_S/8
I
I
+6dB
Q
Q
+6dB
COMPLEX REAL/I
TO
REAL
Q
2
Q
+6dB
–f_S/32
–f_S/16
DC
f_S/32
f_S/16
DOWNSAMPLE BY 2
6dB GAIN TO
COMPENSATE FOR
NCO + MIXER LOSS
–f_S/32 f_S/32
–f_S/8 –f_S/16
DC
f_S/16 f_S/8
Figure 59. DDC Theory of Operation Example (Real Input, Decimate by 16)
Rev. 0 | Page 33 of 64