AD9684 Datasheet, PDF (38/64 Pages) Analog Devices – Dual, 14-Bit, 1.25 GSPS, 1.2 V/2.5 V, Analog-to-Digital Converter

AD9684
Product
Overview
Online
Documentation
Design
Resources
Discussion
NCO FREQUENCY TUNING WORD (FTW) SELECTION
12-BIT NCO FTW = MIXING FREQUENCY/ADC SAMPLE RATE × 4096
QUADRATURE ANALOG MIXER +
2 ADCs + QUADRATURE DIGITAL
MIXER + NCO
COMPLEX INPUT—SAMPLED AT f_S
REAL
I
ADC
SAMPLING
AT f_S
90°
PHASE
Q
ADC
SAMPLING
AT f_S
IMAGE DUE TO
ANALOG I/Q
MISMATCH
QUADRATURE MIXER
I
I
+I
I _Q–
12-BIT
NCO
Q
90°
0°
Q
COMPLEX
I
Q
QI
+_Q
+
BANDWIDTH OF
INTEREST
Sample
& Buy
Data Sheet
–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
POSITIVE FTW VALUES
12-BIT NCO FTW =
ROUND ((f_S/3)/f_S × 4096) = +1365 (0x555)
–f_S/32 f_S/32
DC
Figure 61. DDC NCO Frequency Tuning Word Selection—Complex Inputs
DDC NCO PLUS MIXER LOSS AND SFDR
Setting Up the NCO FTW and POW
When mixing a real input signal down to baseband, 6 dB of loss
is introduced in the signal due to filtering of the negative image.
The NCO introduces an additional 0.05 dB of loss. The total
loss of a real input signal mixed down to baseband is 6.05 dB. For
this reason, it is recommended to compensate for this loss by
enabling the 6 dB of gain in the gain stage of the DDC to
recenter the dynamic range of the signal within the full scale of
the output bits.
When mixing a complex input signal down to baseband, the
maximum value that each I/Q sample can reach is 1.414 × full
scale after it passes through the complex mixer. To avoid an
overrange of the I/Q samples and to keep the data bit-widths
aligned with real mixing, introduce 3.06 dB of loss (0.707 × full-
scale) in the mixer for complex signals. The NCO introduces an
additional 0.05 dB of loss. The total loss of a complex input
signal mixed down to baseband is −3.11 dB.
The worst case spurious signal from the NCO is greater than
102 dBc SFDR for all output frequencies.
NUMERICALLY CONTROLLED OSCILLATOR
The AD9684 has a 12-bit NCO for each DDC that enables the
frequency translation process. The NCO allows the input
spectrum to be tuned to dc, where it can be effectively filtered
by the subsequent filter blocks to prevent aliasing. The NCO
can be set up by providing a frequency tuning word (FTW) and
a phase offset word (POW).
The NCO frequency value is given by the 12-bit, twos
complement number entered in the NCO FTW. Frequencies
between ±fS/2 (+fS/2 excluded) are represented using the
following frequency words:
• 0x800 represents a frequency of −fS/2.
• 0x000 represents dc (frequency is 0 Hz).
• 0x7FF represents a frequency of +fS/2 − fS/2¹².
Calculate the NCO frequency tuning word using the following
equation:
NCO_
FTW
=
round^{} 2¹²

mod( f_C ,
f_S
f_S ) 

where:
NCO_FTW is a 12-bit, twos complement number representing
the NCO FTW.
fC is the desired carrier frequency in Hz.
fS is the AD9684 sampling frequency (clock rate) in Hz.
mod( ) is a remainder function. For example, mod(110,100) =
10, and for negative numbers, mod(−32, +10) = −2.
round( ) is a rounding function. For example, round(3.6) = 4,
and for negative numbers, round(−3.4) = −3.
Note that this equation applies to the aliasing of signals in the
digital domain (that is, aliasing introduced when digitizing
analog signals).
Rev. 0 | Page 38 of 64