AD9680 Datasheet, PDF (55/97 Pages) Analog Devices – Dual, 14-Bit, 1.25 GSPS, 1.2 V/2.5 V, Analog-to-Digital Converter

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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
–f_S/2
–f_S/3
–f_S/4
–f_S/32 f_S/32
–f_S/8 –f_S/16 _DC
f_S/16 f_S/8
f_S/4
f_S/3
Sample
& Buy
AD9680
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 149. 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.
An additional 0.05 dB of loss is introduced by the NCO. The
total loss of a real input signal mixed down to baseband is 6.05 dB.
For this reason, it is recommended that the user compensate for
this loss by enabling the additional 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 each I/Q sample can reach is 1.414 × full scale
after it passes through the complex mixer. To avoid overrange
of the I/Q samples and to keep the data bitwidths aligned with
real mixing, 3.06 dB of loss (0.707 × full scale) is introduced in
the mixer for complex signals. An additional 0.05 dB of loss is
introduced by the NCO. 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 AD9680 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 and 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.
The NCO frequency tuning word can be calculated 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.
fS is the AD9680 sampling frequency (clock rate) in Hz.
fC is the desired carrier frequency 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. C | Page 55 of 97