Datasheet

AD9122
Rev. B | Page 42 of 60
DATA RATES vs. INTERPOLATION MODES
Table 22 summarizes the maximum bus speed (f
BUS
), supported
input data rates, and signal bandwidths with the various combi-
nations of bus width modes and interpolation rates. The real
signal bandwidth supported is a fraction of the input data rate,
which depends on the interpolation filters (HB1, HB2, or HB3)
selected. The complex signal bandwidth supported is twice the
real signal bandwidth.
In general, 2× interpolation is best supported by enabling HB1,
and 4× interpolation is best supported by enabling HB1 and HB2.
However, in some cases, power dissipation can be lowered by not
using HB1. If the bandwidth required is low enough, 2× interpo-
lation can be supported by using HB2, and 4× interpolation can
be supported by using HB2 and HB3.
COARSE MODULATION MIXING SEQUENCES
The coarse digital quadrature modulation occurs within the
interpolation filters. The modulation shifts the frequency
spectrum of the incoming data by the frequency offset selected.
The frequency offsets available are multiples of the input data
rate. The modulation is equivalent to multiplying the quadra-
ture input signal by a complex carrier signal, C(t), of the form
C(t) = cos(ω
c
t) + j sin(ω
c
t)
In practice, this modulation results in the mixing functions
shown in Table 21.
Table 21. Modulation Mixing Sequences
Modulation Mixing Sequence
f
S
/2 I = I, −I, I, −I, …
Q = Q, −Q, Q, −Q, …
f
S
/4 I = I, Q, −I, −Q, …
Q = Q, −I, −Q, I, …
3f
S
/4 I = I, −Q, −I, Q, …
Q = Q, I, −Q, −I, …
f
S
/8 I = I, r(I + Q), Q, r(−I + Q), −I, −r(I + Q), −Q, r(I − Q), …
Q = Q, r(Q − I), −I, −r(Q + I), −Q, r(−Q + I), I, r(Q + I), …
Note that
2
2
=r
As shown in Table 21, the mixing functions of most of the modes
cross-couple samples between the I and Q channels. The I and
Q channels operate independently only in f
S
/2 mode. This means
that real modulation using both the I and Q DAC outputs can
only be done in f
S
/2 mode. All other modulation modes require
complex input data and produce complex output signals.
Table 22. Summary of Data Rates and Bandwidths vs. Interpolation Modes (DVDD18, CVDD18 = 1.9 V ± 2%)
Filter Modes
Bus Width HB3 HB2 HB1 f
BUS
(Mbps) f
DATA
(Mbps) Real Signal Bandwidth (MHz) f
DAC
(MHz)
Nibble (4 Bits) 0 0 0 1230 153.75 75 153.75
0 0 1 1230 153.75 60 307.5
0 1 0 1230 153.75 37.5
307.5
0 1 1 1230 153.75 60
615
1 1 0 1230 153.75 37.5
615
1 1 1 1230 153.75 60
1230
Byte (8 Bits) 0 0 0 1230 307.5 150 307.5
0 0 1 1230 307.5 120 615
0 1 0 1230 307.5 75
615
0 1 1 1230 307.5 120
1230
1 1 0 1230 307.5 75
1230
1 1 1 615 153.75 60
1230
Word (16 Bits) 0 0 0 1230 615 300 615
0 0 1 1000 500 200 1000
0 1 0 1230 615 150
1230
0 1 1 615 307.5 120
1230
1 1 0 615 307.5 75
1230
1 1 1 307.5 153.75 60 1230