Datasheet

AD9125
Rev. 0 | Page 38 of 56
DATA RATES VS. INTERPOLATION MODES
Table 23 summarizes the maximum bus speed (f
BUS
), the
supported input data rates, and the signal bandwidths for
various combinations of bus width modes and interpolation
rates. The maximum bus speed in any mode is 250 MHz. The
maximum DAC update rate (f
DAC
) in any mode is 1000 MHz.
The real signal bandwidth supported is a fraction of the input
data rate, which depends on the interpolation filter (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 enabling HB1 and HB2.
In some cases, power dissipation can be lowered by avoiding
HB1. If the bandwidth required is low enough, 2× interpolation
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 quadrature
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 mixing functions as
shown in Table 2 2 .
Table 22. 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, …
3 × f
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 22, the mixing functions of most of the modes
result in cross coupling samples between the I and Q channels.
The I and Q channels only operate independently 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 23. Summary of Data Rates and Bandwidths vs. Interpolation Modes
Filter Modes
Bus Width HB3 HB2 HB1 f
BUS
(Mbps) f
DATA
(Mbps) Real Signal Bandwidth (MHz) f
DAC
(MHz)
Byte Mode
(8 Bits)
0 0 0 250 62.5 31.25 62.5
0 0 1 250 62.5 25 125
0 1 0 250 62.5 15.625
125
0 1 1 250 62.5 25
250
1 1 0 250 62.5 15.625
250
1 1 1 250 62.5 25
500
Word Mode
(16 Bits)
0 0 0 250 125 62.5 125
0 0 1 250 125 50 250
0 1 0 250 125 31.25
250
0 1 1 250 125 50
500
1 1 0 250 125 31.25
500
1 1 1 250 125 50
1000
Dual-Word Mode
(32 Bits)
0 0 0 250 250 125 250
0 0 1 250 250 100 500
0 1 0 250 250 62.5
500
0 1 1 250 250 100
1000
1 1 0 250 250 62.5
1000
1 1 1 125 125 50 1000