AD9146 Analog Devices, AD9146 Datasheet - Page 35

• Manufacturer Part Number: AD9146
• Description: Dual, 16-Bit, 1230 MSPS, TxDAC+® Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9146

• Resolution (bits): 16bit
• Dac Update Rate: 1.23GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Byte,LVDS,Nibble

Data Sheet
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
In practice, this modulation results in the mixing functions
shown in Table 20.
Table 20. Modulation Mixing Sequences
Modulation
f
f
3f
f
Note that
As shown in Table 20, 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
means that real modulation using both the I and Q DAC outputs
can only be done in f
require complex input data and produce complex output signals.
QUADRATURE PHASE CORRECTION
The purpose of the quadrature phase correction block is to
enable compensation of the phase imbalance of the analog
quadrature modulator following the DAC. If the quadrature
modulator has a phase imbalance, the unwanted sideband appears
with significant energy. Tuning the quadrature phase adjust value
can optimize image rejection in single sideband radios.
Ordinarily, the I and Q channels have an angle of precisely 90°
between them. The quadrature phase adjustment is used to change
the angle between the I and Q channels. When I Phase Adj[9:0]
(Register 0x38 and Register 0x39) is set to 1000000000, the I DAC
output moves approximately 1.75° away from the Q DAC output,
creating an angle of 91.75° between the channels. When I Phase
Adj[9:0] is set to 0111111111, the I DAC output moves approxi-
mately 1.75° toward the Q DAC output, creating an angle of
88.25° between the channels.
S
S
S
/2
/4
/8
S
/4
C(t) = cos(ω
r
=
2
Mixing Sequence
I = I, −I, I, −I, …
Q = Q, −Q, Q, −Q, …
I = I, Q, −I, −Q, …
Q = Q, −I, −Q, I, …
I = I, −Q, −I, Q, …
Q = Q, I, −Q, −I, …
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), …
2
c
t) + j sin(ω
S
/2 mode. All other modulation modes
c
t)
S
/2 mode. This
Rev. A | Page 35 of 56
Q Phase Adj[9:0] (Register 0x3A and Register 0x3B) works in
a similar fashion. When Q Phase Adj[9:0] is set to 1000000000,
the Q DAC output moves approximately 1.75° away from the
I DAC output, creating an angle of 91.75° between the channels.
When Q Phase Adj[9:0] is set to 0111111111, the Q DAC output
moves approximately 1.75° toward the I DAC output, creating
an angle of 88.25° between the channels.
Based on these two endpoints, the combined resolution of the
phase compensation register is approximately 3.5°/1024 or
0.00342° per code.
DC OFFSET CORRECTION
The dc value of the I datapath and the Q datapath can be
independently controlled by adjusting the I DAC Offset[15:0]
and Q DAC Offset[15:0] values in Register 0x3C through
Register 0x3F. These values are added directly to the datapath
values. Care should be taken not to overrange the transmitted
values.
Figure 45 shows how the DAC offset current varies as a function
of the I DAC Offset[15:0] and Q DAC Offset[15:0] values. With
the digital inputs fixed at midscale (0x0000, twos complement data
format), Figure 45 shows the nominal I
as the DAC offset value is swept from 0 to 65,535. Because I
and I
and I
20
15
10
0x0000
5
0
OUTxN
OUTxN
Figure 45. DAC Output Currents vs. DAC Offset Value
are complementary current outputs, the sum of I
is always 20 mA.
0x4000
DAC OFFSET VALUE
0x8000
OUTxP
0xC000
and I
OUTxN
0xFFFF
AD9146
currents
0
5
10
15
20
OUTxP
OUTxP