AD9148BPCZ AD [Analog Devices], AD9148BPCZ Datasheet - Page 50

• Manufacturer Part Number: AD9148BPCZ
• Description: Quad 16-Bit,1 GSPS, TxDAC+ Digital-to-Analog Converter
• Manufacturer: AD [Analog Devices]

AD9148
DIGITAL DATA PATH
The block diagram in Figure 59 shows the functionality of the
complex digital data path. The digital processing includes a
premodulation block, a programmable complex filter, three
half-band interpolation filters with built-in coarse modulation,
a quadrature modulator with a fine resolution NCO as well as
phase, gain, and offset adjustment blocks.
There are two complex digital data paths that feed the four DACs.
Each digital data path accepts I and Q data streams and processes
them as a quadrature data stream, resulting in two quadrature
data streams. All of the signal processing blocks can be used
when the input data stream is represented as complex data.
The data path can be used to process an input data stream
representing four independent real data streams as well; however,
the functionality is somewhat restricted. The premodulation
block can be used, as well as any of the nonshifted interpolation
filter modes.
PREMODULATION
The half-band interpolation filters have selectable pass bands
that allow the center frequencies to be moved in increments of
½ of their input data rate. The premodulation block provides a
digital upconversion of the incoming waveform by ½ of the
incoming data rate, f
multiplies the incoming data samples alternatively by +1 and −1.
This can be used to frequency shift baseband input data to the
center of the interpolation filters pass band.
PROGRAMMABLE INVERSE SINC FILTER
The AD9148 provides a programmable inverse sinc filter to
compensate the DAC roll-off over frequency. As this filter is
implemented before the interpolation filter, its coefficients need
to be changed depending on the interpolation rate and DAC
output center frequency.
PREMOD
f
S
/2
FILTER
SINC
PROG
Figure 59. Block Diagram of Digital Data Path
–1
DATA.
HB1
Functionally, the premodulation
HB2
HB3
PHASE/GAIN/
OFFSET ADJ
DIGITAL
Rev. PrA | Page 50 of 73
Filter Structure
The programmable inverse sinc filter is a 9-tap complex FIR
filter using complex conjugate coefficients. The z-transfer
function is:
where:
x
filter input, respectively.
y
filter output, respectively.
H
filter coefficients, respectively.
c
complex conjugate.
The filter coefficients need to be calculated and programmed
into the AD9148 registers to perform the operation desired.
Filter Implementation
To perform the complex filtering of the complex input, the filter
is divided in four filters working in parallel, two sets of H
two sets of H
The coefficients for the filter are stored in SPI Register 0x20 to
Register 0x27 in twos-complement format. They have variable
length, 3 bits to 10 bits.
0
I
I
, c
I
and x
and y
and H
1
, c
H
y
X
2
X
I
Q
, c
( )
Q
Q
I
z
+
Q
are the in-phase (real) and quadrature (imaginary)
are the in-phase (real) and quadrature (imaginary)
3
, and c
j
are the in-phase (real) and quadrature (imaginary)
=
×
Q
y
x
y
Q
I
I
(see Figure 60).
Figure 60. Complex Filter Implementation
=
+
=
+
+
c
4
=
c
H
3
are the complex filter coefficient, and c
0
j
j
(
×
I
×
×
H
+
Preliminary Technical Data
×
z
y
x
c
H
H
H
H
I
−
x
Q
1
Q
I
Q
I
Q
5
+
×
I
+
=
j
−
z
×
c
−
H
H
2
1
H
×
I
+
Q
Q
z
+
c
×
) (
−
2
⋅
j
x
6
×
×
x
Q
+
+
z
H
i
c
+
−
+
–
1
Q
2
j
×
j
+
⋅
×
(
+
z
c
H
x
−
3
+
7
Q
Q
×
+
)
×
z
c
−
x
0
3
I
×
+
+
z
c
H
−
4
8
×
I
Y
Y
×
I
Q
z
−
X
x
I
4
Q
their
and
)