AD9878BST Analog Devices Inc, AD9878BST Datasheet - Page 23

• Manufacturer Part Number: AD9878BST
• Description: IC FRONT-END MIXED-SGNL 100-LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD9878BST

• Rohs Status: RoHS non-compliant
• Number Of Bits: 12
• Number Of Channels: 4
• Power (watts): 673mW
• Voltage - Supply, Analog: 3.3V
• Voltage - Supply, Digital: 3.3V
• Package / Case: 100-LQFP

Tx THROUGHPUT AND LATENCY
Data inputs affect the output fairly quickly, but remain effective
due to the AD9878 filter characteristics. Data transmit latency
through the AD9878 is easiest to describe in terms of f
cycles (4 × f
f
the input.
Latency of I/Q data from the time it enters the data assembler
(AD9878 input) to the time of DAC output is 119 f
cycles (29.75 f
input take up to 176 f
propagate and settle at the DAC output.
Frequency hopping is accomplished via changing the PROFILE
input pin. The time required to switch from one frequency to
another is less than 232 f
DAC
A 12-bit digital-to-analog converter (DAC) is used to convert the
digitally processed waveform into an analog signal. The worst-
case spurious signals due to the DAC are the harmonics of the
fundamental signal and their aliases (see the Analog Devices
DDS tutorial at www.analog.com/dds). The conversion process
produces aliased components of the fundamental signal at
with an external RLC filter at the DAC output. It is important
for this analog filter to have a sufficiently flat gain and linear
phase response across the bandwidth of interest to avoid
modulation impairments. A relatively inexpensive seventh-
order, elliptical, low-pass filter is sufficient to suppress the
aliased components for HFC network applications.
The AD9878 provides true and complement current outputs. The
full-scale output current is set by the R
the DAC gain register. Assuming maximum DAC gain, the value
of R
For example, if a full-scale output current of 20 mA is desired,
then R
The following equation calculates the full-scale output current,
including the programmable DAC gain control:
where N
The full-scale output current range of the AD9878 is 4 to
20 mA. Full-scale output currents outside this range degrade
SFDR performance. SFDR is also slightly affected by output
matching—that is, the two outputs should be terminated equally
for best SFDR performance. The output load should be located
as close as possible to the AD9878 package to minimize stray
n
SYSCLK
×
SET
f
SYSCLK
R
I
cycles before the AD9878 output responds to a change in
OUT
SET
SET
for a full-scale I
GAIN
= (39.4/0.02), or approximately 2 kΩ.
=
=
MCLK
±
32
is the value of DAC fine gain control [3:0].
39
MCLK
f
CARRIER
4 .
V
). The numbers provided indicate the number of
DACRSET
cycles). DC values applied to the data assembler
R
SET
SYSCLK
(
n
OUT
×
SYSCLK
=
10
I
, 1
is determined using the equation:
OUT
(
−
clock cycles (44 f
, 2
7
5 .
cycles (58.5 f
. 3
=
+
)
0
39
5 .
These are typically filtered
N
4 .
GAIN
I
SET
OUT
)
20
resistor at Pin 49 and
MCLK
MCLK
cycles).
cycles) to
SYSCLK
SYSCLK
clock
clock
Rev. A | Page 23 of 36
capacitance and inductance. The load can be a simple resistor to
ground, an op amp current-to-voltage converter, or a transformer-
coupled circuit. It is best not to directly drive a highly reactive
load, such as an LC filter. Driving an LC filter without a
transformer requires that the filter be doubly terminated for
best performance—that is, both the filter input and output should
be resistively terminated with the appropriate values. The parallel
combination of the two terminations determines the load that
the AD9878 sees for signals within the filter pass band. For
example, a 50 Ω terminated input/output low-pass filter looks
like a 25 Ω load to the AD9878. The output compliance voltage
of the AD9878 is −0.5 V to +1.5 V. Any signal developed at the
DAC output should not exceed 1.5 V; otherwise, signal distortion
results. Furthermore, the signal can extend below ground as much
as 0.5 V without damage or signal distortion. The AD9878 true
and complement outputs can be differentially combined for
common-mode rejection using a broadband 1:1 transformer.
Using a grounded center tap results in signals at the AD9878 DAC
output pins that are symmetrical about ground. As previously
mentioned, by differentially combining the two signals, the user
can provide some degree of common-mode signal rejection.
A differential combiner can consist of a transformer or an
op amp. The object is to combine or amplify the difference
between only two signals and to reject any common—usually
undesirable—characteristics, such as 60 Hz hum or clock
feedthrough, that is equally present on both signals.
Connecting the AD9878 true and complement outputs to the
differential inputs of the programmable gain cable drivers
AD8321/AD8323 or AD8322/AD8327 (see Figure 27)
provides an optimized solution for the standard compliant
cable modem upstream channel. The cable driver’s gain
can be programmed through a direct 3-wire interface
using the AD9878 profile registers.
PROGRAMMING THE AD8321/AD8323 OR
AD8322/AD8327/AD8238 CABLE-DRIVER
AMPLIFIERS
Users can program the gain of the AD832x family of cable-driver
amplifiers via the AD9878 cable amplifier control interface. Two
(one per profile) 8-bit registers within the AD9878 store the gain
value to be written to the serial 3-wire port. Typically, either the
AD8321/AD8323 or AD8322/AD8327 variable gain cable
amplifiers are connected to the chip’s 3-wire cable amplifier
AD9878
DAC
Figure 27. Cable Amplifier Connection
Tx
CA
CA_EN
CA_DATA
CA_CLK
LOW-PASS
FILTER
3
VARIABLE GAIN
CABLE DRIVER
AMPLIFIER
AD832x
75Ω
AD9878