AD9878BST Analog Devices Inc, AD9878BST Datasheet - Page 21

• 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

TRANSMIT PATH
The transmit path contains an interpolation filter, a complete
quadrature digital upconverter, an inverse sinc filter, and a 12-bit
current output DAC. The maximum output current of the DAC is
set by an external resistor. The Tx output PGA provides additional
transmit signal level control. The transmit path interpolation
filter provides an upsampling factor of 16 with an output signal
bandwidth as high as 4.35 MHz for <1 dB droop. Carrier
frequencies up to 65 MHz with 26 bits of frequency tuning
resolution can be generated by the direct digital synthesizer
(DDS). The transmit DAC resolution is 12 bits, and it can run at
sampling rates of up to 232 MSPS. Analog output scaling from
0 dB to 7.5 dB in 0.5 dB steps is available to preserve SNR when
reduced output levels are required.
DATA ASSEMBLER
The AD9878 data path operates on two 12-bit words, the I and Q
components, that form a complex symbol. The data assembler
builds the 24-bit complex symbol from four consecutive 6-bit
words read over the TxIQ [5:0] bus. These words are strobed
into the data assembler synchronous to the master clock (MCLK).
A high level on TxSYNC signals the start of a transmit symbol.
The first two 6-bit words of the symbol form the I component;
the second two 6-bit words form the Q component. Symbol
components are assumed to be in twos complement format. The
timing of the interface is fully described in the Transmit Timing
section. The I/Q sample rate f
maximum transmit spectrum. This is the familiar Nyquist limit
(hereafter referred to as f
TRANSMIT TIMING
The AD9878 has a master clock and expects 6-bit, multiplexed
TxIQ data upon each rising edge (see Figure 24). Transmit
symbols are framed with the TxSYNC input. TxSYNC high
indicates the start of a transmit symbol. Four consecutive 6-bit
data packages form a symbol (I MSB, I LSB, Q MSB, and Q LSB).
INTERPOLATION FILTER
Once through the data assembler, the IQ data streams are fed
through a 4× FIR low-pass filter and a 4× cascaded integrator
comb (CIC) low-pass filter. The combination of these two filters
results in the sample rate increasing by a factor of 16. In addition
TxSYNC
MCLK
TxIQ
NYQ
TxI[11:6]
) and is equal to half f
IQCLK
puts a bandwidth limit on the
TxI[5:0]
TxQ[11:6]
IQCLK
.
TxQ[5:0]
Figure 24. Tx Timing Diagram
Rev. A | Page 21 of 36
TxI[11:6]
t
SU
to the sample rate increase, the half-band filters provide the
low-pass filtering characteristics necessary to suppress the spectral
images between the original sampling frequency and the new
(16× higher) sampling frequency.
HALF-BAND FILTERS (HBFs)
HBF 1 and HBF 2 are both interpolating filters, each of which
doubles the sampling rate. Together, HBF 1 and HBF 2 have
26 taps and increase the sampling rate by a factor of 4
(4 × f
In relation to phase response, both HBFs are linear phase filters.
As such, virtually no phase distortion is introduced within the pass
band of the filters. This is an important feature, because phase dis-
tortion is generally intolerable in a data transmission system.
CASCADE INTEGRATOR COMB (CIC) FILTER
The CIC filter is configured as a programmable interpolator
and provides a sample rate increase by a factor of 4. The
frequency response of the CIC filter is given by:
COMBINED FILTER RESPONSE
The combined frequency response of the HBF and CIC filters
limits the input signal bandwidth that can be propagated through
the AD9878.The usable bandwidth of the filter chain limits the
maximum data rate that can be propagated through the AD9878.
A look at the pass-band detail of the combined filter response
(Figure 25) indicates that to maintain an amplitude error of
1 dB or less, signal bandwidth is restricted to about 60% or less
of f
Thus, in order to keep the bandwidth of the data in the flat
portion of the filter pass band, the user must oversample the
baseband data by at least a factor of two prior to presenting it to
the AD9878. Note that without oversampling, the Nyquist
bandwidth of the baseband data corresponds to f
the upper end of the data bandwidth suffers 6 dB or more of
attenuation due to the frequency response of the digital filters.
Furthermore, if the baseband data applied to the AD9878 has
t
TxI[5:0]
HU
NYQ
Max BW
IQCLK
H
.
( )
f
TxQ[11:6]
or 8 × f
−
(1dB droop)
⎡
⎢
⎢
⎣
⎛
⎜
⎝
1
4
NYQ
⎞
⎟
⎠
TxQ[5:0]
1
).
−
= 0.60 * f
1
e
−
−
e
j
(
2
j
2
π
TxI[11:6]
π
f
f
( )
4
MCLK
)
⎤
⎥
⎥
⎦
3
/8
=
TxI[5:0]
⎡
⎢
⎢
⎣
⎛
⎜
⎝
1
4
⎞
⎟
⎠
sin
sin
(
π 4
( )
π
NYQ
f
f
)
. As such,
AD9878
⎤
⎥
⎥
⎦
3