AD9148BPCZ AD [Analog Devices], AD9148BPCZ Datasheet - Page 61

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

Preliminary Technical Data
In addition, the P or N output can act as a current source or a
current sink. When sourcing current, the output compliance
voltage is 0 V to 1.6 V. When sinking current, the output compliance
voltage is 0.8 V to 1.6 V. The auxiliary DAC current direction is
programmable via Bit 6, Register 0x33 and Bit 6, Register 0x37
(DAC SPI select is 0 to control AUX1 and AUX2, and DAC SPI
select is 1 to control AUX3 and AUX4). The choice of sinking or
sourcing should be made at circuit design time. There is no
advantage to switching between sourcing and sinking current
after the circuit is in place.
These auxiliary DACs can be used for local oscillator (LO)
cancellation when the DAC output is followed by a quadrature
modulator. More information and example application circuits
are given in the Interfacing to Modulators section.
INTERFACING TO MODULATORS
The AD9148 interfaces to the ADL537x family of with a minimal
number of components. An example of the recommended
interface circuitry is shown in Figure 79.
The baseband inputs of the ADL537x family require a dc bias of
500 mV. The nominal midscale output current on each output
of the DAC is 10 mA (1/2 the full-scale current). Therefore, a
single 50 Ω resistor to ground from each of the DAC outputs
results in the desired 500 mV dc common-mode bias for the
inputs to the ADL537x. The signal level can be reduced by the
addition of the load resistor in parallel with the modulator
inputs (RLI, RLQ). The peak-to-peak voltage swing of the
transmitted signal is
Figure 79. Typical Interface Circuitry Between the AD9148 and ADL537x
V
SIGNAL
AD9148
IOUT1_P
IOUT1_N
IOUT2_N
IOUT2_P
=
I
FS
×
[
[
2
2
RBQN
RBQP
×
×
RBIP
RBIN
Family of Modulators
50Ω
50Ω
50Ω
50Ω
R
R
B
B
×
+
R
R
L
L
]
]
100Ω
100Ω
RLQ
QDAC
IDAC
RLI
OR
Figure 81. DAC Modulator Interface with Fifth-Order, Low Pass Filter
IBBP
IBBN
QBBN
QBBP
ADL537x
50Ω
50Ω
MABACT0043
(OPTIONAL)
Rev. PrA | Page 61 of 73
2pF
33nH
33nH
22pF
Baseband Filter Implementation
Most applications require a baseband anti-imaging filter between
the DAC and modulator to filter out Nyquist images and broadband
DAC noise. The filter can be inserted between the I-to-V resistors
at the DAC output and the signal level setting resistor across the
modulator input. Doing this establishes the input and output
impedances for the filter.
Figure 81 shows a fifth-order low-pass filter. A common-mode
choke is used between the I-to-V resistors and the remainder of
the filter. This removes the common-mode signal produced by
the DAC and prevents the common-mode signal from being
converted to a differential signal, which would appear as unwanted
spurious signals in the output spectrum. The common-mode
choke or balun may not be needed if the layout between the
DAC and IQ modulator is optimized and balanced. Splitting the
second filter capacitor into two and grounding the center point
creates a common-mode low-pass filter providing additional
common-mode rejection of high frequency signals. A purely
differential filter will pass common-mode signals.
Driving the ADL5375-15 with the AD9148
The ADL5375-15 requires a 1500 mV dc bias and therefore
requires a slightly more complex interface than most other
Analog Devices, Inc., modulators. It is necessary to level shift
the DAC output from a 500 mV dc bias to the 1500 mV dc bias
that the ADL5375-15 requires. Level shifting can be achieved
with a purely passive network, as shown in Figure 80. In this
network, the dc bias of the DAC remains at 500 mV, while the
input to the ADL5375-15 is 1500 mV. Note that this passive
level shifting network introduces approximately 2 dB of loss in
the ac signal.
Figure 80. Passive Level Shifting Network for Biasing ADL5375-15 from AD9148
22pF
56nH
56nH
AD9148
IOUT1_P
IOUT1_N
IOUT2_N
IOUT2_P
3pF
6pF
3pF
100Ω
RBQN
RBQP
45.3Ω
45.3Ω
45.3Ω
45.3Ω
RBIN
RBIP
ADL537x
RSQN
RSQP
RSIN
RSIP
1kΩ
1kΩ
1kΩ
1kΩ
3480Ω
3480Ω
3480Ω
3480Ω
RLQN
RLQP
RLIN
RLIP
5V
5V
ADL5375-15
IBBP
IBBN
QBBN
QBBP
AD9148