CN-0016 Datasheet, PDF (1/2 Pages) Analog Devices – Interfacing the ADL5370 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC

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Circuit Note
CN-0016
Devices Connected/Referenced
AD9779A Dual 16-Bit, 1 GSPS DAC
ADL5370 300 MHz to 1000 MHz I/Q Modulator
Interfacing the ADL5370 I/Q Modulator to the AD9779A
Dual-Channel, 1 GSPS High Speed DAC
CIRCUIT FUNCTION AND BENEFITS
This circuit provides a simple, elegant interface between the
ADL5370 I/Q modulator and the AD9779A high speed DAC.
The ADL5370 and the AD9779A are well-matched devices
because they have the same bias levels and similarly high
signal-to-noise ratios (SNR). The matched bias levels of 500 mV
allow for a “glueless” interface—there is no requirement for a
level shifting network that would add noise and insertion loss
along with extra components. The addition of the swing-
limiting resistors (RSLI, RSLQ) allows the DAC swing to be
scaled appropriately without loss of resolution or of the 0.5 V
bias level. The high SNR of each device preserves a high SNR
through the circuit.
CIRCUIT DESCRIPTION
The ADL5370 is designed to interface with minimal
components to members of Analog Devices family of
TxDAC® converters (AD97xx). The baseband inputs of the
ADL5370 require a dc common-mode bias voltage of 500 mV.
With each AD9779A output swinging from 0 mA to 20 mA,
AD9779A
93
OUT1_P
92
OUT1_N
RBIP
50Ω
RBIN
50Ω
RSLI
100Ω
ADL5370
19
IBBP
20
IBBN
OUT2_N
OUT2_P
84
RBQN
50Ω
RBQP
83 50Ω
RSLQ
100Ω
23
QBBN
24
QBBP
Figure 1. Interface Between the AD9779A and ADL5370 with 50 Ω Resistors
to Ground to Establish the 500 mV DC Bias for the ADL5370 Baseband Inputs
(Simplified Schematic)
Rev. A
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a single 50 Ω resistor to ground from each of the DAC outputs
provides the desired 500 mV dc bias. With just the four 50 Ω
resistors in place, the voltage swing on each pin is 1 V p-p.
This results in a differential voltage swing of 2 V p-p on each
input pair.
By adding resistors RSLI and RSLQ to the interface, the output
swing of the DAC can be reduced without any loss of DAC
resolution. The resistor is placed as a shunt between each side of
the differential pair, as shown in Figure 1. It has the effect of
reducing the ac swing without changing the dc bias already
established by the 50 Ω resistors and the DAC output current.
The value of this ac swing-limiting resistor is chosen based on
the desired ac voltage swing. Figure 2 shows the relationship
between the swing-limiting resistor and the peak-to-peak ac
swing that it produces when 50 Ω bias-setting resistors are used.
Note that all Analog Devices I/Q modulators present a relatively
high input impedance on their baseband inputs (typically >1 kΩ).
As a result, the input impedance of the I/Q modulator will have
no effect on the scaling of the DAC output signal.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
10
100
1k
10k
R_L (Ω)
Figure 2. Relationship Between the AC Swing-Limiting Resistor and the
Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors
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