CN0016| Application Note
Circuit Note
CN0016
A
Verified Circuit Designs Using Analog Devices Signal Processing Devices
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Interfacing the ADL5370 I/Q Modulator to the AD9779A Dual-Channel, 1GSPS High-Speed DAC
Contributed November 7, 2008 Devices connected in this Circuit Note: AD9779A Dual 16-Bit, 1 GSPS DAC ADL5370 300MHz to 1000MHz IQ Modulator
Circuit Function and Benefits
The circuit described in this document and shown in Figure 1 provides a simple, elegant interface between the ADL5370 I/Q modulator and the AD9779A highspeed DAC. The ADL5370 and the AD9779A are wellmatched 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 which adds noise and insertion loss along with extra components. The addition of the swing limiting resistor allows the DAC swing to be scaled appropriately without loss of resolution or of the 0.5 V bias level. The similar SNR of each device preserves a high SNR through the circuit. Figure 1 Interface Between the AD9779 and ADL5370 with 50
Circuit Description
The ADL5370 is designed to interface with minimal components to members of the Analog Devices' family of TxDACs (AD97xx). The baseband inputs of the ADL5370 require a DC bias of 500 mV. With each AD9779A output swinging from 0 to 20 mA, a single 50 resistor to ground from each of the DAC outputs provides the desired 500 mV dc bias. With just the four 50 resistors in place, the voltage swing on each pin is 1 Vpp. This results in a differential voltage swing of 2 Vpp on each input pair.
Resistors to Ground to Establish the 500 mV DC Bias for the ADL5370 Baseband Inputs
2.0 1.8
DIFFERENTIAL SWING (V p-p)
1.6 1.4 1.2 1.0 0.8 0.6
By adding resistors RSLI and RSLQ to the interface, the 0.4 output swing of the DAC can be reduced without any 0.2 loss of DAC resolution. This resistor is placed in the 0 shunt between each side of the differential pair, as 10 100 1000 10000 RL () shown in Figure 1. It has the effect of reducing the ac swing without changing the DC bias already established Figure 2: Relationship Between the AC Swing-Limiting Resistor and the Peak-to-Peak Voltage Swing with 50 Biasby the 50 resistors.
Setting Resistors
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 bias-setting resistors are used. Note that all Analog Devices IQ
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