Interfacing the ADL5375 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC
(CN0021)
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Optimized For:. High Speed
Applications:. Communications
Circuit Function and Benefits
This circuit provides a simple, elegant interface between the ADL5375 I/Q modulator and the AD9779A high speed DAC. The ADL5375 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.
Figure 1: Interface Between the AD9779A and ADL5375 with 50 Ω Resistors to Ground to
Establish the 500 mV DC Bias for the ADL5375-05 Baseband Inputs (Simplified Schematic)
Figure 1: Interface Between the AD9779A and ADL5375 with 50 Ω Resistors to Ground to Establish the 500 mV DC Bias for the ADL5375-05
Baseband Inputs (Simplified Schematic)
Circuit Description
The ADL5375 is designed to interface with minimal components to members of Analog Devices family of TxDAC® converters (AD97xx). The baseband
inputs of the ADL5375 require a dc common-mode bias voltage of 500 mV. With each AD9779A output swinging from 0 mA to 20 mA, 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.
