Datasheet

4.7.2.1 TRF3703 LO Source
4.7.2.2 DAC-to-Modulator Interface
4.8 Reference Operation
www.ti.com DAC5681/81Z/82z EVM Hardware Description
The DAC5682z EVM requires an external local oscillator (LO) source to drive the onboard TRF3703
modulator. This external LO input needs to be connected to the SMA connector J23. The signal level of
the LO source must comply with the requirements in the TRF3703 data sheet (SLWS184), but typically an
LO power around 6-to-8 dBm is adequate.
The TRF3703 quadrature modulator requires a common-mode dc voltage of approximately 3.3 V. In order
to use the dc-offset adjustment capabilities of the DAC5682z for carrier suppression, it is imperative to
maintain a dc path from the DAC output to the modulator input. The common-mode voltage for the
modulator is maintained with a passive resistor network that is designed to provide the proper operation
point for the DAC5682z and the TRF3703 modulator.
The DAC5682z EVM is configured with enough pads to provide a specific fifth-order differential passive
LC filter. By default, it is only populated with a simple LC low-pass filter to attenuate the higher clock
harmonics. The 3-dB corner of this filter is approximately 300 MHz.
Figure 8. Response of Default Baseband Filter
The DAC5682z EVM also includes a pi pad network on the modulator output to provide some matching or
filtering, if desired. In its default state, the pad is not used and a series capacitor is used on the RF output.
The DAC5681/81z/82z full-scale output current is set by applying an external resistor (R18) between the
BIASJ pin of the device and ground. The full-scale output current can be adjusted from 20 mA down to 2
mA by varying this resistor. The full-scale output current, IOUTFS, is defined as follows:
SLAU236A – November 2007 – Revised October 2008 DAC5681/81z/82z EVM 11
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