Datasheet

AD9122
Rev. B | Page 49 of 60
REDUCING LO LEAKAGE AND UNWANTED
SIDEBANDS
Analog quadrature modulators can introduce unwanted signals at
the LO frequency due to dc offset voltages in the I and Q baseband
inputs, as well as feedthrough paths from the LO input to the out-
put. The LO feedthrough can be nulled by applying the correct
dc offset voltages at the DAC output. This can be done using the
auxiliary DACs (Register 0x42, Register 0x43, Register 0x46, and
Register 0x47) or by using the digital dc offset adjustments
(Register 0x3C through Register 0x3F).
The advantage of using the auxiliary DACs is that none of
the main DAC dynamic range is used to perform the dc offset
adjustment. The disadvantage is that the common-mode level
of the output signal changes as a function of the auxiliary DAC
current. The opposite is true when the digital offset adjustment
is used.
Good sideband suppression requires both gain and phase
matching of the I and Q signals. The I/Q phase adjust registers
(Register 0x38 through Register 0x3B) and the DAC FS adjust
registers (Register 0x40 and Register 0x44) can be used to calibrate
the I and Q transmit paths to optimize sideband suppression.