AD8348

The AD8348 is a broadband quadrature demodulator with an integrated intermediate frequency (IF) variable-gain amplifier (VGA) and integrated baseband amplifiers. It is suitable for use in communications receivers, performing quadrature demodulation from IF directly to baseband frequencies. The baseband amplifiers have been designed to directly interface with dual channel A-to-D converters such as the AD9201, AD9283, and AD9218 for digitizing and post-processing.

The IF input signal of AD8348 is fed into two Gilbert-cell mixers through an X-AMP VGA. The IF VGA provides 45dB of gain control. A precision gain-control circuit sets a linear-in-dB gain characteristic for the VGA and provides temperature compensation. The LO quadrature phase splitter employs a divide-by-two frequency divider to achieve high quadrature accuracy and amplitude balance over the entire operating frequency range.

Optionally, the IF VGA can be disabled and bypassed. In this mode, the IF signal is applied directly to the quadrature mixer inputs via pins MXIP and MXIN.

Separate I & Q-channel baseband amplifiers follow the baseband outputs of the mixers. The DC common-mode voltage level at the baseband outputs is set by the voltage applied to the VCMO pin. Typically VCMO is connected to the internal VREF voltage but it can also be connected to an external voltage. This flexibility allows the user to maximize the input dynamic range to the A-to-D converter. Connecting a bypass capacitor at each offset compensation input (IOFS & QOFS) nulls DC offsets produced in the mixer. Offset compensation can be overridden by applying an external voltage at the offset compensation inputs.

The mixers' outputs of AD8348 are brought off-chip for optional filtering before final amplification. Inserting a channel selection filter before each baseband amplifier increases the baseband amplifiers' signal handling range by reducing the amplitude of high-level, out-of-channel interferers before the baseband signal is fed into the baseband amplifiers. The single-ended mixer output is amplified and converted to a differential signal for driving ADCs.

The AD8348 is a broadband quadrature demodulator with an integrated intermediate frequency (IF), variable gain amplifier (VGA), and integrated baseband amplifiers. It is suitable for use in