Analog Devices AD605 X-AMP test

Introduction from Analog Devices AD605 Datasheet:

"The AD605 is a low noise, accurate, dual-channel, linear-in-dB
variable gain amplifier (VGA), optimized for any application
requiring high performance, wide bandwidth variable gain
control. Operating from a single 5 V supply, the AD605 provides
differential inputs and unipolar gain control for ease of use.
Added flexibility is achieved with a user-determined gain range
and an external reference input that provide user-determined
gain scaling (dB/V)."

It sound's very powerful, if this can replace the discrete transistor version of VGA in my receiver projects, it save a lots of board spaces and greatly improve the dynamic range. Let's make a evaluation board to try try.

Setup:

The AD605 evaluation board, a 68 ohm resistor is loaded with the input to make up of a near 50 ohm input impedance.

Test setup: Gain control voltage is from adjustable power supply. From 0.1V to 3.0V.

Signal source is from signal generator, 455KHz, -60dBm, quite a small signal.

Output power is measured from spectrum analyzer, it's dynamic range way better than an oscilloscope.

Gain-Control voltage plot from AD605 datasheet. In this test, FBK(open) configuration is using.

Result:

Gain-Control voltage plot from actual test, which is very closed to the plot in datasheet. A nearly linear-in-db curve between 0.1V to 2.5V control voltage. and tuning range of about 45dB. Certainly a very respectable figure.

45dB of gain is not enough for a typical receiver, there are another amplifier unit in the AD605 package, may be I could cascade them and put a ceramic filter in between them, and share same control voltage.