AD9218 interferences at L-band

Document created by analog-archivist Employee on Feb 23, 2016
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We are developing a bistatic radar using the AD9218.
We are using the evaluation board of AD9218, and have a sampling
frequency of 80MHz. The processing technique we use can detect
weak signals. We have seen that the AD9218 circuit generates a
series of radiated interferences at L-band that disrupts our measurements.
The radiated interferences are captured by our instrument's antennas.

 

The AD9218 is just a straight pipeline converter released ~10yrs that predates
current SPI CLK divider options so unfortunately I can’t think of anything
internal to the ADC that would cause this.

More likely it’s a harmonic or intermod artifact from the 80MSPS CLK and Ain,
or other system signal that’s coupling into the ADC.

10MHz spaced spurs w/ 80Msps CLK would suggest an Analog Input either
intentional, or coupling of 10MHz, 30MHz, 50MHz, 70MHz, 90MHz, etc…

One test to see if the spur is resulting from some parasitic ADC input coupling
is to short the AIN terminals together on each ADC to force a 0V differential.
Just remember not to GND the Ain pins, but connect them to each other….

With the inputs shorted the ADC output will assume a midscale value +/- the
inherent DC offset of the ADC. If the spurs go away, it could suggest some
system noise was coupling and being converted by the ADC.

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