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AD9259 Analog Input range above supply?

Question asked by alexdes on Sep 12, 2014
Latest reply on Sep 23, 2014 by DougI

Hi,

 

I want to use the quad ADC AD9259 for an imaging application, which requires high DC accuracy and low noise across the whole ADC input range.  Reading the datasheet, the maximum (and recommended) input range is 2V peak-to-peak, which is above the 1.8V supply.  This means that the body diodes will start conducting in the uA range when the analog input approach or goes above 1.8V.  This will lead to an input signal compression between ~1.8V and 2V; Am I correct in thinking that way??  Note the use of a RC anti-aliasing filter in front of the ADC, a LC filter is not suitable because we need clamp diodes to protect ADC from +/-5V differential op-amp.

 

Is there a way to see this compression in the datasheet?  Most graph are specified with a 1.89Vp-p input signal (-0.5dBFS). 

 

What would be the maximum analog input recommended to avoid compression?  A body diode SPICE model would be perfect for this situation. 

 

 

Would another Quad ADC with LVDS output be better suited for my application?

Typical sampling rate is 10MSPS, but we were planning for 4x oversampling to get ~1 more bit out of the AD9259 (12 ENOB to 13 ENOB).  Our application requires ~12 ENOB (+/-0.5bit) in term of accuracy across the whole ADC input range.

 

Regards,

 

Alex

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