AD9259 Analog Input range above supply?

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

  • 0
    •  Analog Employees 
    on Sep 23, 2014 11:34 PM

    Hi Alex,

    Thank you for your interest in the AD9259. The AD9259 full-scale input swing of 2Vpp is a differential magnitude. At full-scale, the single-ended signal on one analog input (let's say VIN+) would swing +/-0.5V above and below the common mode voltage, which is nominally 0.9V (AVDD/2). Similarly, the single-ended signal on the other (complementary) analog input (let's say VIN-) would swing -/+0.5V below and above 0.9V. If these are truly differential signals, they would be swinging in a complementary fashion with respect to each other. The difference between the +/-0.5V on VIN+ and the -/+0.5V on VIN- results in 2Vpp_differential.

    With a full-scale (2Vpp_differential) input, and the input common mode voltage = 0.9V, the voltage on the analog inputs would only swing between 0.4V (0.9V - 0.5V) and 1.4V (0.9V + 0.5V), so you do not need to worry about input diodes conducting in this case.

    Some other quad LVDS ADCs (lower power, better performance) to consider are:

    AD9253 (14bit 125MSPS)

    AD9653 (16bit 125MSPS, note that minimum sample rate is 20MSPS)

    Thanks again.

    Doug