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high-performance ADC driver; is VGA an option?

I need a driver for a high-performance high-speed ADC.  The input BW requirement is DC to 175MHz;
the magnitude could be as low as ~100mV, but could be higher as well.  Low noise and low distortion
are very important.  My ADC is a single-rail 5V analog input, 2.2Vp-p VCM=3.1V (400Msps 14-bit).  I
started looking at VGAs to accommodate the potentially wide range of input signal amplitude.


1. The AD8370 datasheet says "LF to 750MHz".  I want to dc-couple the inputs; but there is no
common-mode input range given. Does this mean the part must be ac-coupled, and I must rule out
the AD8370? Is "LF" quantifiable? 


2. If the AD8370 is not an option, I would then look for a std fixed-gain diff amp; any suggestions
would be appreciated.  I need gain around 12dB, input as above (+/- 100mV input), very flat gain
through 175MHz, > 5V supply, low noise, low distortion. 

Thanks in advance.



  • Actually, if you have the capability to use dual supplies, the AD8370 can be used as a dc accurate VGA. Using dual, symetric supplies, connect the VOCM pin and one of the inputs to gnd. This will set the internal bias point to 0V. I'm trying to find out right now what the permissible input swing is in this configuration. There is quite a bit of information regarding this on page 18 in the datasheet.

    If you are looking for simple gains block ADC drivers, I suggest the ADL5562 and ADL5561 amplifiers. This is exactly what these parts have been designed for.

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