AD9234 coplanar ground

Customer is asking the following question. Please share your experience :

"We are trying to find a reliable way to manage signal Gnd return between a very wide bandwidth T & H and AD9234, through use of Grounded CPWs.  AD9234 has Pwr-S-S-Pwr arrangement of input lines, where we usually rely on Gnd-S-S-Gnd to preserve signal integrity at high frequency.  ADI’s pin arrangement can be more problematic if the source signal is delivered by a part with VDD that is different from 3.3V on AD9234 – Pwr and Gnd are both AC-ground and Pwr can be used for high-frequency ground return, but when AC-ground at one line end is at a different potential than at the at the other line end, we cannot easily preserve a uniform low-resistance return path for the sampled signal.  A non-uniform Gnd return induces signal distortion. 

   Has ADI investigated this issue at all, and if so, what is the recommended way to preserve signal ground return?"

Thanks

  • 0
    •  Analog Employees 
    on Jul 22, 2016 5:28 AM over 4 years ago

    Hi YKA,

    The pins of the AD9234 do follow a Pwr-S-S-Pwr arrangement. But both the "Pwr" pins are AVDD3 (3.3V). In the layout of the AD9234, the differential input traces are referenced to GND, but then have decoupling capacitors from these Pwr pins to GND at the DUT. Please refer to the AD9234 board layout.

    EVALUATING THE AD9680/AD9690/AD9234 ANALOG-TO-DIGITAL CONVERTER [Analog Devices Wiki]

    The AD9234 also has the exposed paddle which should be connected to GND. Between the decoupling capacitors and the exposed paddle the signal should be safe. We have tested this board and arrangement up to 6GHz to register a 3dB roll-off of roughly 2-2.2GHz in the analog front end. What is the frequency of interest in this case? Usually, when one utilizes a T/H in front of the ADC, for all intents and purposes, the ADC is seeing a low frequency signal, or a "sliced" version of the signal at the T/H input.

    Hope this helps.

    Umesh

  • 0
    •  Analog Employees 
    on Jul 25, 2016 9:01 PM over 4 years ago

    Thanks Umesh, this is very helpful. As a follow on question, do they need to use all bypass caps if they use multiple ADCs:

    "We chose an external T&H to mitigate bandwidth differences.  Nevertheless, we intend to fully rely on available half-power bandwidth at each AD9234.

    I want to add a follow-up to this one.  We plan to use a pair of AD9234s on the same fab; I essentially copied supply bypass capacitors as-described in AD9234 documents.  Of course, that leaves me with quite a lot of bypass for two parts, and I begin to wonder whether I will need that much supply bypass duplication.  Both parts will be running at full sample rate, at the same time, for duration of a system evaluation test.  So, does ADI recommend that I still use full bypass for both parts?  At present, bypass for one AD9234 is this:

  • 0
    •  Analog Employees 
    on Jul 26, 2016 5:53 AM over 4 years ago

    Hi Yasmin

    We recommend using bypass caps for optimum performance. Honestly we haven't tried removing these caps to see which ones matter. At this moment we can only recommend what's in the schematic

    Thanks

    Umesh