Common mode matching between AD9253 and AD8352

Hi,

We are using IF sampling ADC (AD9253) in our system. ADC driver, AD8352 is used to drive this ADC.

In the datasheet of the ADC, it is shown that it is better to drive the input signal with the common mode voltage of 0.9V. How to make sure that ADC driver output is centered around 0.9V?

Should we just connect Vcm pin in ADC to ground through 0.1uF cap (to get 0.9V Vcm) and connect this Vcm pin to the Vcom pin in AD8352? Will this scheme work? Or should we only use transformer based coupling as shown in datasheet of the ADC?

The frequency of operation is 140MHz with BW of 40 MHz. In this case is it even necessary to think about common mode voltage, since our signal has zero DC component?

Thanks in advance.

  • 0
    •  Analog Employees 
    on Sep 17, 2012 6:18 PM

    Hello Veera,

    Thank you for posting the question regarding the AD9253 on E-zone. For your application I would suggest AC coupling the outputs of the AD8352 with the inputs of the AD9253.

    - Connect a 0.1uF decoupling capacitor to the VCM pin of the AD8352. This will make sure that the common-mode for the driver will be at mid-supply.

    - AC couple the outputs of the AD8352 to the inputs of the AD9253.

    - Connect a 0.1uF decoupling capacitor to the VCM pin of the AD9253. Then use  termination resistors to connect this DC voltage to the AINx lines of the ADC.

    I have attached a drawing to this effect. Let me know if you have any further questions.

    Good luck with your design.

    Umesh

  • Hi Umesh,

    Thanks for the suggestion.

    Unfortunately, we cannot add anti-aliasing filter to the input of the ADC.

    Can I use Termination resistance RL in this case? This resistance may reduce the input impedance of the ADC.

    In Data-sheet it is suggested to use small value resistance in serial arm to the ADC.

    For quad channel ADC like this, how to provide common mode DC voltage to all four inputs. Can I connect Vcm pin to all four termination resistances through four different Rcm resistors?

  • 0
    •  Analog Employees 
    on Sep 18, 2012 5:18 PM

    Hi Veera,

    Please be aware that if you do not add an AAF, the broadband noise generated by the amplifier gets fed into the ADC and will affect the SNR performance of the ADC.

    Using termination resistors as recommended in the amplifier datasheet should be ok.

    For hooking up the CM on the quad channel, I would recommend using a Kelvin connection and using a small (~100ohm) series resister on each of the four paths to control the crosstalk. I would also use 0.1uF decoupling capacitors at each of the four hook-up points on the channels.

    Please refer to the evaluation board schematic in the "Design and Integration Files" section below:

    http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad9253/products/EVAL-AD9253/eb.html

    Let me know if you have any further questions.

    Umesh