I have some problems for designing a circuit based on AD9218. This is my first time to learn this IC, and make PCB layout by myself.

The datasheet of AD9218 shows that pins 13,30,31,48 are the same pin for Analog Supply (Vd), but they aren't short between (13,48) and (30,31). Would you tell me how do I supply the voltage? When I supply 3V for pin (13,48), the other pin's (30,31) voltage is 2V which is lower than minimun Vd (2.7V) in datasheet.

Secondly,the datasheet provide a driver schematic for inputs but I don't have this IC. Can I use the AD8056 to driving AD9218?

There is a difference between this two pictures, AD8138 has a capacitor between differential inputs and AD8055 doesn't. Is the capacitor required? Should I use the same capacitor (15pF) between the differential inputs?

Thanks for your help.

  • 0
    •  Analog Employees 
    on Apr 7, 2013 2:42 AM

    Hello zhe-xin,

    Please note that the capacitor you reference in the AD8138 circuit is not across the differential input, but is actually between the Vocm input and ground.  A capacitor across the AD8138 differential input will cause problems, so this should not be done.  The 15 pF capacitor following the series resistors on the AD8138 outputs is correct.

    Best regards.


  • Hello Jonathan,

    Thanks for your answers. What I means is:

    1. Can AD8056 drive the AD9218? I only have AD8056.

    2. Should I add the 15 pF capacitor between Ain and Ain-bar to AD8056 circuit if 1. is allowed?

    3. The Vd has four pins (13,30,31,48). They are shorted between pin 13 and 48, and also shorted between pin 30 and 31. Why they aren't shortet between (13,48) and (30,31)?

    4. How do I supply the voltage?

    [1]only pin 13 (or 48)

    [2]only pin 30 (or 31)

    [3]both pin 13 (or 48) and 30 (or 31)

    I'm sorry make you misunderstood. Thanks for your help.

  • 0
    •  Analog Employees 
    on Apr 9, 2013 2:06 AM

    Hello zhe-xin,

    I've addressed questions 1 and 2 below.  Questions 3 and 4 are best answered by someone in the ADC group.

    1)  Yes, it's possible to drive the AD9218 with the AD8056.  They are reasonably well-matched with regard to HD and noise in the 10 MHz neighborhood.  The criss-crossed differential driver is not as convenient to use as a fully-differential amplifier such as the AD8138, but it is certainly possible to use it.  To adjust the output common-mode level with the criss-crossed arrangement you have to add the desired output common-mode level to the main input and lower non-inverting input.  The AD8138 Vocm input directly sets the output common-mode level, and the internal common-mode feedback loop produces excellent output balance.

    2)  The two AD8056s can drive the differential output circuit with the 25 ohm series resistors and differential 15 pF capacitor, but I would check with somebody in the ADC group to find the best RC values to use with the AD9218.

    Best regards.


  • 0
    •  Analog Employees 
    on Apr 12, 2013 12:30 AM

    Hi zhe-xin,

    I support the AD9218 and here's my responses to your questions relating to the ADC:

    Questions 3&4) Each of the (4) Vd pins (13, 30, 31, 48) on the AD9218 should be connected to a common 3V Vd supply bus on your PCB design. As you noted, some of these Vd pins may share common connection nodes within the AD9218 device but this does not eliminate the requirement to externally bias all (4) Vd pins in your PCB design.

    Question 2) Adding to Jonathan's feedback above, yes, from the ADC perspective 25ohm series resistors and a 15pF diff cap per AD9218 datasheet figure 43 is a good recommended configuration.

    Best Regards,

    Tony M