AD9629 Vref question

Hello EZ,

Very quick question: I am looking at using the AD9629-20 in an application, and I am trying to determine if a voltage follower is required for fast current response on the VREF input of the AD9629-20.

My external reference circuit consists of a LDO with 1.8Vout tied through a resistor to a 1V zener diode, which is tied directly to Vref of the AD9629-20, and nowhere else.  The Vref output is decoupled as shown in Fig. 42 of the datasheet, with both (1) 1uF and (1) 0.1uF capacitors.

However, on your evaluation board for the AD9629, you have a voltage follower (AD822) on your external reference.  Is that necessary and why?

Thanks!

Ryan

  • 0
    •  Analog Employees 
    on Feb 24, 2016 12:30 AM

    Hi Ryan,

    Thanks for your interest in AD9629.

    In your application, is the SENSE pin tied low or high? If it is tied low, VREF is generated internally and an external reference is not needed. The capacitors shown in Figure 42 are needed for this internal voltage source.

    If the SENSE pin is tied high, then VREF is provided from an external source. On our evaluation board, the SENSE pin can be tied low or high with a jumper. The AD822 is a holdover from our engineering evaluation and was used to allow for characterizing performance at different VREF levels. You do not need this.

    Thank you.

    Doug

  • Thanks for getting back, Doug.

    VSENSE is tied high, and the reference is external.  But now I'm a little more confused.  Are you saying that both the capacitors and the voltage follower are not needed?!  Should I infer that the max current draw is 1V/7.5k?

    Ryan

  • 0
    •  Analog Employees 
    on Feb 24, 2016 11:59 PM

    Hi Ryan,

    The capacitors are there for stability when the internal VREF generator is on (SENSE tied low). When the VREF generator is off, the capacitors are not needed for stability, but they provide some noise filtering and so are still serving a purpose. If your LDO is OK with the capacitance, I would keep the capacitance.

    The nominal current draw will be about 1V/7.5kOhm.

    Thank you.

    Doug