VFB as a voltage reference

Hi -

I am using an ADA4505 VFB as a voltage reference to obtain Vdd/2 (Vdd = 3.3V) as a midpoint reference in an active filter circuit, using a pair of 1% 10K resistors at the NI input. the 4505 is being supplied by an LDO, which is preceded by a switcher supply.

My question is, how stable would this VFB output be for a comparator? The comparator requires a threshold of around 200mV, so I would need to take the VFB output and divide it down to feed Vref of the non-inverting comparator.

My thinking is, perhaps the instability of the 3 different devices (switcher, LDO, VFB) plus the resistor divider may contribute a lot of instability so my Vref at comparator could vary by some conjugate of those numbers that could end up too low or high. I can tolerate around 50mV on the threshold.

So...bad idea? Should I use a dedicated voltage reference?

Thanks for your help!

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  • Gary,

      Which comparator?

    The Filter Wizard is really good;  I know the guy that wrote it.

    I would have to see the full design to comment further, but I would guess they use an AD8031 which is a high current

    bipolar that will drive a Tesla motor......

    What is the frequency of your filter?  You might save some current with something other than the 4096.

    Also, the industry makes a lot more types of duals than quads, so don't use the 4096-4.  Two duals is

    a lot easier to lay out, especially with active filters that have lots of R's and C's around them.  The additional

    space and money is minimal and the performance is better.  I  would look at the LT6078 and the LT6020.

    What threw me off is your dual use of the midpoint.  For the AF, and also for the comp ref.

    If the comparator input is well behaved, you could just divide down again from the 3.3V/2 and again, put

    a small cap on the 200 mV node.

    Harry

Reply
  • Gary,

      Which comparator?

    The Filter Wizard is really good;  I know the guy that wrote it.

    I would have to see the full design to comment further, but I would guess they use an AD8031 which is a high current

    bipolar that will drive a Tesla motor......

    What is the frequency of your filter?  You might save some current with something other than the 4096.

    Also, the industry makes a lot more types of duals than quads, so don't use the 4096-4.  Two duals is

    a lot easier to lay out, especially with active filters that have lots of R's and C's around them.  The additional

    space and money is minimal and the performance is better.  I  would look at the LT6078 and the LT6020.

    What threw me off is your dual use of the midpoint.  For the AF, and also for the comp ref.

    If the comparator input is well behaved, you could just divide down again from the 3.3V/2 and again, put

    a small cap on the 200 mV node.

    Harry

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