• Not Answered

State Variable Filter - Single supply

Can UAF42 work with single supply voltage?

If it's not possible, which IC have a State Variable topology that allows single supply.

I'll use it for notch filter.


4 Replies

  • Hi Rosario,

    The UAF42 uses dual supply operational amplifiers internally is intended for use with a single supply source. Therefore, in a single-supply system the negative supply(presumably) would have to be generated by a dc-to-dc converter. 

    A buck/boost inverting switching regulator such as TI's MC3306A could be used to create the opposite polarity supply. It may be overkill if you don't need all the current that this regulator solution will provide. If the UAF42 output load current requirements are light and you don't intend to use a single supply greater than 10 V, then TI's LMC7660 switched capacitor voltage converter will provide just enough current to power the UAF42. Note that the LMC7660 produces about -9 V with a +10 V input when it is loaded at 6 to 7 mA - the quiescent current level of the UAF42.

    Regards, Thomas

    PA - Linear Applications Engineering


  • In reply to Thomas Kuehl:

    Hi Rosario,

    Looking back at my response to you I somehow mixed up the wording. The first sentence should have read, "The UAF42 uses dual supply operational amplifiers internally is not intended for use with a single supply source."

    Regards, Thomas

    PA - Linear Applications Engineering


  • In reply to Thomas Kuehl:

    It looks like to me that the "ground" pin and signal input(s) could be biased to mid-supply along with AC-coupling the inputs and outputs.

    I'm not sure why the internal op-amps would care if V- is tied to power ground and the "signal ground," pin 11 (DIP), tied to 1/2 V+.

    In figure 1 of the datasheet, for example, the "ground" end of Rq would also need to be tied to 1/2 V+ along with pin 11. Pin 2, if used, might also need to be AC-coupled. The 1/2 V+ supply should provide a low AC impedance to ground via either an op amp output, rail splitter, or resistive divider with a bypass cap.

  • In reply to Wayne Kirkwood:

    Hi Wayne,

    Yes, I had considered the options of biasing the inputs at half the supply using an ac bypassed dc reference level, or using an active rail splitter such as the TLE2426, and then ac coupling the UAF42's input and output. But then while thinking about it, the thought occurred to me that the most often applied filter function is the low-pass. A low-pass requires dc coupling so that dc and the lowest ac frequencies can be coupled in and out of the circuit. If the UAF42 input pins were biased at half the supply, the interfacing circuits migth have a problem with the dc, half-supply bias level. Therefore, I elected not to mention the idea.

    I do agree that for the high-pass and band-pass state-variable topologies that the ac coupling approach could be applied. Also, if the active low-pass didn't require dc coupling, then the capacitor coupling could be applied. But in reality a band-pass response is created by this approach.

    Thanks for contributing your ideas!

    Regards, Thomas

    PA - Linear Applications Engineering