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UAF42: UAF42 Square Wave to Sine Wave Filter

Danilo A.
Mastermind 25555 points
Part Number: UAF42

Hi Team,

Our customer is designing a filter to generate sinewaves using the circuit in Figure 2 of SBFA003 application note. He need to determine the values of resistors RF1 and RF2 to produce 40.6Hz and 528Hz. He will create 2 separate circuits for each frequency. How can we compute for the values of RF1 and RF2?

Regards,

Danilo

  • 0
    Guru 140200 points

    Hi Danilo,

    haven't you seen the design equations in the datasheet?

    For 40.6Hz I get:

    RF1 = RF2 = 3M9201

    For 528Hz I get:

    RF1 = RF2 = 301k43

    See also the appnotes on the product page of UAF42:

    Kai

  • 0
    TI__Guru* 93105 points

    Hi Danilo,

    I did a quick UAF42 circuit simulation using the RF1 and RF2 values that Kai derived for 40.6 Hz and 528 Hz band-pass filters. RQ remained at 2.8 k. The results from a TINA Spice gain vs frequency sweep are seen here:

      

    Keep in mind that the resistors and capacitors internal to the UAF42 do have component tolerances so the center frequency (fc) can vary a little between devices. Use low tolerance precision resistors for RF1 and RF2 to minimize response variations caused by them.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Mastermind 25555 points

    Hi Kai and Thomas,

    Thank you for sharing the values of resistors for these requirements and for the confirmation thru simulation.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Mastermind 25555 points

    Hi Team,

    The customer is requesting for the formula used to find the values of RF1 and RF2. I used the formula of Noninverting Pole-Pair at page 6 of the datasheet but can't figure out how to arrive with these values.

    For 40.6Hz

    RF1 = RF2 = 3M9201

    For 528Hz

    RF1 = RF2 = 301k43

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Guru* 93105 points

    Danilo,

    Applying the equations listed in the UAF42 Figure 1. Noninverting Pole-Pair circuit and running the algebra for the band-pass here are the results:

    First use Eq. 6 to set band-pass gain:

    ABP = R4/RG   

    and since RG = R4 , ABP = +1V/V

    Then to find RF1 = RF2 it is derived From Eq. 1:

    RF1 = RF2 = [1 / ( 4π2∙fC 2∙C12) ] 0.5

    For example if fc = 40.6 Hz

    RF1 = Rf2 = [1/ ((39.48) (40.6)2 (1∙10-9)2) ]0.5 = 3.92∙106 Ω

    The same value that Kai provided.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Mastermind 25555 points

    Hi Thomas,

    Thank you for these information. The value of RQ is 2.8kΩ in the application note. Should the customer maintain the value of RQ for 40.6Hz and 528Hz?

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    Guru 140200 points

    Hi Danilo,

    the UF42 implements the so called "state variable filter". One of its advantages is that the frequency of operation and the Q factor can be varied independently. So, if you still want a Q=10, then there's no need to change the RQ=2k8 resistor.

    Kai