High Pass Sallen Key - Single Supply Confusion

Discussion in 'General Electronics Chat' started by madscientistdan, Mar 7, 2012.

  1. madscientistdan

    Thread Starter Member

    Mar 6, 2012
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    Hello Everyone,

    I'm not sure how to handle the low pass filter characteristics from the voltage divider to get 6V.
     
    Last edited: Aug 20, 2012
  2. madscientistdan

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    Mar 6, 2012
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    5546 hjjkjijjk kook
     
    Last edited: Aug 20, 2012
  3. Jony130

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    Feb 17, 2009
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    OMG Simply remove R3 and R8 (replace them with short-circuit) also remove R9, R10 C7 C8.
    Reduce R4 and R5 to 10K and use one voltage divider for bias two op amps.
    C3 put in parallel with C4.
    As for C4 value

    C4 = 0.16/( R4||R5 * Fc) = 0.16/( 5K * 0.1Hz) = 330uF
     
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  4. Audioguru

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    Dec 20, 2007
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    I cropped your schematic so it is no longer bigger than my neighbourhood.
    I removed the useless parts.
     
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  5. madscientistdan

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    Mar 6, 2012
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    ok thanks again
     
    Last edited: Aug 20, 2012
  6. Audioguru

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    Why? The existing opamp already has an extremely high input impedance the same as a buffer.
    But I think the opamps will have difficulty driving the very low values of your frequency determining resistors. Use resistors 10 times higher and capacitors 1/10th what they are now.

    C7 also has a fairly high value and will usually be a polarized electrolytic type.

    No.
    C9 is not needed. C3 should be 10uF to 100uf. It is a hum filter and does not create a cutoff frequency.

    If you have hum then the value of C4 should be high. If the biasing resistors are 100k each then C4 can be 33uF. Then it is a good filter to reduce the hum.
    If you don't have hum then C4 is not needed and C3 can be 0.22uf.
     
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  7. madscientistdan

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    Mar 6, 2012
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    Thanks so much for your help!
     
    Last edited: Aug 20, 2012
  8. Audioguru

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    The opamp has a low output impedance which is important. But what is the impedance of a 33uF filter capacitor at 35kHz? Only 0.14 ohms which is almost the same as the opamp. So the buffer opamp is not needed.

    At 35kHz, a 10uF capacitor has a reactance of only 0.46 ohms. But the minimum load for the circuit is only 1k ohms so a 0.022uF film capacitor will be fine. A 0.1uF film capacitor can also be used. Besides, a ceramic capacitor is a microphone that picks up vibration, plus it adds distortion.

    C8 is 0.1uF which is good for high frequencies and C9 is 10uF (or 100uF) which is good for low frequencies. Then C9 is not needed.

    A battery does not produce hum so C4 is not needed.

    C3 simply filters the bias voltage at high frequencies. At 35kHz its reactance is only 46 ohms but it is in parallel with C4 that has a lower reactance. Both do not affect the cutoff frequency of 35kHz.
     
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  9. madscientistdan

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    Mar 6, 2012
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    Thanks Audioguru!
     
    Last edited: Aug 20, 2012
  10. Jony130

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    In this case buffer doesn't help to much.
    But I will also change as Audioguru suggested filter resistor values.
    R1 = 3.9K ; R2 = 5.1K ; R6 = 11K ; R7 = 2K and all filter capacitor to 1nF
    And then change R3 and R4 to 100K

    Simply, all AC current will be shorted to the GND by low impedance of a 33uF filtering capacitor.


    Op amp distort much heavier when output load is increased.
    So you should not use load smaller then 1K.

    You can leave C4 = 10uF if you want.
     
    Last edited: Mar 8, 2012
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  11. madscientistdan

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    Mar 6, 2012
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    thanks again
     
    Last edited: Aug 20, 2012
  12. madscientistdan

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    Mar 6, 2012
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    I have a couple of questions still:
     
    Last edited: Aug 20, 2012
  13. Jony130

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    Why you change component value ? Why you not use a standard E24 resistor values ?
    Also don't forget about component tolerance. So you real Fc will be slightly off from that you calculated.

    Normally we don't use any extra resistor.
    http://www.analog.com/library/analogDialogue/archives/41-05/input_protection.html
    And sometimes we add the extra resistor to compensate offset current.

    Input resistance should be at least 10 times greater then source output resistance.

    No,
    C7 = 0.16/ ( F * Rload)
    To reduce the C7 effect on the circuit we should choose F = Fsignal_min/10

    But maybe the next stage has a pull down resistor.

    Use tantalum capacitor or electrolytic. Or use high load resistance then you can use film capacitor.
     
    Last edited: Mar 8, 2012
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  14. Audioguru

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    A final dsiscussion about using a buffer opamp for the bias:
    1) The impedance of the bias source should be 1% or less than the frequency-determining resistors (R2 is 500 and R6 is 1.1k but they should be higher) because the impedance of the bias source is in series with the frequency-determining resistors. The two resistors in parallel are 344 ohms and 1% of them is 3.4 ohms.
    2) The TL081 opamp has Jfet inputs that need no bias current so they are an extremely high resistance. Two 100k resistors can easily provide a 6V reference voltage for them.
    3) The output resistance of a TL081 buffer is about 200 ohms which is reduced by its negative feedback voltage gain. At 35kHz its negative feedback voltage gain is 100 so its closed-loop output impedance is 2 ohms.
    4) A 33uf capacitor has a reactance of only 0.14 ohms at 35kHz which is much better than a buffer opamp.
    The TL081 has a fairly high frequency response. Stray capacitance to ground on its output (maybe caused by a shielded audio cable) combines with its 200 ohms output resistance to cause phase shift which causes the opamp to oscillate at a high frequency. The oscillation does not occur if the circuit has a series 100 ohms resistor.

    The OP talks about using a 10uf output capacitor to pass 50Hz. Then its load can be as low as 320 ohms to 1600 ohms which is very low. Most opamp loads are 10k ohms or higher then the output capacitor can be a 0.22uF unpolarized film capacitor.

    A tantalum capacitor is not reliable and it causes distortion. A ceramic capacitor is a microphone since it picks up vibration and it causes distortion. They can be used for power supply filters but should not be used in a signal path.

    I am confused about the function of this highpass filter. What is the ultrasonic input to the piezo-ceramic sensor?
     
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  15. madscientistdan

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    Mar 6, 2012
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    Hi Audioguru,

    Thanks for making those points. I will try to think about them and understand them.
     
    Last edited: Mar 10, 2012
  16. madscientistdan

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    Mar 6, 2012
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    Hi Jony130,

    I chose the resistor values
     
    Last edited: Aug 20, 2012
  17. Audioguru

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    Does anything communicate at ultrasonic frequencies beneath the surface of the ocean? Dolfins use sound frequencies but some scientists "guess" that they hear 30kHz to 40kHz. Whales use very low sound frequencies.

    Submarines use very low radio frequencies.
     
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  18. madscientistdan

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    Mar 6, 2012
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    Hi Audioguru,




    What is the best solution?
     
    Last edited: Aug 20, 2012
  19. Audioguru

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    An opamp can drive a low power voltage divider that feeds a low signal voltage to 50 ohms.
    An opamp can drive a transistor that can feed high power into 50 ohms.

    Take your pick deterrmined by how much power is needed into 50 ohms.
     
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  20. madscientistdan

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    Mar 6, 2012
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    Thanks audioguru,

    I'm having a tough time knowing which of the two solutions I need to go with, and I'm not even sure how I would implement it.
     
    Last edited: Mar 10, 2012
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