MAX7400 8th order filter problem

Discussion in 'General Electronics Chat' started by Dritech, Apr 22, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
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    Hi all,

    I am using the MAX7400 lowpass filter IC, where I am inputting a signal at 50Hz, 1Vp and a 2.5V offset.
    The external clock is set to 10kHz, which according to the datasheet should set the cutoff frequency to 100Hz.
    Why is the output staying at 0V?

    I also tried using a capacitor instead of an external clock, but it still did not work.

    Datasheet: http://www.mouser.com/ds/2/256/MAX7400-MAX7407-90493.pdf

    Below is the circuit used:

    [​IMG]
     
    Last edited: Apr 22, 2016
  2. Papabravo

    Expert

    Feb 24, 2006
    10,140
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    With the limited information you have provided, my first guess is failure to read and understand the datasheet. How about a schematic and a link to the datasheet so we can be more helpful.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Sorry forgot to upload the image. Please see updated post.
     
  4. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    Are you doing this in a simulator, or on a real breadboard?

    You might need a load on the output. Datasheet recommends 10 KΩ || 50 pf.

    This is not uncommon with filters that they require a load, in the same way that a transmission line has different behavior when open, shorted, and terminated in it characteristic impedance.
     
    Last edited: Apr 22, 2016
  5. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    As you use a single supply, you will need to connect the OS input with a voltage of about 1/2 Vcc.
    This circuit is given in the by you attached datasheet:

    Max7400_offset.png

    Bertus
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,140
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    Good catch. I failed to appreciate from the datasheet that requirement. I was looking for a valid input signal range and found only the OS input range.
     
  7. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply.
    I don't have the IC yet. I am trying to simulate the circuit on simulation software first.
    The circuit is still not working with a 10 KΩ || 50 pf connected at the output.
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    @bertus , Thanks for the reply. Isn't that pin adjusted to set the output offset? The input voltage already has an offset of 2.5V.
    I tried it with a potential divider but the output ramained at 0V.

    [​IMG]
     
    Last edited: Apr 22, 2016
  9. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    Then I would question the behavioral model in the simulator. Where did you get the model from? I hope the answer is Maxim.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    From the datasheet:

    Offset and Common-Mode
    Input Adjustment
    The voltage at COM sets the common-mode input volt-
    age and is internally biased at midsupply by a resistor-
    divider. Bypass COM with a 0.1μF capacitor and
    connect OS to COM. For applications requiring offset
    adjustment or DC level shifting, apply an external bias
    voltage through a resistor-divider network to OS, as
    shown in Figure 4. (Note: Do not leave OS unconnect-
    ed.) The output voltage is represented by the following
    equation:
    VOUT = (VIN - VCOM) + VOS
    with VCOM = VDD / 2 (typical), and where (VIN - VCOM)
    is lowpass filtered by the SCF, and VOS is added at the
    output stage. See the Electrical Characteristics for
    COM and OS input voltage ranges. Changing the volt-
    age on COM or OS significantly from midsupply
    reduces the filter’s dynamic range.

    Try making the OS voltage adjustable as in fig 4 I posted.


    Bertus
     
  11. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again,

    Can someone help me determine the maximum peak-to-peak voltage that can be applied to the input of the MAX7400?

    I cannot really understand the paragraph on page 11. There is also a table showing a Vcc of 3V and an Fc of 1kHz. It indicates Vin(Vp-p) is 2V. Is that the maximum peak-to-peak when having the provided specifications?

    I was planning to use the IC with a 3.3V single supply rail, with an input signal that should not exceed 2kHz and with a maximum peak-to-peak of around 3V (with a 1.65V offset). I am guessing that this is not possible to achieve, right?

    Datasheet: http://www.mouser.com/ds/2/256/MAX7400-MAX7407-90493.pdf
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The MAX7400 is indended to be used at 5 Volts.
    The input voltage will be 4 Vp-p.
    The MAX7400 will not work correctly at 3.3 Volts.
    For 3.3 Volts use the MAX7404/7407, with an input voltage of 2 Vp-p.

    Bertus
     
  13. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Sorry I was referring to the MAX7404. So it would be best to first filter the signal ans then apply the amplification.
     
  14. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    The datasheet states:
    " For applications requiring offset adjustment, apply an external bias voltage through a resistor-divider network to OS such as shown in Figure 3."
    Can I connect the OS pin to the REF3318 voltage reference IC instead of using a resistor-divider?
    If yes, can I use one REF3318 to drive the OS pins of five filters? Also, regarding the 0.1uF capacitor recommended in the MAX7404 filter IC datasheet, should I connect one at the output of the REF3318, or one at every OS pin for all filters?
     
  15. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Does anyone know if the OS pin can be driven from the REF3318 reference voltage IC? or does it have to be a high input impedance? (since the schematic shows quite large values for the voltage divider)
     
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