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  1. #1
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    Understanding active filter datasheet specs

    I'm looking at the MAX292 active filter datasheet here: http://datasheets.maxim-ic.com/en/ds/MAX291-MAX296.pdf and there are a few numbers I don't understand after getting as far as I could with Google.

    1. Insertion Gain - This means the overall signal gain?
      • So if the insertion gain is -1dB, I should expect a 1Vpp input signal to result in a 0.891Vpp output signal?
      • The datasheet lists insertion gain in terms of Fo. What is Fo... the corner frequency?
      • Since insertion gain is a function of Fo, is this just telling me the filter response curve (it does seem to match up with the frequency response graphs later in the document), or is it telling me some other additional signal loss spec?
    2. What is DC Swing?
    3. Output offset voltage is listed as +/- 150mV typical. Does that mean if I were to use this filter to convert PWM to a steady voltage, it could be off by up to 150mV?
    4. Clock feedthrough is 6 mVpp. Does this mean I should expect a 6mV signal at the clock frequency in the output? That seems high...


    Thanks!
    J

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  2. #2
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    Re: Understanding active filter datasheet specs

    1. Insertion gain is indeed the gain at a specific frequency. Since the circuit has a gain of about -3dB at Fo, then Fo would be considered the corner frequency. The insertion gain is showing the intended filter rolloff with frequency.

    2. DC swing is the output swing for a DC (very low frequency) input.

    3. Output offset is the DC offset that you may see. The output DC level may be off by that amount (400mV maximum).

    4. Yes, you may see up to 6mV of clock noise at the output. That value is not unusual for switched capacitor filters.

    If you want a switched-cap filter with lower offset and noise you might check the LTC1569-7. They also make a LTC1569-6. Not sure what the difference is.
    Zapper
    Curmudgeon Elektroniker


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  3. #3
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    Re: Understanding active filter datasheet specs

    Awesome thank you so much, and for pointing out the LTC filters too.

    Quote Originally Posted by crutschow View Post
    If you want a switched-cap filter with lower offset and noise you might check the LTC1569-7. They also make a LTC1569-6. Not sure what the difference is.
    They both have RRC response. It looks like the -6 has lower max cutoff (64 kHz), lower current draw (3-12 mA), lower clock feedthrough (0.1 mV RMS), and slightly lower SNR, while the -7 has a higher max cutoff (256 kHZ), higher current (6-27 mA), higher clock feedthrough (0.4 mV RMS), and slightly higher SNR. The -6 looks amazing for lower frequency high accuracy applications. Thanks again!

    J



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