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Dynamic Gain Control - FilterPro Bandpass Filter

Ron Thomas
Prodigy 20 points

Other Parts Discussed in Thread: OPA1652, OPA1642, OPA211

With the help if TI filterpro  I was able to design a low frequency band pass filter. The filter design was spot-on and yielded very desirable results. I have a total of 1000x gain across the seven stages.

I need the gain to to be adjustable dynamically to help differentiate patterns in strong signal environments. To achieve that I imagine that the gain need to be dropped proportionally across the seven stages. Now I understand (I believe I do) that if the resistor values are adjusted to drop the gain, the filter characteristics also get altered.

I would greatly appreciate any suggestions on implementing a dynamic gain control for the above-mentioned system.

  • 0
    TI__Guru* 93105 points

    Hello Ron,

    I am happy to hear that FilterPro helped you successfully synthesize your band-pass filter. Indeed you would want to distribute that high gain across as many stages as reasonable to reduce the gain sensitivities.

    The equations that set the gain and response for FilterPro's Multiple-Feedback and Sallen-Key topologies are highly interdependent. As you have seen changing the gain recalculates all the components such that the response remains intact with the higher gain.

    I have seen some low-order active filter designs having an analog multiplier, or divider, in the feedback used path to control the filter's cutoff, or center, frequency. But nothing that would be practical for a high-order filter like you have designed. It would seem that it would be best to keep the gain control function separate from the filter function. I would precede the filter by an AGC stage and let it take care of the gain control.

    Regards, Thomas

    PA - Linear Applications Engineering

     

     

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hello Thomas,

    Thank you for your prompt response. I am planning to pick up extremely weak signals and pass it through the filter. I am just concerned that implementing a gain stage U/S of the filter will saturate the signal with unwanted noise.

    I could implement the gain control after passing through a filter stage with a nominal 10x gain and then run it through a gain stage and then run the signal through another stage of filter with unity gain. We're talking about an extremely high order filter here, though.

    I would appreciate any thoughts.

    Thanks,

    Ron

  • 0 in reply to Ron Thomas
    TI__Guru* 93105 points

    Hi Ron,

    Thank you for the additional information about your circuit application. Yes, if you reduce the overall gain of the band-pass filter and set it such that it will accommodate the full input range matter, the dynamic range issue is addressed. Then, it is just a matter of selecting very low noise operational amplifiers for the filter, and the gain-stage that follows the filter, so that the very low-level signals are not lost in noise.

    One of our audio, SoundPlus, operational amplifiers such as the OPA1642, or OPA1652, might be a good choice providing their gain-bandwidth (GBW) is sufficient to support the required filter response. FilterPro provides the recommended minimum GBW by stage so that helps make it easier to select the correct amplifier. Here's the link for viewing their data sheets:

    http://www.ti.com/product/opa1652

    http://www.ti.com/product/opa1642

    If the filter and/ or amplifier requires even lower noise and higher bandwidth, the OPA211 (OPA2211 dual) is one to consider:

    http://www.ti.com/product/opa211

    Regards, Thomas

    PA - Linear Applications Engineering