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THS4551: TI pdf document contains error? Band pass filter details

Greg Kallfa
Prodigy 40 points
Part Number: THS4551

Hi,

There is a specific TI pdf that details an active pass band circuit using the THS4551.  

www.ti.com/.../tiducc9a.pdf

Table 2 is on page 6.  It lists R2 as feedback resistor and R3 as gain resistor.  Table 6 is on page 9.  It lists R2 as the attenuation resistor and R3 as the feedback resistor...? 

Additionally, there is a circuit (figure 5 on page 6) that outlines R2 and R3 and the component values listed are mixed up from the info on Table 6 as well.  Is this an error?  I'm trying to use the formulas on page 8 to calculate my component values but I am getting confused reading this TI document.  

I am no circuit whiz so I am unsure which resistors would control an overall op amp gain of 2500.  I see the formula listed for pass band gain which depends on the ratio of the input and gain resistors.   Is that different from the regular op amp gain that typically depends on the ratio of gain and feedback resistors?   

If the pass band gain only refers to the shape of the filter then I do not necessarily want to increase that.  I just want an overall op amp gain of 2500 and I'm not sure how to plug in my resistor values to do so. 

Also if the pass band gain is a different than op amp gain, does it still add/multiply with the op amp gain or are they completely independent?  Any help clearing this up is really appreciated!!    

  • 0
    TI__Expert 7875 points
    Hi Greg,

    Table 6 refers to the component definitions listed in equations 3 through 7 where R3 is the feedback resistance. Table 2 is referring to the components as showing in Figure 5 as you have noted. I see how this is confusing and apologize that it was not more clear. I will work the original authors to get it edited for consistency.

    In regards to the gain, the "pass band gain" is the maximum gain of the active filter circuit in the pass band. So if you want to have a gain of 2500 at the frequency where the filter is passing the signal than you will want to set the pass band gain to 2500. This gain is conceptually similar to the gain you would get from a standard op-amp configuration where the gain is set by Rf and Rg, but for a band pass filter we also have capacitors that are increasing and then rolling off the frequency around you pass frequency to attenuate undesired signal frequencies.

    Regards,
  • 0 in reply to Jacob Freet
    Prodigy 40 points

    Aha I was using Figure 5 to plug into the formulas!   Whoops.

    Thanks Jacob, that clears up my confusion and I understand the pass band gain much better!