Part Number: THS4130
Other Parts Discussed in Thread: THS4131
Hi there,
I read this slyt721 and found the following infomation is confusing. See below picture, I marked out with two blue rectangles.
I thought Figure6 is MFB rather than Sallen-Key configuration. And how should I get the fc of this filter ? Thanks very much!
Yes sir, this is an MFB unity gain filter followed by a big attenuator in the postfiler,
The MFB itself is just doing this (using an extract section of my MFB design flow). This is a very old circuit, normally, I would put the attenuation into the MFB itself to avoid unnecessary Vpp at the FDA output pins.
And more briefly Rust, your question on where the 24.84MHz came from - inside the MFB is a noise gain profile that peaks around the Fo frequency. The approach in that 3rd article is to find that noise gain peak (and the full flow tries to reduce that peak as well, sort of described in the 1st article) and adds a margin at that frequency to set a minimum loop gain inside the filter - I use 20dB and add that to the peak noise gain at its peak frequency, then the single pole Aol op amp response needs to hit 20dB above that at that frequency which allows a min GBP to be developed. still using the RC values in the app note, here is what that looks like - since the THS4131 is much faster than the min, the min LG is much higher than that 20dB, a gain of 1 FDA stage has a DC noise gain of 2V/V or 6dB. I was reading that section of the article, really kind of vague on how they are setting up the filter frequencies - and I would have done this as a attenuating FDA stage without the post RC attenuator. Normally, this kind of stage is a 3rd order butterworth, but that is not what it is doing it seems.
