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Change capacitors in WEBENCH

Is it possible to change capacitors in WEBENCH, and then see how it affects the circuit?

I know I can click components and choose an alternative, but I want to change it to something that isn't listed in the alternatives. That is, I'm designing a low-pass filter for 4.2KHz using Sallen Key, and the second stage shows C2 as 27nF. The closest cap I actually have in the lab is 22nF, and I want to know how if I can use it or not. The only alternatives in the list are all 27nF.

Thanks

  • Yes, it is possible.

    1. Go to the schematic view of the tool

    2. On "cap tolerance" choose IDEAL/EXACT (this is to say custom or non-real component)

    3. Put your cap seed value in the box (let's say 1.234E-8)

    4. See your capacitor value on schematic as well as the BOM section below the schematic is now being changed to 1.234E-8


    See attached picture. (see area circled with green color)

    Herman

  • Thank you very much for the reply and instructions. I actually have two caps in my design (C2 between A1 output and R1 input), and when I change the CapSeedValue for C1, I see that C2 gets calculated to a new value. Is there a way to specify both of them (or hold one constant while changing the other), and then see what that does to my cutoff frequency?

  • Mike,

    Thank you for using our WEBENCH Filter Designer solution for your filter needs. As I understand it you want to adjust both capacitors to be equal. At this time, Filter Desinger is not capable of making both caps equal. They are dependent on the seed, and as you saw, calculated to differnt values to meet the filter requirements.

    One option that you may want to try is to export the circuit into TINA. Once you have the filter circuit in that envirnment you can change the components to any value and simulate to see what happends to the cutoff frequency. Please note that if you make this modificaiton, the approximation (Bessel, Butterworth, etc) response will not  kept.

  • Bonnie,


    Thank you for the suggestion. I have imported the design into TINA and changed the caps as I wanted to. I'm new to TINA, and I'm not sure how to now run a simulation to see the cutoff frequency.  Can you please provide some guidance?

    Thanks

  • Mike,

    Sure can. After you get your circuit diagram squared away, you start the evaluation by:

    Once you do that you need to program in your preferences:

    Good luck, please let me know how it goes.