# How can I approximate the capacitance of a circuit, prior to building it?

I have recently gotten into FPGAs, and am interested in interfacing it with other, high-speed components (in general, not any one specific components).

Since time is a factor, and I have both resistance and capacitance, I have (as I understand it) an RC circuit, which will affect my signalling signal rise and fall time, limiting my speed.

The ICs & FPGA I will be using mention their capacitance & resistance in their data sheet, but how do approximate the capacitance of my circuit? What is the capacitance of perfboard (aka verboard), solderless breadboard, of a a solder joint? Of wire? a resistor?

Obviously, this depends hugely on the quality of the components, how much solder I use, the distance between the wire and surrounding wires, which is why I'm not asking about this specific circuit. I am trying to get an order of magnitude.

One of the comments suggested I build the circuit, and there is some value in that (especially for solderless breadboards), but I don't want to have to buy many components, to solder them up, and then throw them out, only to have to repeat the process with the next idea.

I have rewritten this question, because I do not want an exact answer for (this) one circuit, I want a general method. I don't want to come back to this forum to ask for each individual circuit, I want a general method/values. I also want to know, if the circuit I am thinking of is too slow, what are the major factors, how close am I, what changes would make a significant difference, and is the very idea even close to feasible?

• I can tell you that those really cheap solderless protoboards present near $5\:\text{pF}$ between adjacent rows. But your question seems really ... odd ... to me, given that you are asking about solder joints and wiring distances and perfboard in the context of an FPGA and "high speed." Can you discuss your application somewhat better? The only thing that comes to mind right now is that you have an FPGA board someone else built, it provides a header, and you want to bring wires from this header out to a personal circuit you make. Is that it? And how fast are we talking?
– jonk
Nov 15, 2018 at 21:45
• Your description is pretty well spot-on. The FPGA is a Spartan 3E, on a Papilio One board - papilio.cc/index.php?n=Papilio.PapilioOne The Ram I have is HM628128ALP-10 (100 ns, according to the datasheet), and I'm hoping for 2 megabytes/second (at 8-bits wide, so 2 megahertz data transfer). I've just realized I can get away with 1 megabyte/second, if I put specific parts in the Papilio block ram. There's not (quite) enough block ram to put everything in block ram - I'm 5kb short. Nov 15, 2018 at 22:32
• so, you want to connect RAM at 2 MT/s to an FPGA using breadboard? I'm using that term rarely, but: LOL.$$\\$$No. Breadboard can't be used for this. Nov 15, 2018 at 23:04
• No one will tell you this, but sometimes you are better off just building and testing the damn thing. You can calculate theothiclally things to the nth degree sometimes but it becomes unrealistic off real world. Nov 15, 2018 at 23:13
• The problem is that you're looking for details on a situation which you should be avoiding to begin with. Nov 19, 2018 at 21:21