[sdiy] Voltage divider with RC filter - help with the basics, please

[Tom Wiltshire]

Hi All,

I've got the following situation:

http://www.electricdruid.net/FilterDivider.png

It's a voltage divider, with a cap across the lower resistor. It will feed another input, which will also have an input impedance, which is going to affect things.

Now, what I already know is that for a voltage divider, the voltage out is Rb/(Ra+Rb) * Vin, and also that for a simple RC filter, the cutoff would be 1/2PiRC - in this case Ra and C. But I also know that putting them together isn't that simple.

How do I combine these two to make a voltage divider filter so that I can work out how to get 80% volume with cutoff at 100Hz, for example? Furthermore, if I feed a low impedance, that'd affect the situation too, so how do I allow for that?

My specific case is a MCP4822 DAC (which outputs 0V to 4.095V) feeding a SSM2164 CV input, which only accepts 0V to 3.3V, and has an impedance of 5K. So I need a voltage divider that gives me 3.3/4.095 = 0.806 of Vin. Fc of the filter isn't decided yet, but think CV filtering, so probably <1000Hz.

Some pointers would be appreciated. I mostly do pretty well, but there's times where some formal education in this stuff would be helpful, and that's when I start asking you all!

Thanks,
Tom