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I am designing low pass active filter (Sallen-Key structure) and I have come to a point where I have to decide what should be the value of C (C1 = C2 = C) and then calculate values of resistances. I know that capacitance can not be very low because then I would have to take into consideration parasitic capacitances of other elements on the PCB. I also know that it is not recommended to take high value of C but I do not know why.

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High values of C would imply low values of R, therefore high signal currents.

In simple circuits (e.g. emitter followers) when the signal current is a large proportion of the quiescent current, you have to consider the device's nonlinearity (i.e. distortion). In op-amp based circuits, as long as the opamp has sufficient bandwidth and drive capability, this is a lesser concern.

In either case, high signal current means high supply current, i.e. heat and wasted power.

However, go too far in the other direction, and low signal currents mean high resistor values, and higher noise (both shot noise from the lower currents, and Johnson noise from the higher resistances).

The right balance depends on the application.

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Define high. Anyway, there is a thumb rule: the bigger is the capacitor, the bigger ESR and ESL it has. Of course, ceramic caps are always better than electrolytes of same capacity, but the rule still works. Maybe i could tell more if i had more info on the application.

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  • \$\begingroup\$ fc = 8 kHz, I have assumed that C1=C2, R1=mR, R2 = R. I have count m based on Q and K (gain). Now I wonder should I use C = 12n and R approx 3k or C approx 1u or maybe 1n. \$\endgroup\$ – Al Bundy May 31 '15 at 11:23
  • \$\begingroup\$ 10nF is a nice value, but 1nF or 1uF are OK too. One consideration that i recommend: standard values. Another: if you care, think about output resistance, don't make it to high. \$\endgroup\$ – Gregory Kornblum May 31 '15 at 11:29
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High value capacitors have lower leakage resistance associated with them. Certainly this depends on the type of capacitor used. If you use paper/mica capacitors their leakage is relatively lesser but they are often too bulky.

So if you want to generate 1 Hz, it is not difficult, but generating 0.001 Hz will be. So the method used to use a digital divider. Similarly row frequency (0.001hz) ramp is generated by digital means. Active filters can amplify the capacitor by miller effect and effectively solve lower frequency issues with not so high a capacitor. Some of the newer Farad capacitors may possibly solve many problems.

I have not tried these. But repeatability of an oscillating frequency will still be a problem due to their tolerance and temperature dependence.

It will be a good exercise to attempt to make a ramp growing from 0 to 10 volt in say 1000 secs. You will have to have a lot of patience to determine the repeatability if you change the temperature!

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