RC low-pass filter: pros/cons of various R & C values for a given frequency? - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 24, 2016, 02:45:07 am
content: I've been fiddling around with a low-pass RC filter to smooth out some noise from a signal. Just by messing around with the values of resistors and capacitors I had to hand I settled on a 33K resistor and 0.1uF cap, which I believe should filter to approx. 48Hz. This works pretty well for my application.However, playing around with an RC filter calculator, I realised I could make up my 48Hz filter using a variety of combinations of values for resistor and cap. For example, 100K & 33nF.So I was wondering: for a given filter frequency, what are the pros and cons of having different values for the components?

author: Ammar
author_rank: Regular Contributor
timestamp: August 24, 2016, 03:33:32 am
content: If you look at the transfer function it would appear that the product of R and C is all that matters. However, a practical filter will have something on the input and the output. Say you wanted to drive a 100 ohm load and you had a 10k resistor in your filter. The filter will become loaded and its characteristics will change significantly. You could probably figure out by how much with a thevenin equivalent.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 24, 2016, 03:48:21 am
content: Your filter circuit doesn't float on its own in a vacuum. Presumably it is connected to some source circuit, and it feeds into some destination stage "downstream". The output impedance of the stage feeding your filter circuit plays a significant part in the scaling of the filter elements. And the destination load impedance also must be factored into the equation.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: August 24, 2016, 07:11:16 am
content: The input to my filter is the output of what is essentially a differential amplifier (a current-sense IC). The output is consumed by an ADC port on an ATmega328. I can't find any numbers for the former; datasheet just says "low impedance". And the latter is stated as having an Analog Input Resistance (not sure if that counts as the same thing) of 100M. Interestingly, the Atmel datasheet says that the "ADC is optimized for analog signals with an output impedance of approximately 10k or less".So, if I can assume that the source is of such low impedance as to be negligible (am I right?), is it as simple as saying that the resistor in my filter should be <10K for optimal ADC performance? And that if I'm otherwise using a higher value than that, it may have a negative effect on the sampling rate and accuracy of the ADC readings? (Too much impedance slows the charge of the internal store-and-hold cap?)

author: b_force
author_rank: Super Contributor
timestamp: August 24, 2016, 07:19:58 am
content: I'd actually go for a 2nd order filter most of the time (still don't get why most people don't use that for filtering PWM as well).In this case, most important is the resistor.The value is a tradeoff between input impedance and noise.The higher the value, the more noise you'll get

author: tggzzz
author_rank: Super Contributor
timestamp: August 24, 2016, 08:30:33 am
content: Quote from: HwAoRrDk on August 24, 2016, 02:45:07 amPhysical size.Cost.Parasitic components, especially for RF.Tolerance.Component availability.Impedance relative to source and load, critical for RF.Noise.