Your thoughts on powering analogue supply pins through an RC filter? - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: February 15, 2023, 09:04:41 pm
content: I have found that powering analogue supply pins through an RC filter can often provide impressive results. I don't recall ever seeing this in application notes or other designs and so I've always felt a bit naughty doing it, even though I see nothing wrong with it. I'm wondering what other people think about it?It should go without saying that this is only appropriate for low currents, and that the resistor value should be chosen carefully given both the expected typical current, and peak current.As an example, I am powering the ADXL345 (triple-axis accelerometer) this way as shown in the schematic below. The ADXL345 typically current is 145uA. The 3.3V rail is supplied by DC-DC converter and shared with a Wi-Fi module and other potentially noisy devices. The following graph shows the accelerometer output (RGB lines = XYZ measurements) for both R35 = 22Ω (left half), and R35 = 0Ω (right half). The RC filter reduces the measurement noise significantly.

author: Someone
author_rank: Super Contributor
timestamp: February 15, 2023, 09:13:46 pm
content: Filtering DC supplies with RC is explicitly mentioned in data sheets of various mixed signal devices (say microcontrollers) and even switching regulators.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: February 15, 2023, 09:29:33 pm
content: Yeah? I've not come across it. Plenty of ferrite beads, inductors, Pi filters, but never an RC filter. Do you have an example to hand?

author: RoGeorge
author_rank: Super Contributor
timestamp: February 15, 2023, 09:31:28 pm
content: It is not unusual to use an RC filter for power. That's how usually audio preamplifiers (especially the ones with discrete transistors) used to be powered from the same power supply as the final power stage: through an RC filter. As long as the current through the filter does not vary much, it's OK.When the current varies, a Zener diode might be needed in parallel with the capacitor, or the R in the RC filter might be substituted with an L.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: February 15, 2023, 09:46:51 pm
content: Interesting. I was expecting replies to pick holes in the approach. For example, if a device has both digital and analogue pins (such as a microcontroller) then the RC filter would cause digital and analogue pins to power up at different times, and potentially risk damage or unexpected behaviour. Or perhaps some finer nuance of EMC would advise against unnecessary impedance in supply rails.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 15, 2023, 10:09:31 pm
content: It's the cheapest, not the best.Main disadvantage is that the current must be constant, or else the voltage might vary too much.Power up timing probably won't matter much, and the RF is supposed to go to ground through the capacitor while resistors rather dissipate as thermal than radiates as radio, so EMC shouldn't be a problem either.

author: TimFox
author_rank: Super Contributor
timestamp: February 15, 2023, 10:19:10 pm
content: In analog systems, especially "class-A" where the DC current draw of each stage is constant, the series resistor also acts as a "decoupling" impedance, reducing the effect of one stage's AC component of supply current on the supply voltage to the other stages, reducing possible unwanted feedback between stages.In vacuum tube circuits, this technique was known to the ancients with cascaded audio or RF signal-level stages.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2023, 10:21:36 pm
content: Must be new to analog? Shows up in many places. I think ADC appnotes often include it. Lots of signals things: Ethernet PHYs too, PLLs, etc.See also ferrite bead, which, isn't great for a lot of reasons, but so is the resistor (for different reasons). Between those sets of reasons, one or the other often works out. At worst, a full filter network can be used (with proper inductors and damping).Appnotes in general, range from good and well-researched, to written by foreign intern. They follow the same power law distribution as everything else: 80% of it is mediocre to crap, 20% is usable to good; and of that 20%, 20% again is well researched and immediately useful. And so on. As with all things, they must be read critically: cross-reference against other papers and books, most of all support it with theoretical and empirical knowledge where possible (and -- that we should be so lucky! -- almost everything* in EE can be developed from first principles, or measured and quantified -- given enough time and effort that is), always stop to consider what's being covered and what isn't -- and why, why some aspects might be left out intentionally. Especially when the article discusses a product for sale by the manufacturer: the bias is obvious. Do not mistake them for any kind of authority: they are simply products from a corporation, big or small, doesn't matter.Compare to books by well-respected authors: they are more likely to have experience with multiple parts from multiple suppliers, and less bias towards any one of them.*Basically the exceptions are equivalences of the Halting problem. Which ranges from the condensed-matter version (the analysis of any general material is provably unprovable; simple solids like diamond are...easy enough, but we already struggle to describe materials like ferroics, let alone something like "computronium"), to the explicit case (computers, as we use them today, are a branch of EE). In contrast, linear circuits and EM fields are easily simulated, and almost all nonlinear circuits too.Anyway -- well spotted use!Tim

author: Psi
author_rank: Super Contributor
timestamp: February 15, 2023, 10:25:55 pm
content: Just don't mess up and use an LC filter in the wrong order.You end up with a voltage boost circuit every time your MCU draws a current pulse and it will fry the MCU.Learnt that the hard way.

author: nctnico
author_rank: Super Contributor
timestamp: February 15, 2023, 10:28:45 pm
content: Quote from: sebmadgwick on February 15, 2023, 09:46:51 pmWell, there are cases where it does more damage than good. With loads that draw high peak currents for example. In such a case the power regulation will suffer and cause distortion in the output signal.