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Multiple DC voltages, regulators, and capacitance multipliers reducing ripple
Posted by Nominal Animal on 19 Sep, 2018 14:40 -
In many microcontroller projects (especially with precision ADCs), it'd be nice to have a dedicated low-ripple supply for the ADC and related things like opamps and precision voltage references. As Dave showed in #1116, a capacitance multiplier followed by a voltage regulator is one simple and cheap option. These projects typically consume less than 1A on all rails combined. However, I'm not sure how to combine the capacitance multipliers when several voltage regulators are used to provide the final voltages.
There are two main cases where I'd like some advice as to how to significantly reduce power supply ripple:- Using a switch-mode boost converter to provide a low-current higher voltage for opamp rails and voltage regulators, for analog input buffering.
For example, say I have 5 VDC supply voltage (with at least 1.5 A current capability), and I'd like to use a cheap switchmode boost converter to raise that to say 15V, then regulate down to 12V, for some opamps and voltage references, for a maximum budget of say 50 mA. However, I'd also like to regulate the 5 VDC down to 3.3 VDC for a microcontroller. Let's say I put a capacitance multiplier between the supply and the 3.3V regulator. How should I connect the boost converter: in series or in parallel with the capacitance multiplier? - Using a 15-20 VDC voltage supply, and regulating it down to 10 or 12 VDC (for opamps etc.), 5 VDC, and 3.3 VDC, with the 3.3 VDC generating the worst ripple.
A capacitance multiplier before the 10 or 12 V regulator makes a lot of sense. But, how to add the other two stages for the 5 VDC and 3.3 VDC regulators, preferably with ripple reduction as well?
All I can think of for stopping the ripple from propagating to the higher voltage supplies, would be to use two 10/12V regulators and two 5V regulators, with the second 10/12V regulator in parallel with the first one, followed by a capacitance multiplier, followed by the two 5V regulators; the other one followed by yet another capacitance multiplier and a 3.3V regulator. However, this seems like a really convoluted way to do this; I'm pretty sure the experienced ones here know better ways of doing this.
(This is actually related to my project of showing how to create good applications that interface to USB or serial-connected microcontroller projects, using e.g. Python or C, keeping the GUI responsive even when communications take a human-perceived time to complete. The software side is progressing nicely -- I've already got a pretty nice skeleton with Python3 (threading and queue modules) and Gtk+, with asynchronous communications via USB/serial using worker threads and queues, even though the default Python interpreter only executes Python code in a single thread at a time -- but the way to properly reduce ripple for multiple VDC voltage rails is confounding me.)
I do believe such circuits are quite common, so I'd love to see good, simple examples. However, if you have difficulty visualizing what I'm asking, I can create a couple of example schematics to illustrate. My basic intent is to create low-ripple voltage regulator break-out boards I and others could use with ADC modules and microcontrollers and such that are available at the usual suspects. - Using a switch-mode boost converter to provide a low-current higher voltage for opamp rails and voltage regulators, for analog input buffering.
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I've been using the capacitance multiplier with my 10V reference (DC source -> cap_mult -> 12V regulator -> REF01)
There is a voltage drop across the transistor, the voltage value depends on its beta, the collector-base resistor value and the Ice current used. So start with identifying that voltage drop. -
So start with identifying that voltage drop.
Oh, a single capacitance multiplier is not a problem; I can handle that. My problem is how to have more than one, from a single supply, to produce multiple different ripple-reduced regulated DC voltages. -
You may use as many capacitance multipliers as you wish.
The only issues are:
1. the voltage drop (and therefore a power loss = Vce*Ice)
2. they do not regulate (unless you include a regulation into its base)
3. the output voltage ramp-up speed depends on k*R*C.
PS: the LM317 might work as a combination of capacitance multiplier and voltage regulator, imho. Not tested in practice, however. -
You seem to be envisioning a complex power supply system but to just focus in on one part you mention of: “ a capacitance multiplier followed by a voltage regulator is one simple and cheap option”, it would be possible and practical to combine the two.
A capacitance multiplier has a charged capacitor connected to the base of a pass transistor to use the beta of the transistor to multiply the effective capacitance. A simple zener/transistor regulator has a zener connected to the base of a pass transistor to use the beta of the transistor to multiply the current capabilities of the Zener.
By combining both circuits (which is commonly done, see photo) you keep the parts count down without any real downside. Any of the integrated regulator ICs combine these features as well. It would still be good practice to have an adequate size cap on the input to any regulators.
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PS: the LM317 might work as a combination of capacitance multiplier and voltage regulator, imho. Not tested in practice, however.
LM317 is not low noise at higher frequencies, e.g. LT3042/45 or TPS series from TI would be better if very low noise is required... -
You may use as many capacitance multipliers as you wish.
The question is how, without creating additional problems.
Consider for example the following two circuits, that regulate 5 VDC and 3.3 VDC rails from a higher DC supply:
(Note that I believe C2 and C5 are either omitted, or 1µF to 4.7µF ceramic caps.)
Will the two capacitance multipliers interact if I connect them as shown, in parallel? Is there a better way to combine such capacitance multiplier + regulator circuits?
At the core, the question is how this kind of circuit can be used in a modular fashion; and if the VIN is a noisy switch-mode supply, are there any gotchas to look for.
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Should be good, but when dealing with high precision/low noise you have to pay attention.
If there is no special requirements or limitations you could go from 5V to 3.3V instead of parallel design, this will get down the power losses in 3V3 regulator, but rising losses at 5V.
For high precision/low noise supply it probably helps to go with dedicated supply as far as possible and pay attention to board layout. -
You seem to be envisioning a complex power supply system
Well, yes. A typical case would be a 32-bit ARM microcontroller (blue pill, Teensy, etc.) connected via USB to a computer, with a couple of 5V peripherals, and a 24-bit ADC module like an ADS1232 or AD7793BRU using a break-out board, and possibly a digital isolator, sampling some low-frequency signal or DC level via a voltage divider or such, or for example reading a Pt100 or similar resistive temperature sensor via a suitable breakout board. From eBay, the parts cost is around $10-$15 USD for that kind of designs, but power supply noise is an issue. (Another issue is that 5 VDC supplies are ubiquitous, and deriving a 10 - 15V low-ripple voltage for opamps and voltage references and such, say at 50 mA or less, is problematic for us hobbyists. Something modular would be optimal.)By combining both circuits (which is commonly done, see photo) you keep the parts count down without any real downside.
Very interesting. How precise is the Zener voltage over a reasonable temperature range and load current? For e.g. opamp high rail (where the exact voltage does not matter much) this seems spot on. Would something like this work for filtering and regulating the output of a (noisy) boost converter, for say 0-50 mA loads?For high precision/low noise supply it probably helps to go with dedicated supply as far as possible and pay attention to board layout.
Note that these are for us hobbyists, so it's more about trying to compensate for the limitations of existing break-out boards for hobby and prototype uses, rather than achieving anything specific -- unless there is a simple board that could be used and populated/adjusted as needed (for 12V, 5V, and 3.3V; optimally from a 4.5-6 VDC noisy supply). For example, I was thinking that instead of choosing the transistors and resistors carefully, one could use a trimmer, and some procedure to adjust the resistance for each transistor.
Perhaps I should ask that -- 4.5-6VDC @ 2A in; 12VDC @ 0-50mA + 5VDC @ 500mA + 3.3VDC @ 250mA very low-ripple output -- as a separate question; the one I asked here might be too vague to effectively answer. -
Design proceeds as any other case:
1. How much attenuation do you need? On what rails?
2. Design the filter or regulator accordingly.
So easy, right?
Point being, you don't need to go out of your way to design an 80dB filter when only 20dB will do (for which an LDO or RC or ferrite bead will do fine).
Now, if you want to have a number of standard options available, that's fair game.
I would be just as, or more, concerned about improving the PSRR of those shitty stages. Why do you need 80dB+ of supply attenuation -- does any particular circuit actually have zero (or less!?) PSRR? Come on, this is solid state we're talking about, we don't have to use shitty triode amplifiers with transformer coupling anymore!*
*The best worst example that comes to mind. Triodes have a significant output resistance component, so a class A common-cathode stage has nearly 0dB PSRR. Audiophools like them for whatever reason...
Regarding crosstalk, indeed a cap mult. circuit "cascodes" the load current into the supply, so the reverse PSRR (input voltage change as a result of load current change, for a given input impedance of course) is limited by that. To isolate noise currents as well as noise voltages, use a multi-stage filter on each one, before the dropper.
Tim -
Note that these are for us hobbyists, so it's more about trying to compensate for the limitations of existing break-out boards for hobby and prototype uses, rather than achieving anything specific -- unless there is a simple board that could be used and populated/adjusted as needed (for 12V, 5V, and 3.3V; optimally from a 4.5-6 VDC noisy supply). For example, I was thinking that instead of choosing the transistors and resistors carefully, one could use a trimmer, and some procedure to adjust the resistance for each transistor.
Perhaps I should ask that -- 4.5-6VDC @ 2A in; 12VDC @ 0-50mA + 5VDC @ 500mA + 3.3VDC @ 250mA very low-ripple output -- as a separate question; the one I asked here might be too vague to effectively answer.
Hm, I am a bit confused as you stated precision ADCs and references in the first post...
Breadboarding and precision does kind of exclude each other. The layout plays a huge role for this kind of stuff.
If you want 24bits sampled with good quality than you have to go down the rabbit hole
If not than take a cheaper ADC with less bits, this will relax the design around it.
It is quite not easy to answer to vague questions, bit more details and the chances would be better to get a specific answer
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Quote
Will the two capacitance multipliers interact if I connect them as shown, in parallel? Is there a better way to combine such capacitance multiplier + regulator circuits?
What do you mean by "interact"??
The capacitance multiplier(s) as shown above (not tried with your PNP/NPN combo) and the voltage regulator(s) will not "interact" provided the voltage regulators will be happy with their minimal input voltage, and, the capacitance multiplier(s) ("CM") can provide the required max current.
That means - you must know the voltage drop at the CM(s) such the Vin minus the CM_voltage drop_at_max_current is bigger than the minimal input voltage the Voltage regulator requires.
Also mind the CM's output voltage ramp-up speed - it takes several seconds after a power up with larger R and C to get the voltage out of the CM.
PS:
Example1:
Vin = 10V
CM_voltage drop_@200mA = 3V (NOTE: !!! we do not know your CM voltage drop - you must measure it !!!)
VIN_regulator_min = 7.5V
VOUT_regulator = 5V
10V - 3V = 7V
7V < 7.5V ----> not good
Example2:
Vin = 10V
CM_voltage drop_@200mA = 2V (NOTE: !!! we do not know your CM voltage drop - you must measure it !!!)
VIN_regulator_min = 7.5V
VOUT_regulator = 5V
10V - 2V = 8V
8V > 7.5V ----> ok -
Nominal Animal - “Very interesting. How precise is the Zener voltage over a reasonable temperature range and load current? For e.g. opamp high rail (where the exact voltage does not matter much) this seems spot on. Would something like this work for filtering and regulating the output of a (noisy) boost converter, for say 0-50 mA loads?”
Ah, the bottom line is you can either get simple or good (or maybe somewhere in between), but not both. It’s like an auto gasoline engine, the engine is really pretty simple but when you go for performance by adding emission controls, gas efficiency controls, etc., you end up with a monster almost no one can understand or repair.
The circuit I showed with a simple zener could be improved with a higher grade temperature compensated diode and a good cheap choice might be a TL431 adjustable one. A higher gain darlington pair would probably help as well as the example shows a gain of 10X for each transistor which is pretty low.
https://www.onsemi.com/pub/Collateral/TL431-D.PDF
If you’re looking for high accuracy of your finished product then others have pointed out that the power supply may not be the only limiting factor.
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Now, if you want to have a number of standard options available, that's fair game.
That is it exactly. I do not have any specific target I am aiming at. I am only looking for what can be done using simple "modules", for hobbyist and prototyping use.
If I could make a small board that takes 4-6 VDC in, and provides nice low-ripple 5 VDC and 3.3 VDC rails, and optionally a 10VDC or something like that, I could get a batch of those manufactured and use them in my projects. I haven't seen anything like that, yet. (I can read circuits and datasheets, and adapt circuits for my own needs to produce my own boards, but my electronics knowledge is mostly theoretical; I have very little practical experience.)
Similarly, a small supply board that takes a higher voltage, say 15 to 20 VDC, from a switchmode power supply (like a laptop charger), and filters and regulates that to 3.3V, 5V, and 10V, would be extremely useful.Why do you need 80dB+ of supply attenuation
I don't, of course!
It's just that in existing break-out boards and example circuits, the filtering is just supply bypass caps, and maybe some bulk capacitance. In reality, many 5V USB wall warts have a couple of hundred millivolts of ripple at 100/120 Hz (depending on the mains frequency), and many microcontroller circuits and display modules generate additional relatively low-frequency noise (say, 100 Hz to 20 kHz). Bypass caps don't help much there.
If I want to use a boost converter module to get a low-power 10 to 12 V rail from 5V, the noise is a significant problem. I don't have practical experience in dealing with switch-mode supply noise (on either side, load or supply!), so I'm asking for advice: ideas, references, and particularly practical experience on what has been found to work. The EEVBlog #1116 gave me an idea that perhaps using capacitance multipliers and several voltage regulators would work here.To isolate noise currents as well as noise voltages, use a multi-stage filter on each one, before the dropper.
That was exactly what I was grasping at. An example of how to do that, without doing the typical newbie mistakes, would be hugely appreciated!Hm, I am a bit confused as you stated precision ADCs and references in the first post...
That is because 24bit ADCs with 22bit precision and accuracy at slow sample rates are easily available as $5 to $20 break-out boards (essentially using the manufacturers example application circuits). If you use a lab power supply, something close to the datasheet specs can be achieved. With typical switchmode supplies in practical (hobbyist) circuits, not so much.It is quite not easy to answer to vague questions, bit more details and the chances would be better to get a specific answer
I know, and apologise for the vague question. I'm having a hard time putting the core question into words. I don't have any specific target yet, and am basically trying to "map" the subject, as I haven't been able to find anything similar on the web. (Besides, me fail English often.)What do you mean by "interact"??
The example circuit above uses one supply source for two separate capacitance multipliers and voltage regulators; the two circuits are essentially in parallel. By "interact", I mean voltage and current ripple from one load (after the voltage regulator) causing a ripple or noise in the other; and whether the two capacitance multiplier circuits affect each other.
This entire thread is not about one capacitance multiplier or its behaviour, but about how to use more than one of them, from a single shared supply voltage, to produce low-ripple voltage rails, without making the most common mistakes. If there are common mistakes, that is. I'm having a hard time finding advice on what kind of filter topologies work well here (single supply, multiple filters, each followed by a voltage regulator).Also mind the CM's output voltage ramp-up speed - it takes several seconds after a power up with larger R and C to get the voltage out of the CM.
Yes. If the microcontrollers can handle a slow supply ramp-up that'll be okay (I think so, but haven't checked yet) as a software start-up delay suffices; otherwise I'll need some kind of "all supplies good" signal.Ah, the bottom line is you can either get simple or good (or maybe somewhere in between), but not both.
Very true. From EEVBlog #1116 I understood the main limitations (voltage drop, rise time, and gain varying between individual transistors), but I haven't been able to find anything related to using more than one (such circuit) to provide different DC rails from a single supply.If you’re looking for high accuracy of your finished product
No, no, just trying to find out what we can do to switch-mode power supply ripple, and the ripple caused by microcontrollers and especially PWM'd display modules. Especially with something cheap, that we could use as a module itself, when doing prototyping or hobbyist stuff. I don't even have any specific product in mind, really: just something I think could be done better.
I know I'm not the first one to look at this, so I'm trying to find out if anyone has related experience they're willing to share here. That said, although there is no answer to my initial question (probably because it is too vague), the responses have been very helpful. At minimum, I can post a better question (after checking out the leads -- the ideas, circuits, and appnotes -- shown here, of course). -
That is because 24bit ADCs with 22bit precision and accuracy at slow sample rates are easily available as $5 to $20 break-out boards (essentially using the manufacturers example application circuits). If you use a lab power supply, something close to the datasheet specs can be achieved. With typical switchmode supplies in practical (hobbyist) circuits, not so much.
If it is cheap, build/buy the circuits, test & measure them, compare to various options and if it meets your expectations you are done
The most valueable experience is experience made by your own.I know, and apologise for the vague question. I'm having a hard time putting the core question into words. I don't have any specific target yet, and am basically trying to "map" the subject, as I haven't been able to find anything similar on the web. (Besides, me fail English often.)
You are not alone
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the only problem with designing noise to spec is interference
you make a light chair a fat guy is gonna break it. no need not to over engineer non mass produced things. Overkill might be cheaper on your time then engineering a specification requirement. The industrial standard for noise is what you can sell LOL (well and maybe intimidating the competition a bit)
the least hair pulling way to determine it is to figure out what bit count you want on your ADC (decimal the customer sees) and determine where the noise should be to barely meet the specification. If your going crazy trying to reach that goal you might be at conventional engineering limits or you might not know what your doing.
Maybe a sanity rule is to avoid massive parallelsim (say 8 LT1028's) of op-amps/transistors to lower noise floor unless you have a target goal of really low noise, because at this point either you made the best with the parts you got (so you decide to parallel) or you might have other areas to engineer that are less 'brute force'.
Not to say that composite amplifiers are bad or uninteresting, but their kinda power houses you develop a circuit with in mind, or 'tricks' to get your specification, because of the diminishing losses. Or if you want something with lower design effort, bulletproof but expensive (i.e. parallel lt1010 *which i like alot* power amplifier on a big heat sink).
Another good one is to keep RF as far away as possible from the circuit, filter it first so it does not get into everything (i.e. put LC filter before your LReg)
we need a web page that has like ven diagrams and shit on it to explain capacitance multipliers/active noise cancellation in detail and what design choices should lead to its use. -
Alright, cool.
1. Supply ripple
Use bigger capacitors. Add moderate (LC) filtering. Add a regulator or converter stage that rides through the ripple.
Mains PFC is an extreme example of this: the input voltage is fluctuating between 0 and 100%. You literally can't draw any power at 0%, and you also can't draw the power required most of the time, because of the restriction of high power factor (current draw proportional to voltage). Rather, the power varies up and down, with the average controlled to meet demand. The power output from the PFC stage is varying 0-200%, and goes into a storage capacitor, which is usually sized for a small ripple voltage (say 10% of total). The ripple voltage is very nearly a sine wave at twice the mains frequency (i.e., the power ripple frequency). To get clean power from this, use a converter (which also gives important mains isolation..) which regulates the output. Its power input is fairly steady (thanks to the ~10% ripple) so it's not a big deal using a conventional control loop based regulator.
You can't get stable power from a single stage and finite filter capacitance, nor can you do the inverse, a DC-AC inverter stage drawing low input ripple. Conservation of energy.
2. Low frequency filtering
Use a regulator or converter. LDOs have okay PSRR below 10kHz or so, and HDOs are usually better. If you can't afford throwing away some voltage, or need some boost (Vin(min) is near or below Vout), consider a SEPIC or buck-boost ("flying inductor") stage.
If you can afford a lot of size and cost, an LC filter is still just fine, and doesn't cost the voltage drop or noise of an active solution. Iron core chokes are available from Hammond and others. It's a traditional method in tube amps, of course -- which are still being made today, if more for amusement than production.
There's also a kind of hybrid approach: you can shunt the ripple by coupling an amplifier into the output rail, feeding the amplifier with the inverse ripple so it cancels out. This doesn't lose DC voltage, but does draw some current (to power the amplifier). This is sometimes done for very quiet high voltage supplies, where the amplifier can run on a fraction of the total supply voltage (otherwise if it's running from the same supply, it would be worse than just pass-regulating the supply).
3. High frequency filtering (light)
LC filtering is efficient. Ferrite beads or chip inductors are cheap and compact.
LDOs suck at high frequency, probably don't bother. (There are a couple specialty LDOs designed for RF applications, that have good PSRR up to the low MHz. Pricey though.) C mult. is good.
4. High frequency filtering (heavy)
a. Shielding
Conceptually speaking: start with a shielded enclosure, with feedthroughs for each low-bandwidth connection (e.g., power, bias, control voltages, slow data, etc.). If you don't need enterprise-grade shielding and filtering, pare it back. Open up holes in the shield (this lets in/out more radiation), but keep the filtered connections within the perimeter of the remaining shield. Shrink shielding from whole-board to critical areas, or even critical traces. (Topologically speaking, as long as the shield surrounds the signals of interest, it doesn't much matter how it's actually shaped. That's why coax works -- and why it must be grounded on both sides!)
Presumably, you still need some high frequency connections: make sure those are well shielded or CM filtered. Minimize crosstalk between signals, and to the body of the circuit. Don't be afraid to apply the same shielding scheme recursively.
Note that a board with solid ground plane counts as this. It's not at all perfect, what with everything exposed on the board face -- but signals coming in around the edges meet the ground plane immediately, to which their shields are tied, and so common mode noise is conducted around the periphery of the board, not through it.
b. Filtering
Consider higher order filters, and common mode filters.
Signal filters: design all your signals for a characteristic / system impedance. There must always be a resistance somewhere, whether it's the source, load or both. If your system has a lot of mismatch (common in switching circuits, where the on-resistance is very much smaller than the load resistance, and the off-resistance very much larger), something like a constant-resistance filter can be used to introduce the necessary damping/matching resistance. There are tables for single and double terminated filters, and unequal resistances.
Common mode filters are constructed the same way, but usually to higher impedances (100s of ohms), with poorer control (the CM impedance of a single box on the end of a wire isn't very resistive!), and a less precise filter profile. You still want to check that the response is not peaky, given typical situations. Mostly you design it around adequate stopband attenuation, rather than a precisely flat passband or a sharp transition band. Pay attention to differential mode response, because CM chokes come in different types and you need to use the right one.
Power line filters should be low impedance, to keep ripple voltage nominal under changes in load current (Z ~= dV/dI). At least, for CV supplies. CC supplies of course must take the opposite approach (keep ripple current nominal under changes in load voltage --> higher Z).
A PDN (power distribution network) typically has an LC chain topology, with therefore a characteristic impedance and cutoff frequency, and that chain shall be terminated with resistance. It would be terrifically inefficient to terminate it with a bulk resistor that draws DC -- so we use a coupling capacitor in series with that resistor. The coupling capacitor has to be large enough that the RC pair isn't reactive at the cutoff frequency, i.e., Cbulk > 3 * Ctotal (Ctotal being the total bypass capacitance along the network).
It's a common misconception that bulk caps are for storing energy. In actuality, we only care that it acts as a coupling cap, terminating the network with just the right impedance and frequency. Indeed, the fact that it's there to reduce ripple voltage, is proof that it's not for energy storage: energy depends on voltage squared, so an energy storage capacitor is only useful if its voltage is changing a lot.
(Which brings us full circle! Remember that mains PFC stage with the bulk capacitor? The ripple on that capacitor is precisely the energy filled in between cycles. As long as the subsequent converter stage can handle the ripple, and the capacitor itself is okay, that ripple can be fairly large. The main reason we go for a relatively low ripple like 10%, is simply because electrolytic capacitors are not very good. More ripple voltage makes them boil and explode. Film capacitors would be perfectly happy, but they're bulky and expensive. Another issue is hold-up time, which again is an energy storage issue.)
As for filter design itself, that's a very complex issue of course. I would recommend starting with typical numbers and tables, then tweaking in SPICE.
Remember these relationships:
Supply filter impedance: Z = dV/dI, where dV is peak ripple voltage, resulting from peak load current change dI.
Characteristic impedance: Zo = sqrt(L/C)
Cutoff frequency: Fo = 1 / (2*pi*sqrt(L*C))
The last two are used to scale filter tables; filter calculators don't actually know anything about filtering, they just apply these to give you a filter at the Zo and Fo you requested. Ratios only. They will quite happily give you nonsense results (say, nH inductors, or mH inductors surrounded by pF capacitors, etc.), it's up to you to sanity check your inputs and your outputs.
There are also transformations that can be used to make an unreasonable filter practical. Example, using series-parallel transforms, to convert a series-parallel (ladder) bandpass into a coupled-resonators topology. And using impedance matching (of various types), so the resonators can be practical impedances (100s ohms?), while the port impedances are nominal. But this is mainly applicable to RF circuit design.
Tim -
What is capasitance multiplier advantage?Why you put it BEFORE regulator?
The two serial LM317 is quite good too, isnt it? -
What is capasitance multiplier advantage?Why you put it BEFORE regulator?
CM is a device which multiplies the effect of a blocking (decoupling) capacitor C1 by k*R1 ("k" is somehow related to the beta of the Q1).
The two serial LM317 is quite good too, isnt it?
See the simplest form of a CM below.
It is put before a voltage regulator because the CM itself does not regulate, and the voltage drop across the CM (the voltage between its collector and emitter) varies (based on the actual current).
In practice it also means the transistor amplifies the "RC effect" such the output voltage is much higher with a CM than when a simple RC is in use.
See below an example:
100Hz 2Vpp ripple on a 10V DC with 100ohm load (R2) - the ripple filtered with a CM, and, with an RC only.
The ripple suppression is similar (2Vpp-->3.5mVpp), but the output voltage with CM is much higher than with simple RC (5.625V against 99mV at the 100ohm load).
With the simple RC you would need R1=100ohm and C1= 100*100uF to achieve the same effect like with the CM in this simulation.
You have to design the CM such it filters your ripple, and, it provides the necessary current.
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The two serial LM317 is quite good too, isnt it?
No. LM317 is old and noisy regulator. Don't use it in "low noise" circuits. Before building unregulated capacitance multipliers, modern Low Noise LDO shall be considered first.
Something like this: http://www.analog.com/en/products/lt3042.html
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whats the correct way to quantify the difference between energy storage in a big capacitor and its filter effect? Do you base the explanation on source impedance change?
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no need not to over engineer non mass produced things
I love to overengineer things. I love robust, reliable things more than even more useful but fragile things. In AvEs words, I too am an Uncle Bumblef*ck.the least hair pulling way to determine it is to figure out what bit count you want on your ADC (decimal the customer sees) and determine where the noise should be to barely meet the specification.
I am doing the opposite: trying to see what kind of resolution and accuracy I can get in practice. Preferably with existing modules, with as little custom design as possible. (I can do it, but I suck at it.)we need a web page that has like ven diagrams and shit on it to explain capacitance multipliers/active noise cancellation in detail and what design choices should lead to its use.
Whenever I have time, I trawl through old threads here, to find out what kind of experiences and circuits others have worked on. Even stuff like different types of DC-DC converters, and their benefits and limitations -- something akin to a selection guide -- are not easy to find. In Dave's videos, I especially love the "traps for young players" parts, even though I'm neither young nor a player.. 1. Supply ripple: Use bigger capacitors. Add moderate (LC) filtering. Add a regulator or converter stage that rides through the ripple.
Good advice. I can definitely use something like an LC filter followed by say LT3042 (as ogden suggested as an example) to generate a very nice 3.3V from 5V for ADCs and other 3.3V sensors, like MAX31865 for Pt100/Pt1000 resistive temperature sensors.consider a SEPIC or buck-boost ("flying inductor") stage
I suck. A SEPIC or a buck-boost, followed by an LC filter and a low noise LDO, is most of what I need for these microcontroller projects. Time to research these!
I do wonder why there are no 5VDC (USB; say 4V to 15V input) non-isolated SEPIC modules providing filtered, low-ripple 5VDC and/or 3.3VDC. You could even add some diodes, so one could power the project from USB or a separate power supply. A lower ripple (especially at lower frequencies) would help a lot with all kinds of sensors.
I believe, but have no proof, that supply ripple or glitches cause a large part of stability issues in hobbyist/maker microcontroller projects. I know they are a big problem with many SBCs.Ferrite beads or chip inductors are cheap and compact.
I noticed some time ago that the "better" USB-connected microcontroller development boards tend to have ferrites on the supply side. They may not be necessary per se, but I would not be surprised if they were actually useful in practice, in avoiding occasional glitches (as in specific situations, or with specific hardware) due to noise on the USB +5V line. They're usually heavily filtered on desktops and laptops, but especially on single board computers there may not be much filtering there. (Olimex even has an USB dongle, basically just two bulk capacitors between the USB power and ground, to be used with SBCs and USB devices with occasional spikes of high current need, like 3G modems.)Shielding
I love Hammond die-cast aluminium enclosures. They're cheap and nice, and if you mount the board upside down to the lid, you can easily use the lid as a heat sink, without limiting access to the innards. I also like the brushed aluminium look you can easily get. (I'd like to find out more about coating the inner surfaces, with some kind of spray lacquer or synthetic rubber, to reduce the risk of shorts due to my own stupidity (like forgetting a loose screw in the enclosure).Shrink shielding from whole-board to critical areas, or even critical traces. (Topologically speaking, as long as the shield surrounds the signals of interest, it doesn't much matter how it's actually shaped.)
For those Hammond diecast enclosures, I'll probably put sub-assemblies on separate boards, with each one having the backside basically a continous ground plane. Like cards on a back plane. Should help with modularity, too: no need to respin one large board for each experiment; can just swap "modules".A PDN (power distribution network)
Ah yes; that's the correct term for this!It's a common misconception that bulk caps are for storing energy.
Nothing in reality is that simple
They do store potential energy in an electric field.
However, just like most things in physics, there is more than one way to describe it. For example, in geometrical optics you can either use Fermat's principle (which states that a ray of light between two points takes the path that takes the least time), or describe the path of the ray using reflections and refraction as described by Snell's law; the math is the same either way. (You can derive Snell's law from Fermat's principle.)
Reality is weird, and that's why I'm always looking for others practical experiences.They will quite happily give you nonsense results (say, nH inductors, or mH inductors surrounded by pF capacitors, etc.), it's up to you to sanity check your inputs and your outputs.
My own field is computational materials physics, so that is basically second nature to me now. Whenever I find a new formula describing some interesting phenomena, I do a quick dimensional analysis to see if it makes any sense at all. (So much utter garbage in the air these days.) Then I test it with known (or expected) results. If they differ, I start looking at why. If they don't, I'll be surprised and very suspicious.
It definitely beats trying to fix code written by others, especially those who believe error checking and security is something that can be added on top, later, if needed. (I did quite a bit of that, when I was younger.) -
if you want cutting edgeish low noise systems that are approachable look at the linear technologies appnote for nanovoltage noise measurement. There are two, one old and one from 2016 or so.
completely different then tiny amperage measurement but its a good start to see what a decent high end solution is (but they take cost liberties too). One app note covers 0-10Hz and the other is like 10 or 100-100KHz and some unregulated wideband
http://www.analog.com/media/en/technical-documentation/application-notes/an83f.pdf
http://www.analog.com/media/en/technical-documentation/application-notes/an124f.pdf
http://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf
now the last 159 one is basically an updated 83 with better noise floor. I built the 83, and its a decent preamplifier to have in the lab in general, and I plan to build the new one. I have yet to shell out for the 124 app note. If you are interested in the 83 amplifier I put some military tantalum caps on it and gave it a dual switch so you can power it off a bench supply if you want and I added another port on it that feeds out the amplified unfiltered signal from the lt1028s. It's not outdated because the parts count is pretty low and its cheap compared the newer one.
But if you study those 3 circuits and read the app notes and read the related datasheets for the parts used you start to get an understanding of what 'impressive' is.. granted its not a hydrogen maser filter or Joseph junction but the specifications are defiantly impressive.
124 has some of what you are looking for because it has a power system and goes over the isolation transformer and stuff for the best measurements -
I forgot why the filter IC in the app note is being used in 4th order mode. You can do 8th order on it, it has two sections *IC has 4, but the schematic by JW uses 2 circuits to make a 'balanced' signal but only one is used. , one is connected but not loaded.. I thought maybe this is done for some kind of high performance reason like current level balance inside of the IC. You can easily turn it into a 8th order filter if you wanted to, but its not written in the note or the datasheet for the IC IIRC.
I think I investigated it. I don't remember if I changed my circuit or not.
I like those LT circuits btw, they are great for lab work, even if you can get a dedicated system to do the thing, its large.. those things can be built pretty cheap and if you want to do something complicated with lots of inputs to really test something you can have an entire arsenal, It saves ALOT of money and space. very much worth it to go through those app notes and build a few circuits, I have alot of them on my list. You can really torture something properly with all those hammond boxes or alotids tins -
http://www.analog.com/media/en/technical-documentation/application-notes/an159fa.pdf
The last page (Appendix B) even describes how and why to use ferrites properly for controlling spikes from high-frequency switchmode power supplies.But if you study those 3 circuits and read the app notes and read the related datasheets for the parts used you start to get an understanding of what 'impressive' is..
AN83's figures C3 and C6 showing a thermally based AC-DC converter is simply brilliant. We really need a directory of appnotes from different sources -- or rather, a categorized collection of them. I can find an appnote when I know what I am looking for, but for us hobbyists and newbies, knowing which appnotes to read to find out useful and practical approaches to specific types of problems would be extremely useful. I bet I've learned more from reading application notes (especially the parts where they explain the design choices) than I did on any electronics courses. Lab work maybe excepted; practice beats theory.
As an example, I was aware of SEPIC as a converter type before I started this thread, but I just didn't connect it at all; probably because I lack practical experience. (Book learning and all that.)
It is quite clear to me why people have their favourite chips and approaches. It takes a lot of time and effort to find even one good approach to a specific design problem. Researching and testing multiple ones for a single problem takes more time than most can spare. If only we'd get all you experienced ones to share your findings more!