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Questions regarding capacitors, oscillators, jumpers, grounds etc.

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muashr

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Hi,

I have following questions.

  1. Why multiple capacitors of different values are connected between VDD and ground?
  2. What are solder jumpers? What they are useful for?
  3. Why and when external oscillators are used or needed for microcontroller?
  4. What to do with unused pins of CMOS devices or microcontrollers?
  5. Why & when two pins of microcontroller are connected with each other via a (boost) capacitor?
  6. What is the difference between digital and analog grounds?
Thanks
 
1/ Electrolytic caps are used for smoothing but often have inductive qualities, in order to decouple the effect of this a low value is wired in parallel to shunt any HF component that may occur.
3/ In order to make the frequency of operation more flexible.
4/ Connect to power common.
6/ Generally only by symbol to indicate if electrically separate condition exists.
Max.
 
1) Also: Often a very large capacitor is somewhere on the board as a main source of power for the entire board or a portion of it. Simultaneously, many smaller caps are placed all about the board near the critical circuitry (typically each chip package) for "local" bypassing. An analogy I used that made sense to someone years ago was like this: Think of the big capacitors as the supermarket that has lots of electrons for sale - that's where the chips go when they need to stock up on electrons. However, often times a chip needs a few electrons very quickly - it would take too long to run down to the big caps to get them in time for whatever it is they need to do. In those cases, they can go to a local convenience-store that is right around the corner and be back in a flash. That is why there are a lot of smaller caps spread around the board. Many circuits need electrons quickly and the time and natural impedance of a long conductor would cause a quick, temporary voltage starvation.

2) I use solder jumpers because they are the most cost-effective way to make simple one-time changes to a circuit board when it is being built. Tiny switches are often used also but they are never as small, light, and reliable as a good ol' piece of solder or wire. PCB's are often expensive and only become affordable when made in large quantities, so to make it a reasonable idea to buy a bunch of boards, those boards need to be flexible enough to be useful for more than one function. A well thought out design can often have a lot of functional difference with a single jumper. It can re-wire different power supplies, open a conductor for calibration purposes, or with microprocessors - completely change the software routines that are run. Since I do electronics as a hobby I can't afford a lot of boards, so when I do I have to use them whenever possible. At work, I do R&D on circuitry and need to have flexibility for changes in the prototype designs.

Another type of jumper you will often see in older affordable consumer-grade appliances are solid wire jumpers. They often use a single-sided copper circuit board and use a machine to insert a bunch of wires on the other side to make connections that cross all over the board. This is the same machine that puts all the rest of the parts on the board. For consumer products, cost is generally the #1 driving force in the design (hopefully after safety, but that's what regulations and certification agencies are for).

3) Newer microprocessors usually use internal resistor-capacitor timed clocks - again for cost and size efficiency. They work well enough for many functions, but for some more critical operations such as keeping a clock accurate over weeks and months and years requires a specially calibrated oscillator or crystal. User's that need this accuracy can get that by adding their own device, and they don't need to go and design and manufacture or buy a special chip for that one purpose.

4) Connect unused inputs to power or ground - either one works by preventing the input from capturing a charge and "floating" to some other voltage that may introduce random changes on the circuit it works on. Unused outputs are not to be connected. While + or - works for CMOS, older TTL type designs generally it is recommended to tie them to +V so it draws less power.

5) Sometimes these new-fangled chips they make these days are really complex little devices! To get the performance they require, they need specific voltages and a lot of modern designs have built-in voltage converters or regulators or precision sources. These designs are best made with external capacitors that are too large to fit on a silicon chip. For example, that popular RS232 chip runs on a 5 volt supply because that is often what is used in the rest of the logic circuitry, but "real" RS232 needs +12 and -12. Nobody wants to make two more power supplies for this silly little old communication link, so chip makers included it in the chip to boost sales by making it easier and lighter and cheaper to have it on the chip. As a designer, I for sure thought it was a great idea and designed in those chips. I've seen it used on stepper motor controllers and all kinds of other devices like A/D converters etc.

6) Trick question: There is no difference between analog and digital ground! But then again...there is. Often the analog circuitry needs to be very precise in the actual voltage on any given conductor point. External influences can affect these voltages and add "noise" to those signals. In particular, a strong current nearby can set up a magnetic field and induce a voltage change in the analog circuitry. Imagine an audio signal where a change of a millivolt or so can be heard as sound/noise. Also imagine a pot (knob) that is being read by a processor. If the voltage varies too much the processor thinks somebody is fiddling with the knob when it's really just induced noise.
So where does all this noise come from? Often it is traced back to digital circuitry. Digital circuitry tends to switch fast and hard. During those nano-seconds of switching they can produce strong currents. Sooner or later those strong currents need to travel back to the power supply and that is generally through the ground plane. So if the sensitive analog circuitry is place right over the digital ground plane with strong currents going on and off all the time - guess what happens? Those clicks and pops and squeals that are created from the digital switching is picked up and amplified by the sensitive analog circuitry and you hear those clicks and pops and squeals! Having a separate ground plane for analog prevents this from happening. Even if using a single ground plane, a good PCB design is made to make sure that the various ground-return currents (as well as all other currents) are routed separately and distanced from each other. Often there are way more than two grounds!

I work with a circuit that has isolated power supplies for each of eight user interfaces that are spread across a wide area. Each are isolated for safety. As a result, internal to the device there is a ground for each of the eight power supplies, and a ground for the main power and it's digital circuitry. These are all isolated with opto-isolators, which are basically LED lights that signal by switching on and off right by a light sensor. This allows digital signals to travel across space with no metal conductors connecting each of the grounds.

Hoped you learned something!
 
...I meant to add: did you notice a common theme in most of these answers? Most electronic designs these days are made in industry for commercial purposes and for sales and profit. Therefore, usually the strongest influence in a design is to keep the costs down. Nobody wants to spend more than they have to to build an electronic device because it all takes away from profit, and would cause more people to do more work to accomplish the same product. This is strongest in the consumer products industry, whereas for example in the medical electronics field reliability is very high on the list and cost is also important but secondary. Each industry has their own unique needs and collectively they influence the state of the art in electronics. I remember when most electronics seemed to be driven by the military designs, and many design/construction specs called "MilSpecs" are either directly military specifications or taken from there. More recently it seemed that cell phone sales and the desire for small and low-power has been the single strongest influence. Now it seems as though inter-connectivity and communication networking and information sharing will be a big motivator, as seen is things like "IoT".

But in summary...cost is usually the #1 factor in electronic designs, and very often in all other forms of engineering, such as building and bridge designs.
 
Hi,

I have following questions.

  1. Why multiple capacitors of different values are connected between VDD and ground?
  2. What are solder jumpers? What they are useful for?
  3. Why and when external oscillators are used or needed for microcontroller?
  4. What to do with unused pins of CMOS devices or microcontrollers?
  5. Why & when two pins of microcontroller are connected with each other via a (boost) capacitor?
  6. What is the difference between digital and analog grounds?
Hy muashr,

You ask some searching questions.:) In addition to the comprehensive information already posted, here is my two cents worth:

(1) Capacitors have three main functions in electronic circuits. They are used for other functions too: integration and timing for example.
(1.1) To store electrical energy, as in reservoir capacitors. Typically in mains power supplies (50Hz or 60Hz), reservoir capacitors range from 1mF to 100mf (1mF= 1000uF).
(1.1) To couple AC signals while removing the DC component: widely used in audio amplifiers to couple the various gain stages.
(1.3) To decouple to 0V various points in a circuit, mainly supply lines. This is to ensure that the supply lines are a low impedance and do not have signals on them which would upset the circuits. You often see a high value electrolytic capacitor in parallel with a low loss capacitor. The electrolytic capacitor provides low frequency high current decoupling, while the low loss capacitor provides a path to 0V for high frequency currents. You can think of capacitors in this deployment as little local batteries. A typical set up might be a 100uF aluminum electrolytic capacitor in parallel with a 100nF ceramic capacitor. You may ask why can't the electrolytic capacitor decouple all frequencies. The reason is that, because of their construction, their impedance rises rapidly with frequency.

(2) Solder jumpers are normally used to provide a position for an additional component if needed for another configuration. They are alo used like switches to configure a circuit.

(3) Internal oscillators are convenient compact and cheap and are quite adequate for many applications. On the other hand, external oscillators can have a far superior performance: absolute frequency, frequency stability, low jitter, etc. Fundamentally, there are two types of oscillator: timed and crystal controlled. An LM555 timer configured as an oscillator is an example of a timed oscillator. Many chips are half way houses and provide connections for attaching a crystal, but even these cannot ultimately match the performance of an external oscillator.

(4) Unused output pins on devices, inevitably, can be left unconnected, but input pins must always be connected in such a way as to define a stable state for a device, or the device may act strangely. This applies to all circuit functions: both digital and analogue: gates, counters, shift registers, opamps, comparators, linear voltage regulators. For example, a floating CMOS gate input can assume any voltage and will pick up electrostatic and electromagnetic signals in its vicinity. The gate can also oscillate and take excessive current from the supply line. Ultimately it may destroy itself along with the other gates on the same substrate. In general, the inputs of simple logic gates should be connected to 0V. A single inadvertent floating input on a MOS memory chip cost the company I worked for £250K UK and delayed the program three months.:arghh:

(5) There could be many reasons for this. It is best to check the data sheet for the chip in question to find the answer.

(6) Grounds (0V) in real circuits are not the nice solid lines you see on schematics. Instead they comprise thousand a small resistors, capacitors and inductors. This applies to PCB traces and wires.

As a consequence, when a current flows down a conductor it causes signals to be imposed on the conductor. Take an example, if there was 20mili Ohms resistance between the power supply and the circuit and the circuit were consuming 1A, there would be 20mV voltage drop. As the frequency goes up the impedance of the conductor will go up due to the additional effect of the inductance and capacitance.

Analog circuits, by definition, amplify signals as opposed to just switching up and down like digital circuits, so they are sensitive to signals on their 0V line line and, to a lesser extent, on their other supply lines. When they are switching, digital circuits can pass massive current spikes down the OV line, which consequently generate large voltages. Decoupling capacitors, as mentioned above, help this but the OV signals can still trouble analogue circuits. To give you an idea of the problem remember that a modern opamp can have a DC open loop voltage gain of one million (120dB).

For this reason it is best to separate the OV lines for analog and digital circuits. Typically you would have completely separate conductors going back to the power supply 0V. Incidentally this principle also applies to the other supply lines.

On a final note: as I have said many times before on ETO, it is imperative in electronics to understand the difference between a schematic and the actual physical circuit. A piece of wire is not a perfect conductor; the same principle applies to all components: resistors, capacitors, inductors, transistors etc etc.

To illustrate the point here is the schematic of an aluminum electrolytic capacitor:

20140724203949326.png

spec
 
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I wouldn't call it a schematic, but an "equivalent circuit". Sometimes it matters, most of the time it doesn't. You'd be surprised what's important at high frequencies.

3 resistors in series won't equal one of the series combination.
SMT resistors may be mounted on the edge to minimize inductance.
 
...I meant to add: did you notice a common theme in most of these answers? Most electronic designs these days are made in industry for commercial purposes and for sales and profit. Therefore, usually the strongest influence in a design is to keep the costs down. Nobody wants to spend more than they have to to build an electronic device because it all takes away from profit, and would cause more people to do more work to accomplish the same product. This is strongest in the consumer products industry, whereas for example in the medical electronics field reliability is very high on the list and cost is also important but secondary. Each industry has their own unique needs and collectively they influence the state of the art in electronics. I remember when most electronics seemed to be driven by the military designs, and many design/construction specs called "MilSpecs" are either directly military specifications or taken from there. More recently it seemed that cell phone sales and the desire for small and low-power has been the single strongest influence. Now it seems as though inter-connectivity and communication networking and information sharing will be a big motivator, as seen is things like "IoT".

But in summary...cost is usually the #1 factor in electronic designs, and very often in all other forms of engineering, such as building and bridge designs.

While I agree with what you say Rich- keeping costs down is the driver for commercial items- that does not imply shoddy design. There is nothing more expensive than a mass produced item that is difficult to produce because it rolls of the production line non operative, or worst still, fails in the field. In the past there was some horrific commercial items, especially TVs, which were not only badly designed but also used sub standard components. I have never found that this situation arises because of cost cutting- incompetence and stupidity, not necessarily by the designer, were the main cause. You even get lousy equipment where the cost is not relevant, in the military arena for example.

Good quality components are unbelievably cheap now, especially in bulk, and many of the functional blocks in equipment are designed by the chip manufacturers who simply must get it right.

spec
 
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You even get lousy equipment where the cost is not relevant, in the military arena for example.
Hmm, In the military sector cost is very relevant, but I know what you are saying. I worked at a company that produced products for the military. The project is based on bids, and the contract goes to the lowest bidder that can meet the schedule and has the qualifications to produce what they are commissioned to do.
Due to the fact that the project is a bid, cost becomes a big issue as to not exceed the budget in order to earn a profit. I hate to admit this, but when schedules slipped, I often seen corners cut to stay within the budget. Engineers were expected to work ungodly long hours with no added compensation, and at the completion of the project many hires are let go. Personally I would never work for a government contractor again. Well, just my two cents.
 
Hmm, In the military sector cost is very relevant, but I know what you are saying. I worked at a company that produced products for the military. The project is based on bids, and the contract goes to the lowest bidder that can meet the schedule and has the qualifications to produce what they are commissioned to do.
Due to the fact that the project is a bid, cost becomes a big issue as to not exceed the budget in order to earn a profit. I hate to admit this, but when schedules slipped, I often seen corners cut to stay within the budget. Engineers were expected to work ungodly long hours with no added compensation, and at the completion of the project many hires are let go. Personally I would never work for a government contractor again. Well, just my two cents.

Yes Mike,

Cost is crucial in winning Military contracts, but the cost of components and, to a less degree hardware design time, is not normally huge compared to the other activities: specifications, meetings, PERT charts, design reviews, safety, trials, environmental, handbooks, training... And then you get the software costs, which on most of the big projects represented about 70% of the costs. Sometimes an opamp that a commercial firm may pay ten cents for would be twenty dollars for a Mil Spec version.

I worked 12 hours a day 7 days a week for around five years at one time- we didn't get paid overtime. I even had a female manager from another project tell me that I didn't work late much.

I am afraid that, in general, engineers are just tools. When you need them you take them out of the cupboard and when they have finished you put them back. One of the managers more or less said that.:eek:

spec
 
6. What is the difference between digital and analog grounds?

There is not big difference between them, as we know digital component are sensitive about noises can make problems in the system. Digital circuits are stouter to noise, that have only two value 0 and 1. Analog circuits working on specific working points dependent on requirements so working point is easy to drift due to noise.
 
"...the contract goes to the lowest bidder that can meet the schedule and has the qualifications to produce..."
Yes cost is an important aspect of a military product. But the above quote gives away the true priority of military design: "that can meet the schedule...". Not indicated but obviously too obvious to mention is that the product generally must do what it was intended to do, so cost would not be #1. Adequate function, schedule, and contractor reputation are likely ahead of the final cost, making that #4 at best.

For mass-quantity consumer goods, which are arguably the least pre-purchase critically examined product, a non-competitive price will leave the maker with a warehouse full of landfill. Knock the price down a bunch and you will get plenty of people to take a gamble, and many of those will just eat the cost of a poor product if it was cheap enough. A non-working $1000 television or cell phone will be trouble for the manufacturer, but how many people do you think have bothered to return a $5 string of busted Christmas lights? Of the few that may be returned to the retailer, how many of those do you think were shipped back to China for a refund or repair?

For consumer use, final cost is #1 in determining sales success or failure at the low price end of the spectrum, while the more expensive devices tend to sell more on features and brand reputation. For example, for every high-end Apple cell phone sold, there were apx, 7 phones sold by Samsung, Motorola, Huawei, Lenovo, LG,...etc. So while Apple may drive the features, most of the dollars goes to the other makers who offer cheaper models, and that is who the electronic component makers are listening to.
 
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