Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Analog design: Some things to remember

Status
Not open for further replies.

Roff

Well-Known Member
Here's a post I copied from another forum:
**************************************

Here is a list of things (rules) to-know that I found helpful over the years.

1/ Capacitors and resistors have parasitic inductance, about 0.4nH for surface mount and 4nH for a leaded component.
2/ If you don't want a high bandwidth transistor to oscillate place lossy components in at least 2 of the 3 leads. Ferrite beads work well.
3/ When taking DC measurements in a circuit and they don't make sense, suspect that something is oscillating.
4/ Opamps will often oscillate when driving capacitive loads.
5/ The base-emitter voltage Vbe of a small signal transistor is about 0.65v and drops about 2mV/deg C. Vbe goes down with increasing temp.
6/ Multiply 0.13nV by the square root of the ohmic value of a resistor to find the noise in a 1Hz bandwidth. Then multiply by the square root of the BW in Hz gives the total noise voltage.
7/ Johnson noise current goes down with a increase in resistance.
8/ The impedance looking into the emitter of a transistor at room temp is 26Ohm/Ie in mA
9/ All amplifiers are differential in that they are referenced to ground somewhere.
10/ Typical metal film resistor has a temp coef of about 100 ppm/deg C
11/ The input noise voltage of a quiet op amp is 1nv/sqrt(Hz) but there are plenty available with 20nV/sqrt(Hz). Op amps with bipolar front-ends have lower voltage noise and higher current noise than those with FET front-ends
12/ Using an LC circuit as a power supply filter can actually multiply the power supply noise at the filter's resonant frequency. Use inductor with low Q to overcome this.
13/ Use comparators for comparing and op amps for amplifying and don't even think of mixing the two.
14/ Ceramic caps with any other dielectric other than NPO should only be used for bypass applications.
15/ An N-channel enhancement-mode FET needs +ve voltage on the gate-source to conduct form drain-source.
16/ Small signal JFETS work very well as low-leakage diodes by connecting drain & source together in log current-to-voltage converters and low leakage input protection. Small signal bipolars with b-c tied together will also make nice low-leakage diodes.
17/ With low pass filter use Bessel for least amount of overshoot in the time domain, and Cauer (or elliptic) for fastest rolloff in the freq domain.
18/ dB is always 10 times the log of the ratio of 2 powers.
19/ At low frequencies, the current in the collector of a transistor is in phase with the applied current at the base. At high frequencies the current at the collector lags by 90deg. You must appreciate this simple fact to understand high frequency oscillators.
20/ The most common glass-epoxy PCB material (FR4) has a dielectric constant of about 4.3 To make a trace with a characteristic impedance of 100 Ohm, use a trace thickness of about 0.4 times the thickness of the board with a ground plane on the opposite side. For a 50Ohm trace make it 2 times the thickness.
21/ If you need a programmable dynamic current source, find out about operational transconductance amps. Most of the problem is figuring out when you need a programmable dynamic current source.
22/ A CMOS output with an emitter follower can drive a 5V relay nicely as the relays normally have a must-make spec of 3.5V. This saves power and require no flyback components.
23/ Typical thermocouple potential is 30uV/degC. Route signals differentially, along the same path, avoid temp gradients. DPDT latching relays won't heat up when multiplexing these signals.
24/ You SHOULD be bothered by a design that looks messy, cluttered or indirect. This uncomfortable feeling is one of the few indications that there's a better way.
25/ Avoid drawing any current from the wiper of a potentiometer. The resistance of the wiper contact will cause problems (local heating, noise offsets etc.)
26/ Most digital phase detectors have a deadband where the analog output does not change over the small range where the 2 inputs are coincident. This often-ignored fact has helped to create some very noisy PLL's (Use a high val bleeding resistor to always ensure current flow in the deadband)
27/ The phase noise of a phase-locked VCO will be at least 6dB worse than the phase noise of the divided reference for each octave between the comparison frequency and the VCO output frequency. Avoid low-comparison frequencies.
28/ You can almost always determine the leads of a bipolar transistor with an ohm meter. b-e and b-c junctions will measure like a diode with the b-c junction reading slightly lower than the b-e junction when forward biased.
29/ For low distortion, the drains (or collectors) of a differential amp's front-end should be bootstrapped to the source (or emitter) so that the voltages on the part are not modulated by the input signal.
30/ If your design uses a $3 op amp, and you will be making a thousand of them, you have just spend $3000. Are you smart enough to figure out how to use a $.30 op amp instead?
31/ The Q of an LC tank circuit is dominated by the losses in the inductor in terms of series R. Q=omega.L/R
32/ Leakage current doubles for every 10degC increase in temp.
33/ When inputs to most JFET op amps exceed the common-mode range for the part, the output may reverse polarity. This artifact will haunt the designers of these parts for the rest of their lives, as it should!
34/ Understand the difference between "make-before-break" and "break-before-make" when you specify switches.
35/ 3 Terminal voltage regulators in the TO-220 packages are wonderful parts. They are cheap, rugged, thermally protected and very versatile. Use them virtually any place where you need a protected power transistor. They also make nice AM power-modulators.
36/ Use step recovery diode where you need fast edges under 100pS (hot-carrier is even faster)
37/ The old 723 regulator is still one of the lowest noise regulators around! (2.5uVrms 100Hz-10k)
38/ You can make a very simple oscillator with one diac, cap and a resistor.
39/ NPN transistors are normally superior to their PNP counterpart in performance.
40/ Typical spec in some databooks should read "Seen it once". Always work with the worst spec of the part when doing a design.
41/ Don't just copy circuits from application notes without understanding completely how it operates, and the reason for the choice of values.
42/ Dealing with crystals, make sure you understand the difference between series and parallel resonant. In a circuit, crystal frequency can generally be slightly lowered by placing a inductor in series and increased by a capacitor in series.
43/ Power MOSFETS on-resistance will have a -ve temp coef and not +ve at low current levels. This is important to remember when paralleling devices.
44/ Lowest noise figure of a RF transistor is not normally where the input is perfectly matched.
45/ Many un-stable RF devices can be made stable by loading the input or the output by a simple resistor, either in series or parallel.
46/ You trade gain for bandwidth.
47/ Push-pull power invertors using bipolars are risky and can saturate the core because of hysteresis stepping (use power fets)
48/ The Al value of a core will increase up to 50% or more under current transients.
49/ Be aware of leakage inductance when switching. V=L(dI/dt)
50/ The harder you turn-on a power transistor, the longer it will take to turn off.( the part where you burn the joules in the device)
51/ Always remember the Miller guy.
52/ In fault-finding a circuit, don't overlook the obvious. (is there power?)
53/ What is a ground loop, and how to avoid it.
54/ 120 is a better number than 240 when using LM3XX type adjustable regulators.
55/ The lower comparator in the old 555 may have quite a long storage time.
56/ ZERO-ESR caps may do more harm than good.
57/ A correctly configured audio power amplifier will give more distortion in Class-AB, not less, because of the abrupt gain changes inherent in switching from A to B every cycle.
58/ Be a STAR when it comes to ground matters.
59/ Know when you need to use a Zobel network.
60/ Use current mirrors and mirror your current.
61/ Heatsink eff decreases with height above sealevel.
62/ A matt-black heatsink is much better than a shiny one.
63/ Ignoring secondary breakdown can be costly.
64/ Understand fuses and fuse ratings, fast and slow. Do you know when to use a semiconductor-fuse?
65/ Charge balancing resistors are a must when stacking serie-parallel high voltage capacitor banks.
66/ You must understand DC-restoration otherwise you will have a hard time designing Z-modulation in CRT circuits.
67/ Display 6 vert div low freq on a scope, increase the freq (make sure the source is constant amplitude) until display drops to 4.2 div. That is the true 3dB BW of the scope. (scope-source impedance should be matched)
68/ Doing a measurement with your DMM in the ACV position on your DC circuit will give a quick indication of any excess ripple on the supply when you don't have a scope at hand.
69/ Dly timebase on a scope is very useful once you figured out when, why and how to use it.
70/ Know what to expect before you measure, otherwise any measurement is meaningless.
71/ Op amps. Output will swing in the direction that will force the inv-input level to try come closer to the non-inv input level.
72/ Understand virtual ground, slew-rate, CMRR and PSRR. (CMRR decrease with increase in freq)
73/ Making measurements near a spec-analalyzer's noise floor will give 3dB errors.
74/ Understand the phase-noise limitations of the analyzer when making such measurements on oscillators.
75/ In a LC oscillator add some C with -ve temp coef to cancel the +ve temp coef of the L for min drift with temp.
76/ Less drift will result from making C with a few parallel caps, to reduce the heating effect of the oscillating current when spread out over a larger plate area.
77/ You will get more tuning range with the same LC combination in a Clapp than in a Colpitts circuit.
78/ High-Q tuned LC filters will have more insertion loss.
79/ Williams's Rule (Guru at Linear Tech) for precision op amp circuits: " Always invert (except when you can't)"
80/ Cuk is not a kind of locomotive.
81/ If you don't know how to make a design better, find out what makes it worse.
82/ Sometimes you know just enough to be dangerous.
83/ Impedance will reflect back as the square of the turns ratio.
84/ If you could design a component with the characteristics of a finger it could cure many design problems and you will be rich.
85/ Get nervous when the customer you are trying to help doesn't even have a scope.
86/ Specs quoted by reps always exceed those by Engineering.
87/ A bad (Engineer) workman always blames his tools.
88/ Don't believe everything that a SPICE program spits out.
89/ It is easy to get the color code of a 1kOhm and 12Ohm resistor mixed up when you are in a hurry.
90/ I bet one could write a thesis about the ability of probes to get tangled-up on a bench.
91/ DMM can upset sensitive circuits from noise generated inside it.
92/ When probing directly on a crystal of a uP, use 10kOhm or so resistor in series with the probe tip to prevent loading from stopping the osc.
93/ It is easier to see what is happening on the ports using a scope when you trigger one chan against the cpu clock.
94/ National once made a bad op amp many years ago that some Engineers referred to it as "Jelly Beans"
95/ The moment you can start to notice distortion on an oscilloscope it is already way past being acceptable.
96/ Be big enough to say "I don't know", people will respect you more.
97/ The best designer is often working in the marketing department.
98/ Some remarkable discoveries/inventions were made by people that knew very little about the subject. Don't fall into a groove in you thinking process.
99/ The Peter-Principle : Everybody will be promoted up to his own level of incompetence. h**p://pespmc1.vub.ac.be/PETERPR.html
100/ END-Enjoy


List (37%) as from a very clever guy named "John Willison" founder of Stanford Research Systems and some bits added by myself from experience and other.
*****************************
Note: I added nothing to this list.

Ron
 
Soo very true.
My boss at work has been told he needs to get a PhD if he is to go any further in the company, we keep pushing him to do it on cables getting twisted.

its uncanny, even evenly layed out cables undisterbed will magically get themselfs tangled
 
Thanks, Ron
I am dying to know which one:
94/ National once made a bad op amp many years ago that some Engineers referred to it as "Jelly Beans"
I vote for the lousy LM324. It doesn't do anything well.
 
audioguru said:
Thanks, Ron
I am dying to know which one:
94/ National once made a bad op amp many years ago that some Engineers referred to it as "Jelly Beans"
I vote for the lousy LM324. It doesn't do anything well.
Yeah, I don't know. All I could find with a Google search is that in the late 1980s National "built an array of high-volume, low-margin, commodity chips -- 'jelly beans' ".

I realized I should have added one entry:
101/ You can't switch capacitors in a resonant circuit with the outputs of a CD4024 - or any other logic output, for that matter. :roll:
 
Ron H said:
I realized I should have added one entry:
101/ You can't switch capacitors in a resonant circuit with the outputs of a CD4024 - or any other logic output, for that matter.
But somebody says he did it already.
I think the positive supply wire for the CD4024 was a very long 1/2 wavelength, and the ground was 1/4 wavelength. So switching the logic state of a tuning capacitor caused it to resonate at different frequencies with the inductance of the long supply wires.
Breadboards cause weird effects at high RF frequencies, don't they. :lol:

Maybe National's LM833 dual "audio" opamp was their jelly-bean. I tried some but nobody else did so they weren't produced in high volume. They have slightly less noise than a TL072 but no big deal.
 
audioguru said:
Ron H said:
I realized I should have added one entry:
101/ You can't switch capacitors in a resonant circuit with the outputs of a CD4024 - or any other logic output, for that matter.
But somebody says he did it already.
I think the positive supply wire for the CD4024 was a very long 1/2 wavelength, and the ground was 1/4 wavelength. So switching the logic state of a tuning capacitor caused it to resonate at different frequencies with the inductance of the long supply wires.
Breadboards cause weird effects at high RF frequencies, don't they. :lol:
Yep. But it doesn't explain away the sizeable resistance of the output transistors, which also are in series with the caps.
Maybe he'll some day be famous for discovering FM tunneling. And we can say, "I used to poo-poo that guy when he was making his very early discoveries. Boy, were we ignorant".
 
Ron H said:
But it doesn't explain away the sizeable resistance of the output transistors
Maybe he is using a Texas Instruments Cmos counter and is operating it at its absolute max supply voltage of 22V. Then those little output Mosfets are turned on so hard they don't have much resistance.
 
Great info. I'd be curious to see what the older, more knowledgeable members here think of it.

In case you didn't know, Ron matches that description exactly. You won't find a member who's older. That pic he sticks on his header has nothing to do with Mars --- it's a copy of what's on his wife's desk at work.

Oh, yeah. Or more knowledgeable, for that matter.


Ron, although I didn't comb out the entire list, I did notice one error. Those ceramic caps (Item #14) should be NP0, not NPO. I believe the abbreviation stands for Negative/Positive Zero, not Negative/Positive Oh, my!

Sometimes I'm just so trivial.


Dean
 
Dean Huster said:
I did notice one error. Those ceramic caps (Item #14) should be NP0, not NPO. I believe the abbreviation stands for Negative/Positive Zero, not Negative/Positive Oh, my!
Hi eagle-eyes Dean. My phone number has three zero's in it. When I tell it to a voice-recognition attendant, it answers me saying, "Huh?".
Sometimes I say zero's but most of the time they are oh's.

I'm pretty old too. My age is a six followed by an oh. :lol:
 
audioguru said:
I vote for the lousy LM324. It doesn't do anything well.
.
Well I have to disagree in part on the LM324. I have used them and its little brother LM358 in a lot of gel-cel battery chargers as control for a series pass transistor and as a comparator for charge current and they worked well.
Course I have been known to do things that were odd, unusuall, against general accepted practices. Like using silicon diodes for reference sources, putting multiple LED's in parallel with only one current limit resistor for all the the LED's, using LT1054's below minimum voltage, just to name a few.
I may be a little older than Ron H, but I know for sure I am not smarter. :lol:
 
k7elp60 said:
I may be a little older than Ron H, but I know for sure I am not smarter. :lol:
I'm not smart. When you're as old as I am (64), you just have a lot of stuff on your hard drive.
Hmmm, that doesn't sound so good. :oops:
 
It's time for an agressive de-frag, Ron. You should also dump the junk thoughts in your temp files and Norton "protected" file or recycle bin. :lol:
 
audioguru said:
It's time for an agressive de-frag, Ron. You should also dump the junk thoughts in your temp files and Norton "protected" file or recycle bin. :lol:
I'm waiting for Alzheimer's. It automatically erases anything more than 15 minutes old.
.
.
.
.
.
.
.
.
.
.
.

I'm waiting for Alzheimer's. It automatically erases anything more than 15 minutes old.
 
Your're not going to get Alzheimer's. I'm not going to get Alzheimer's.
We are exercising our minds chatting about this complicated stuff.
I think poor old folks who sit around doing nothing and thinking about nothing get Alzheimer's.
 
audioguru said:
Your're not going to get Alzheimer's. I'm not going to get Alzheimer's.
We are exercising our minds chatting about this complicated stuff.
I think poor old folks who sit around doing nothing and thinking about nothing get Alzheimer's.
I think you're right. I HOPE and PRAY you're right. :)
 
audioguru said:
Your're not going to get Alzheimer's. I'm not going to get Alzheimer's.
We are exercising our minds chatting about this complicated stuff.
I think poor old folks who sit around doing nothing and thinking about nothing get Alzheimer's.

That's right, you go crazy instead. Mumbling to yourself under your breath about opamps everywhere you go, talking about the 1960's as if it were last week... hee-hee... well, its really not funny. The best thing you can do is to impart your wisdom on the young so they can take it further one day.
 
Optikon said:
That's right, you go crazy instead. Mumbling to yourself under your breath about opamps everywhere you go, talking about the 1960's as if it were last week... hee-hee... well, its really not funny.
My wifey sometimes thinks I'm crazy but other times thinks I'm funny. It's great fun being an electronics geek. Never mind teaching the noobies, I'm sefish and are learning more about this stuff every day! :lol:
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top