The Signal

Knowing Where to Tap

2013-06-25T11:48:00Z

Jake, a respected veteran engineer in a power generation plant, retired with great congratulations and accolades. A few months later the plant suffered a major malfunction—a real meltdown with all systems involved. The engineering staff could not quickly diagnose the problem so with due urgency they called Jake for an emergency consultation.

He surveyed the situation, checking the condition of some indicator lights and made a few measurements. He resolutely marched to a bank of gray electrical boxes, opened one and tapped on a relay. Instantly indicator lights signaled a status change and systems sprang back to life.

Jake submitted a bill for this very brief consultation—$500, a seemingly modest amount for the gravity of the situation. But a bean-counter in the office was not so impressed, and questioned the $500 charge for such short work. He asked for an itemized bill. The old-timer resubmitted a hand-written statement that read as follows:

My apologies to whomever I stole this story. I honestly can’t remember. It’s amusing and confirms the status and mystique of the guru with special knowledge and experience. We’d all like to imagine we could be Jake.

But what if he had done a better job of sharing his knowledge during his employment years? What if he had more thoroughly mentored and trained his junior colleagues? They would have known where to tap.

When I joined Burr-Brown 34 years ago after seven years of previous engineering experience, I was drawn to this company where knowledge sharing was so deeply ingrained in the culture. Experts generously shared their time with junior engineers. Everyone helped one another to advance the analog art. The chemistry of design reviews was scintillating. Multiple gurus would challenge and improve on one another’s ideas. It was hard-core analog but always with the best intent and good humor.

A culture of knowledge-sharing and collaboration requires maintenance and tuning. People come and go. It requires real intent to sustain this culture. I hope that your company has it. If so, nurture it. If it’s waning, rebuild it. If it’s missing, start it.

With that, I say goodbye. I have plans for more grandfathering, bicycle riding and, to be honest, cleaning my garage. It’s been a privilege to have this forum over the past 15 months and 65 blogs. It was challenging. I found myself learning more about topics I thought I knew pretty well. It reminds me of what an old mentor told me—if you want to really learn something, teach it! (Thanks, Jerry.)

The Signal blogs will stay on this site forever (whatever forever may be in the web age). Watch for a new blog, the TI Precision Designs Hub, where you will hear from multiple experts in the area of analog and data conversion.

Thanks for reading and goodbye,

Bruce email: thesignal@list.ti.com

All The Signal blogs are listed here grouped generally by topic.

Thanks Kristina, Aimee. And special thanks to Grace Bauske, RIP.

Pop Quiz!

2013-06-17T20:16:00Z

Put away your books and take out a sheet of paper. Each question relates to one of The Signal blogs over the past 15 months. If you have difficulty answering, click on the link to bootstrap knowledge on the topic. Answers are at the end so you can score yourself. Have fun!

1. A gain of -0.1 (inverting) amplifier…

a) is very likely to oscillate.
b) requires an op amp with special stability criteria.
c) will likely be stable with a unity-gain-stable amplifier.
d) requires a special attenuator at the input to assure stable operation.
The Inverting Attenuator, G = -0.1

2. Using an op amp as a comparator…

a) is okay if you don’t connect for hysteresis.
b) can achieve faster response and reduce power.
c) is necessary if you need push-pull “totem pole” type output drive.
d) may require care to avoid turning on differential input clamps.
Op Amps used as Comparators—is it okay?

3. Power supply bypass capacitor(s)...

a) are not generally required in SPICE simulations
b) are important to get accurate SPICE simulations
c) values can be optimized using SPICE simulations.
d) circuit layout can be determined with SPICE simulations.
SPICE Simulations and Power Supply Bypassing

4. For two equal resistors connected in parallel, the resulting thermal (Johnson) spot noise…

a) is reduced by half.
b) increases by a factor of 1.414.
c) decreases by a factor of 0.707.
d) decreases by a factor of 0.25.
Resistor Noise—reviewing basics, plus a Fun Quiz

5. Dual op amps will…

a) likely have well-matched offset voltage.
b) likely have well-matched offset voltage drift.
c) a and b.
d) may save space and cost.
Matchy Matchy—how alike are dual op amps?

6. For unused op amps in a dual or quad package, it’s best to…

a) connect in G=1 with input connected within its C-M range.
b) leave all pins unconnected.
c) connect both inputs to ground and leave output unconnected.
d) ! This is an unwise practice. You should use a single op amp instead.
The Unused Op Amp—what to do?

7. Photodiodes…

a) have a more linear response when forward biased.
b) generate a linear output voltage with incident light power.
c) provide a light dependent current with zero applied voltage.
d) behave as a light-dependent resistor in photoconductive mode.
Illuminating Photodiodes ;-)

8. The input impedance of a practical transimpedance amplifier (TIA)…

a) is near zero.
b) is virtually infinite.
c) appears capacitive.
d) appears inductive.
TIA Input Z: Infinite… or Zero? What is it, really?

9. Regarding comparators and hysteresis, …

a) high performance types have low hysteresis.
b) it’s possible to add hysteresis to control “chatter” at transitions.
c) hysteresis creates delay in the transition and should be avoided in high speed circuits.
d) hysteresis occurs in comparators driving inductors that use certain core materials.
Comparators—some practical stuff

10. Potentiometers…

a) should not be used as variable resistors.
b) should be used ratiometrically, when possible.
c) cannot be replaced with electronic components.
d) perform best with a logarithmic taper.
When Potentiometers go to Pot

11. When multiple precision signal processing stages are required…

a) it’s generally best to put substantial gain in the first stage.
b) lower gain in the first stage improves temperature stability and reduces its offset.
c) the last stage is generally the most critical and needs greatest attention.
d) gain should be equally distributed in all stages.
Where to Put Your Gain—waxing philosophical

12. The ESD tolerance of an IC component provided in a data sheet…

a) applies to handling and assembly prior to circuit operation.
b) applies to “typical” circuit operation conditions.
c) is tested on every production IC.
d) is sample tested in production of most ICs.
ESD… Zapp!

13. Equalizing the effective resistance at the inputs of an op amp…

a) is standard practice and conforms to specified operating conditions.
b) is often unnecessary.
c) reduces offset voltage created by input offset current.
d) improves op amp stability.
Input Bias Current Cancelation Resistors

14. The output voltage of a thermocouple…

a) is approximately proportional to temperature Kelvin.
b) is generated at the junction of two dissimilar conductors.
c) a) and b)
d) is approximately proportional to the temperature difference of the two junctions.
Thermocouples—stuff that every analog designer should know

15. The noise (flat-band) of an amplifier is 5uV measured from 20kHz to 100kHz bandwidth. Its spectral noise density is…

a) 17.7 nV/rt-Hz.
b) -123 dBV.
c) 35 nV/rt-Hz.
d) 63 pV/Hz.
Resistor Noise—reviewing basics, plus a Fun Quiz

16. Flicker (or 1/f) noise…

a) ends at the corner frequency.
b) rolls off at approximately 20dB/decade of frequency.
c) rolls off at approximately 10dB/decade of frequency.
d) has equal energy in each 1Hz of bandwidth.
1/f, Flicker Noise—the flickering candle

17. Decompensated op amps…

a) provide wider gain-bandwidth than similar unity-gain-stable op amps.
b) are generally more stable in G=1.
c) have greater offset voltage drift over temperature.
d) have higher quiescent current.
Decompensated Op Amps

18. Chopper op amps…

a) are generally less stable than standard continuous-time op amps.
b) have nearly flat (constant) spectral density throughout their useful range.
c) should be used when very low input bias current is required.
d) should be used on big, noisy motorcycles.
Chopper Op Amps—are they really noisy?

19. The stripe on one end of a tubular polyester capacitor…

a) should be connected to ground.
b) should be connected to the more negative potential node.
c) should be connected to the lower impedance node.
d) indicates capacitance tolerance.
PCB Layout Tricks—striped capacitors and more

20. For a low noise op amp stage with low source impedance…

a) the feedback resistor should have a low resistance value.
b) the op amp’s voltage noise is likely to be critical.
c) the op amp’s current noise is likely to be critical.
d) the input impedance of the op amp circuit should be matched to the source impedance.
Op Amp Noise—the non-inverting amplifier and Op Amp Noise—what about the feedback?

Score yourself…

18-20 Bob Pease¹ would be so pleased! (see note below)
16-17 Analog-savvy!
14-15 Analog-capable.
12-13 Possibly a digital designer.
0-11 Are you a software wonk? :-)

This was just a sampling of 60+ scintillating The Signal topics over the past 15 months. I hope this was fun and comments are welcome, as always.

Bruce email: thesignal@list.ti.com

Note: Launch of this blog coincides with the second anniversary of the death of Bob Pease. He would be so pleased with your interest in analog, no matter what your score! RIP RAP

Settling Time

2013-06-11T20:06:00Z

Settling time is the time required for an op amp to respond to an input voltage step, enter and stay within specified error range of the final value. It’s important in applications that drive an a/d converter, digitizing rapidly changing inputs. But let’s look beyond the definition and focus on the character of settling waveforms.

Last week’s blog on slew rate showed how an op amp transitions from a slewing ramp to a small-signal settling portion of the waveform, figure 1. As the gain is increased, you can see the slower closure to final value. This is due to reduced closed loop bandwidth in higher gain.

This example op amp is tuned to have virtually 90° phase margin in G=1. Notice that there is no overshoot, even in unity gain. Its virtually perfect first-order response serves as a benchmark for comparison but you are unlikely to find an op amp with such generous phase margin in G=1.

The response in figure 2 is more realistic (maybe a bit pessimistic). These waveforms are produced by the same op amp but with approximately 35° phase margin at G=1. (The ideal op amp responses are also shown for comparison.) Its small signal overshoot is approximately 32% in G=1. It appears to be this less overshoot with the 1V step shown because only the small-signal portion of the response produces this overshoot behavior. A larger input step would have the same magnitude overshoot but look proportionally even smaller. This is why you should always check overshoot and stability with small input voltage steps.

Figure 3 shows an expanded view of the G=1 small-signal response. Note that the settling of the final humps to a final steady value appear to require two complete up/down cycles. The wiggles continue, smaller and smaller—beyond the resolution of this graph. An additional cycle or two might be required to settle to high accuracy.

When we visualize this final settling behavior, we often tend to imagine a compressed time scale in the final over/undershoots, as if the natural frequency of this ringing is shifting upward with each hump. But every cycle of settling requires the same time. Excessive ringing can be costly—a good reason to select a reasonably well-behaved op amp.

The true settling time to high accuracy, 16-bits or greater often includes other factors. Behaviors produced by fancier phase compensation techniques and thermal effects can add to the settling time. The amplifier can also be perturbed by glitches from input switching of an a/d converter. Optimizing all this can be tricky business—a good future topic. Still, it’s important to visualize the primary effects at work—slew rate combined with a second-order system response.

Thanks for reading and comments are welcome below.

Bruce email: thesignal@list.ti.com

60+ other interesting The Signal topics.

Slew Rate—the op amp speed limit

2013-06-03T23:03:00Z

Slewing behavior of op amps is often misunderstood. It’s a meaty topic so let’s sort it out.

Click Here to read on EDN Magazine web site.

60+ interesting The Signal topics.

Grounding Principles

2013-05-21T18:26:00Z

In a previous blog on supply bypassing, I cautioned that poor bypassing could increase distortion of an amplifier. A reader, Walter, asked an interesting question… where should you connect the ground of a bypass capacitor to avoid problems?

This raises questions regarding proper grounding techniques. Wow. Big topic, but I may be able provide some insight with a couple of simple examples.

Figure 1 shows inverting and non-inverting amplifier stages with unintended, parasitic resistance or inductance in the ground connections (highlighted in red). The nodes A, B and C are all intended to be ground. But if current flows in parasitic ground impedances, these nodes will not be at the same potential. It is these parasitic ground impedances that can allow distorted ground currents to contaminate signals.

Walter’s specific question was, “where should you connect the bypass capacitor [the ground side].” It’s an important point. The currents flowing in op amp supply terminals (and therefore the bypass capacitors) may be distorted because they represent only half a sine wave. If distorted (or other interfering) current flows into a vulnerable ground node it can increase the distortion (or other errors) of the amplifier.

An interfering or distorted current flowing into node_A directly affects the ground reference of the input signal, summing in an error. Likewise, a ground current injected into node_B serves as a direct input to the amplifier stage (inverted, in the first circuit). Ground current flowing into node_C directly sums an error with the output voltage. This node may be less vulnerable because the error signal is not amplified by the circuit gain.

The bypass capacitor should be connected to node_G. Though there may be additional parasitic impedance on its way to other ground points, variation in voltage at node_G affects the critical nodes equally, so it does not inject an error or distortion. I’ve shown an op amp with a single power supply. The ground connection of the op amp (shown on top of the op amp) should also connect to Node_G. A dual (±) supply op amp circuit would have another bypass capacitor for the negative supply and it, too, should connect to node_G.

A solution is to create a circuit board that establishes a ground with the characteristics of node_G. The principle is simple—the circuit trace from the input ground terminal to the ground side of R1 should be a clear path with no connections to contaminating sources of current along the way (figure 2). This input ground trace can join a larger ground connection or ground plane where they meet. With some gain in this stage, output errors are less critical, but you still may want a separate trace to the output terminal connections.

The input ground connection should not connect to equipment chassis at an input connector. This would create an opportunity for other interfering ground noise (such as AC mains ground currents) from impressing current on the clean input ground trace.

A single blog cannot begin to cover all the issues relating to the art of grounding. Woops… did I call this an “art?” It’s science, not art! While, at times, may seem like black magic, Ohm’s law is always at work. Considering where ground currents flow and how they could affect the circuit is always a good start!

Thanks for reading. Comments welcome below,

Bruce

60 other interesting The Signal topics.

Handy Gadgets and Resistor Divider Calculations

2013-05-14T00:34:00Z

Handy gadgets make our engineering life easier—the little special purpose computer programs or spreadsheets that you might find or create yourself.

Back in the old days, engineers used nomographs. These are graphical aids that solve common multivariable problems of all sorts. Calculators and desktop computing caused their decline so you seldom see them today. I still use a variant of one—an old cardboard R-L-C reactance slide rule given to me in my first electric circuits class back in the ‘60s. It helps me find approximate values in the right impedance range when I’m positioning poles and zeros. I think better with it in my hands.

I believe that the graphical nature of a nomograph can aid in visualization and optimization. Has something has been lost when we just plug numbers into a computer?

In previous blogs, I’ve provided gadgets for calculating op amp noise and 1/f noise. Here is another, an Excel sheet that calculates resistor values for a three-resistor divider with a voltage reference to offset the output voltage. For example, if you have a -10V to +10V input and you want attenuate and shift it to a 0 to 3V output, this gadget calculates the resistor values.

e2e.ti.com/.../0083.Voltage-Divider-with-offset-v1.xlsx

It’s a sub-circuit that is often needed in signal processing. The math is a bit messy, so if you solve it once you probably don’t want to do it again. It’s the type of task that is worth the time to create a gadget. The equations are in figure 2, if you don’t want to use the worksheet. I refined it a bit, adding some checking for out-of bound values and minimum required value for the reference voltage. Try it and see. With the annotations I think you’ll find it easy to use.

Excel (or equivalent) is pretty handy for calculations like this but I find it awkward for some types of programs. I have some gadget programs that parse long files to manipulate data. I’ve used various forms of BASIC for this through the years but now I use Excel’s Visual Basic (macros), loading data into the associated worksheet to use its graphing capabilities. I wouldn’t publish these gadgets. Excel macros are so easily written or modified to create serious damage that they’re scary. I only give them to close associates and I’m not even sure they trust me. :-)

What handy gadget design aids have you made? What ones do you wish you had? Do you use any old-style nomographs or slide-rules like mine?

Thanks for reading and your comments are welcome below,

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Check out 60 other interesting topics.

Chopper Op Amps—are they really noisy?

2013-05-06T12:56:00Z

Chopper op amps offer very low offset voltage and dramatically reduce low frequency 1/f (flicker) noise. How do they do it? Here’s a quick-read on the tricks.

Click Here to read on EDN Magazine site.

Bypass Capacitors… yes, but why?

2013-04-23T18:13:00Z

Everyone knows that op amps should have power supply bypass capacitors located near the IC’s terminals, right? But why? Why, for example, is an amplifier more apt to oscillate without proper bypassing? The reasons will increase your understanding and awareness.

Power supply rejection is an amplifier’s ability to reject variations in the power supply voltage. Figure 1, an example, shows that this rejection capability is very good at low frequency but diminishes as frequency increases. Hummm... poorer rejection at high frequency where oscillations occur?

We often think of external power supply-borne noise interfering with an amplifier. But op amps can create their own problems. For example, output load current must come from the power supply terminals. Without proper bypassing the impedance at a supply terminal can be high. This allows AC load current to produce an AC voltage on the supply pin. This creates an unintended, uncontrolled feedback path. Inductance in this power supply connection can magnify the resulting AC voltage at the supply pin. At high frequency, where power supply rejection is poor, this unintended feedback can cause oscillation.

There are internal forces at work, too. Without a solid power supply, internal circuit nodes may talk to one another creating unwanted feedback paths. Internal circuitry is designed to operate with firm, low impedance on the power supply terminals. An amplifier may behave quite differently and unpredictably without the solid base of low impedance supplies.

With a clean sine wave input, the unintended feedback due to poor bypassing may not be a tidy sine wave. The signal currents in the supply terminals, figure 2, are often highly distorted because they only represent one half of sine wave current. With different power supply rejection characteristics on the positive and negative supplies, the net effect will distort the output waveform.

The issues are magnified with high load current. Reactive loads create phase-shifted load currents that may exacerbate issues. Capacitive loads are already at higher risk of oscillations due to additional phase shift in the feedback path (more detail here). These higher risk cases may need higher value tantalum bypass capacitors and extra care in circuit layout, compact and direct.

Of course, not all poorly bypassed amplifiers oscillate. There may not be sufficient positive feedback, or the phase not quite right (or wrong!) to sustain an oscillation. Nevertheless, performance may be compromised. Frequency and pulse response may be affected with excessive overshoot and poor settling time.

As discussed in a previous blog, these behaviors are not well-modeled in TINA-TI or other SPICE programs. Voltage sources in SPICE are perfectly solid, unperturbed by load currents. Modeling the actual source impedance of your supply and board layout with additional components is tricky and imprecise. Power supply rejection magnitude is modeled in our best macro-models, but the phase relationship of this feedback path is unlikely to match reality. Simulation can be tremendously useful but won’t accurately predict this behavior.

All this should not cause you to be paranoid—no need to go crazy with bypassing. Just be alert to particularly sensitive situations and signs of potential problems. Good analog design thrives with a healthy dose of understanding and awareness. :-)

Thanks for reading and comments are welcome below.

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Look here… 55+ other interesting technical topics The Signal blogs.

Rail-to-Rail Inputs—what you should know!

2013-04-16T18:03:00Z

Rail-to-Rail (R/R) op amps are extremely popular, especially useful with low supply voltage. You should know how R/R inputs are accomplished and understand some trade-offs.

Figure 1 shows a typical dual-input R/R stage comprised of both N and P-channel transistor pairs. The P-channel FETs handle the signal through the lower portion of the common-mode voltage range, to slightly below the negative rail (or single-supply ground). The N-channel FETs operate with common-mode voltage near and slightly above the positive rail. Additional circuitry (not shown) directs traffic, determining which input stage signal is processed by the next stage. Most of our dual input stage op amps are designed so that the transition occurs approximately 1.3V from the positive rail. Above this voltage, there is insufficient gate voltage for the P-channel stage so the signal path is redirected to the N-channel stage.

The P and N input stages will have somewhat different offset voltages. If the common-mode voltage moves through this transition (as it does with R/R G=1 operation), it creates a change in the offset. Some op amps are factory trimmed by laser or electronic trimming, adjusted to reduce the offset of the input stages. This reduces the change through the transition but still leaves a residual bobble. The circuitry controlling the transition from P to N input stage is referenced to the positive supply voltage, not ground. On a 3.3V supply the transition moves to an awkward point—mid-supply.

While unnoticed in most applications, this change in offset voltage may be an issue if high accuracy is required. It can also cause distortion in AC applications. But, again, this will only be seen if the common-mode input voltage crosses the transition between stages.

Figure 2 shows a second type of R/R input stage. An internal charge pump boosts the voltage powering a single P-channel input stage to approximately 2V above the positive supply rail. This allows a single input stage to perform seamlessly over the full rail-to-rail input voltage range—below the bottom rail to above the top rail. No transition glitch.

Charge pump… it sounds spooky to some designers. They’re noisy, right? But our most recent ones are remarkably quiet. Very little current is required because it’s only powering the input stage. There are no extra pins or capacitors—it’s all internal. Charge pump noise is below the broadband noise level; rarely can it be seen in the time domain. Applications that analyze the spectral response below the broadband noise level, however, may see some artifacts.

Not all applications need an op amp with R/R input. Inverting op amp circuits or amplifiers in gain greater than unity, for example, often do not require R/R input, yet still have R/R output. (Maybe this needs another blog.) Do you really need a R/R-input amplifier? Many engineers prefer to use them so they don’t need to worry about exceeding the common-mode range. They use the same op amp in various points in their system—some needing R/R input, others not. Whatever your choice, with knowledge of the R/R types and tradeoffs, you can select more wisely. If in doubt, you are welcome to ask us on our E2E forum.

Here are a few example op amps:

OPA340 Two-Input Stage, Trimmed Offset, 5.5MHz R/R CMOS
OPA343 Two-Input Stage, Untrimmed Offset, 5.5MHz R/R CMOS
OPA320 Charge-pumped Input Stage, Trimmed Offset, 20MHz R/R CMOS
OPA322 Charge-pumped Input Stage, Untrimmed, 20MHz R/R CMOS

Thanks for reading and your comments are welcome.

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Check out 55+ other interesting technical topics… The Signal blogs.

Op Amps… G=1 stable & decompensated

2013-04-08T00:15:00Z

You have voted. Unity-gain-stable op amps won in a landslide—they’re far more popular than decompensated op amps. What’s this all about?

Click Here to read on EDN Magazine site.

Paralleling Op Amps—is it possible?

2013-03-26T13:59:00Z

Is it possible to parallel two op amps to get twice the output current?

We get this question periodically on our E2E forums. Though we may answer with a qualified “yes,” it tends to make us shudder just a bit. It can be done… but with great care. So let me come quickly to a key point. Don’t use the simple circuit on the left. Directly paralleling inputs and output of two op amps is sure to start a serious argument between the two. Differing offset voltages will cause them to fight over the correct output voltage. They may burn all their output current capability in the battle with one output current (sourcing) flowing into the other (sinking current).

Figure 1b has a chance. Op amp A1 is the “master” and A2 is the so-called “slave,” replicating the output voltage of the master. R3 and R4 promote reasonably equal sharing of the load current, even though A2’s output may be slightly different. Feedback is connected on the load-side of R3 and R4 so their voltage drop is corrected. You lose some output voltage swing capability in the I∙R drop on these resistors so you will be tempted to make them low in value. But the offset voltage of A2 will cause extra quiescent current equal to Vos/(R3+R4). It’s a tricky tradeoff.

Be very cautious with high speed signals. You want A2 to accurately replicate the output of A1. If the signal moves too fast, the phase shift of A2 will cause differing output voltages and wasted current. It’s important to avoid slewing. If necessary, add an R-C filter at the input so the fastest rate of change on the output of A1 is well below slewing speeds. The dynamic behavior of two amplifiers may not match well during slewing.

Don’t use older generation op amps that have output inversion (phase reversal) behaviors. If A1 can overdrive the input common-mode range of A2 and its output inverts the result is ugly.

Above all, check the behavior of your circuit thoroughly. SPICE may tell you whether you have a basic working circuit, but op amp macro-models may not accurately predict the quirks that could befall this circuit. Build a breadboard and check all signals and conditions carefully. If your op amp is multi-sourced, consider that not all manufacturers’ devices behave exactly the same. (But, of course, you have only one source for op amps, right?)

Do you think I’m a bit leery of paralleling op amps? Well, yes… call me leery. It can be successful but proceed with caution. I recommend that you consider an easier path—using an op amp with more output current. Here are a few possibilities:

TLV4111 300mA, 6V. CMOS Op Amp.
BUF634 G=1 buffer, 200mA, 36V. Used inside the feedback loop of standard op amps.
OPA547 500mA, 60V Op Amp. Adjustable current limit.
OPA564 1.5A, 24V Op Amp, 17MHz GBW.
OPA548 5A, 60V Op Amp. Adjustable current limit.

Have you successfully paralleled op amps? Or do you have scars from trying? Comments welcome.

Thanks for reading,

Bruce email: thesignal@list.ti.com

Table of Contents… with 50+ other interesting technical topics.

Resistor Puzzle Solution... and a rant on schematics

2013-03-19T02:33:00Z

Did you see last week’s resistor puzzle? Check it out if you missed it. Here’s the solution:

We’re not accustomed to reading three-dimensional schematics so the first step is to redraw it clearly. There are three distinct paths from A to B, colored blue, green and red. Each has a series connection of 1Ω—R—1Ω. The 3Ω resistors are effectively in parallel with the “R” resistors. The symmetry of these 3Ω resistor connections makes their effect the same as if each is directly in parallel with one of the “R” resistors.

The total resistance from A to B is 1Ω, so each of the three legs must be 3Ω. With 1Ω on each end of the legs, the middle parallel combination must also be 1Ω. So R must be 1.5Ω in parallel with 3Ω to make 1Ω.

Was this fun? Maybe you missed a previous puzzle, the infinite resistor network. More fun.

It’s all so much easier when we have a well-drawn schematic. Huummmm… a well-drawn schematic?

A genius former colleague in my past used to say “circuits work better when the schematic is drawn right.” He didn’t mean “drawn without errors.” He meant that it was easier to understand the circuit when it was drawn well. Nuances are easier to discern, details more easily optimized and problems are more easily resolved. So true!

I’m on my soapbox now! Have pride in your schematics. A well-designed circuit deserves a well-drawn schematic. And a poorly drawn schematic does not inspire confidence in your work.

Take care in laying out your schematic. Signals flow better left-to-right, you know. Currents flow downward. With thoughtful layout, you can minimize confusing crossovers and labeled interconnections that hinder interpretation. If you need multiple pages, make connections clear, preferably so one sheet can lie next to another and connections are obvious. Draw sub-circuits the same way each instance they occur so they are easily recognized.

Use familiar symbols. Op amps are triangles not rectangles—much easier to read this way. When possible, place components in a way that suggests a good circuit board layout. Draw symmetrically if this is desirable in the PCB layout. Label IC part numbers and include all circuit values. Number components so that they can be identified easily in e-mails or phone conversations.

So often, I see schematics without a single word of helpful annotation. A few words of explanation can add so much! A footnote explaining why a certain component is chosen or how a value is calculated can be so valuable to a support engineer a few years later. How about labeling some nominal voltages at key nodes? Show gain values and nominal signal levels. Label major blocks. If you last as long as I, you may be the beneficiary of your own good documentation as you retrace your own steps.

Some thought and empathy will make life easier for those who later must decipher your circuit. Make the quality of your schematics a signature feature of your excellent work. Oh, yes… signature! A valued colleague here at TI insists that you should put your name on your schematic. (Thanks, Jim.) You may find that this simple act will cause you to recheck your work one last time. ;-)

Schematics… what’s your pet peeve? What’s your rant? Leave your comments.

Thanks for reading,

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Check out 50+ interesting topics… Table of Contents for all The Signal blogs.

Resistor Puzzle—the sequel

2013-03-12T04:18:00Z

It’s time for some fun! I’ve known a few folks who have tormented colleagues with a resistor cube—equal resistors on all sides. So in case you’ve solved that one, let’s add a twist. In this cube, not all the resistors are equal. The resistance from A to B is 1Ω. Resistor values are indicated, except for those marked “R?” in red. What is the required value for R?

Spoiler alert! We’re sure to get the answer posted in reader comments below. No peeking. ;-)

This blog marks #53, the start of a second year. It’s been great fun and a challenging gig. A week feels like three days! In the coming months, I may skip a week here and there. It will give me a chance to catch my breath… and maybe do some grandfathering, too.

I’d also like to ask for your topic suggestions. I still have a list of my own but I’d like to respond to your ideas. Of course, your suggestion could lie outside my knowledge or experience so no promises. You can post in comments below or send me an email.

And BTW… If you enjoy quizzes, here’s another one for your amusement.

Thanks for reading,

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Check out 52 other interesting topics… Table of Contents for all The Signal blogs.

1/f Noise—the flickering candle

2013-03-03T22:59:00Z

The 1/f (one-over-f) low frequency noise region of amplifiers seems just a bit mysterious. Reader “tweet” asked for a discussion of 1/f noise—a challenging topic for a short blog.

Click Here to read on EDN magazine web site.

Excel noise calculation file here… e2e.ti.com/.../4812.Flicker-Noise-v1.xlsx

Note: This file is different than the one posted a month ago.

Simulating Gain-Bandwidth—the generic op amp model

2013-02-26T13:28:00Z

It may not always be obvious how the gain-bandwidth product (GBW) of an op amp may affect your circuits. Macro-models have a fixed GBW. Though you can look inside these models, it’s best not to tinker with them. What to do?

You can use a generic op amp model in SPICE to check your circuits for sensitivity to GBW. Most SPICE-based circuit simulators have a simple op amp model that you can easily modify. TINA’s is shown in figure 1.

First, set its DC open-loop gain to 1M (120dB). Then, a dominate pole frequency (entered in Hz) will create a GBW of the amplifier in MHz. In this example, a 10Hz dominate pole creates a GBW of 10MHz. Figure 2 shows the open-loop response for three different gain-bandwidths, 5MHz, 10MHz and 100MHz.

Note that that this simple model also includes a second pole (some folks call it a nuisance pole). In some cases, you may want to make this second pole a very high frequency such as 10GHz. This will create an ideal 90° phase margin for any reasonable GBW. In this example, I set the second pole at 100MHz, equal to the highest GBW that I’m simulating. You can see the effect of this second pole in the 100MHz GBW response, causing the open-loop response to bend downward at 100MHz. It causes the unity-gain bandwidth to pull in to approximately 78MHz, similar to what you might see with a real op amp of this GBW. Unity-gain bandwidth and GBW of a real op amp are not necessarily the same number. (Does this need another blog?)

Active filter designs can be tricky to judge GBW requirements and are a good case for use of this technique. FilterPro, used to design the Chebyshev¹ filter in figure 3, provides GBW recommendations but its guidelines may be more stringent than needed in some circumstances. For this design, it recommends a 100MHz or greater GBW to achieve nearly ideal filter design characteristics. I simulated the design using the three gain-bandwidths shown in figure 2, 5MHz, 10MHz and 100MHz. With these results you might decide that a GBW less than 100MHz could be satisfactory. For final simulations, you should use the macro-model for the op amp you select.

I used the parameter stepping function in TINA, varying the dominate pole to change the GBW. Other simulators have similar capability. Of course, parameters could be changed manually, too. Either way, varying the GBW of a generic op amp model will give you insight on its effect in your circuits.

Have you used a generic op amp model to vary other parameters? Comments welcome.

Bruce email: thesignal@list.ti.com (Email for direct communications. Comments for all, below.)

Table of Contents for all The Signal blogs.

Simulations use fee TINA-TI. Filter designs use free FilterPro.

Note 1: Geek fact—Chebyshev (Чебышёв) was a Russian mathematician who died in 1894. Use of his polynomials to create equal-ripple filters came later.