RARELY ASKED QUESTIONS...


The column, "Rarely Asked Questions: Strange but true stories from the call logs of Analog Devices," is published monthly in EDN and Design News.

Technological troubleshooting, insight and ideas from the experts at Analog Devices to help solve your real-world application challenges.

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LEDs are Photodiodes Too

Questions & Answers
and the strange but true stories behind them

Issue 108: LEDs are Photodiodes Too
The Question:
I need a cheap photodetector, but a 1N4148 doesn't seem to work. How should I connect it?
Download this article  [pdf, 181 KB]
Issue 107: The Diamond Plot
The Question:
My signal is within the instrumentation amplifier's specified input voltage range, but the output seems saturated. What's happening? Is the part broken?
Download this article  [pdf, 192 KB]
Issue 106: Overranged Converter Recovers Quickly
The Question:
How do you verify overrange recovery in high-sample-rate converters?
Download this article  [pdf, 540 KB]
Issue 105: Discretion is the better part...
The Question:
How do I choose a discrete transistor?
Download this article  [pdf, 204 KB]
Issue 104: Looking for Trouble in All the Wrong Place
The Question:
I see a lot of ringing and overshoot at the output of my amplifier. I've followed the data sheet guidelines and think that the layout is clean. What could I be doing wrong?
Download this article  [pdf, 196 KB]
Issue 103: Programmable ADC Input Range Provides System Benefits
The Question:
What's the advantage of an adjustable or programmable analog input range?
Download this article  [pdf, 156 KB]
Issue 102: Defying Scrooge
The Question:
In Electronics is Fun, you tell of employers who encourage their engineers by allowing them to use test gear for personal projects. I work for Scrooge. Not only is this forbidden, but my salary is so low I can't afford my own test gear. How can I pursue my hobby?
Download this article  [pdf, 173 KB]
Issue 101: Bits Per Second Make My Head Hertz
The Question:
I'm confused by the use of "Mbps" units when looking at digital isolator data sheets. Why not use MHz?
Download this article  [pdf, 158 KB]
Issue 100: 100 Issues and Counting
The Question:
Your RAQ column has been around for a long time. When did it start? Where can I read more?
Download this article  [pdf, 129 KB]
Issue 99: Overrange Flag Indicates Data Validity
The Question:
What will the output data from an ADC look like during an overrange event?
Download this article  [pdf, 125 KB]
Issue 98: Design Tools and Support Communities Make Analog Fun, not Fearful
The Question:
I'm well versed with digital design, but have been asked to add thermocouple based temperature measurement to our system. My tie is beginning to curl from fright. Where should I start?
Download this article  [pdf, 147 KB]
Issue 97: EngineerZone Makes Getting Answers a Breeze
The Question:
I have a problem that hasn't been covered anywhere. Where can I go to get my tough design questions answered?
Download this article  [pdf, 128 KB]
Issue 96: Soft Clipping vs. Hard Clipping
The Question:
Can 0.1 dB really make that much difference?
Download this article  [pdf, 230 KB]
Issue 95: Do-it-Yourself ASICs
The Question:
What should I do if I can't find an analog IC that performs the required function?
Download this article  [pdf, 152 KB]
Issue 94: Crystal Radio Sets and Op Amps - Both Can Detect RF Signals
The Question:
How are unwanted noise and dc offsets getting into my precision amplifier circuit?
Download this article  [pdf, 285 KB]
Issue 93: Input Magic - Differential Signals Allow Input Swing to Exceed Supply Voltage
The Question:
My ADC has a 1.8-V power supply. How can it have a 2-V p-p input range?
Download this article  [pdf, 307 KB]
Issue 92: Multipliers and Modulators
The Question:
Why can't I use a multiplier as a modulator or mixer? Aren't they the same thing?
Download this article  [pdf, 124 KB]
Issue 91: Noise Gain vs. Signal Gain - What's the difference between noise gain and signal gain?
The Question:
What's the difference between noise gain and signal gain?
Download this article  [pdf, 169 KB]
Issue 90: Resolution vs. ENOB - Still Hazy After All These Years
The Question:
My ADC specifies 12-bit linearity, but 10.5 ENOB. Do your specifications break the rules?
Download this article  [pdf, 117 KB]
Issue 89: Electronic is Fun - Is Analog Electronics Fun?
The Question:
Is analog electronics fun?
Download this article  [pdf, 117 KB]
Issue 88: The Decline of Rounding—or How Significant is Significance?
The Question:
How many digits should I use to represent measured or calculated data?
Download this article  [pdf, 172 KB]
Issue 87: Apples and Oranges—Correlating Specifications Requires Common Test Conditions
The Question:
How is the input referred noise of an ADC related to its signal-to-noise ratio (SNR)?
Download this article  [pdf, 187 KB]
Issue 86: Current Transformers—and Vacuum Cleaners
The Question:
You referred to Current Transformers in a recent RAQ. What are they and how are they used?
Download this article  [pdf, 164 KB]
Issue 85: Great Expectations Come From Basic Understandings
The Question:
What should I do when my circuit performance falls short of my design expectations?
Download this article  [pdf, 117 KB]
Issue 84: Driving Miss ADC
The Question:
I've been told that ADC inputs can be "unfriendly". What should I be worried about?
Download this article  [pdf, 107 KB]
Issue 83: Dual Op Amps and Double Agents: They may not always appear to be who they say they are...
The Question:
Are dual op amps just duplicates of the single version of the op amp? Do they offer the same performance?
Download this article  [pdf, 585 KB]
Issue 82: Transformers: They're Not All Boat Anchors
The Question:
What is the most undervalued analog component?
Download this article  [pdf, 892 KB]
Issue 81: Switching Bits
The Question:
With no signal at the input to my ADC, why are the output data bits switching?
Download this article  [pdf, 1.37 MB]
Issue 80: The Electromotive Force and Op Amps
The Question:
Is it okay to run an op amp on a single +10 V supply, or do I need to use ±5 V supplies?
Download this article  [pdf, 1.21 MB]
Issue 79: Nobody is Perfect - Not Even an Engineer!
The Question:
What is the most common problem with precision analog circuits?
Download this article  [pdf, 835 KB]
Issue 78: Considerations on High-Speed Converter PCB Design, Part 4: Plane Coupling
The Question:
What are some important PCB layout rules when using a high-speed converter?
Download this article  [pdf, 1.05 MB]
Issue 77: Beware of big claims and large numbers...they don't always add up.
The Question:
Are wide bandwidth and fast slew rate the best way to benchmark an amplifier?
Download this article  [pdf, 946 KB]
Issue 76: In and Out - or - Why Have a Capacitor?
The Question:
Why do ICs need their own decoupling capacitors?
Download this article  [pdf, 845 KB]
Issue 75: Crosstalkin' Converters
The Question:
Should I consider crosstalk when choosing an A/D converter?
Download this article  [pdf, 897 KB]
Issue 74: Rotary Potentiometers: There Must Be a Better Option
The Question:
I'm trying to use a potentiometer (pot) in the feedback loop of an op amp circuit, but the adjustment is too sensitive. Do you have any suggestions on how to improve the performance of this circuit?
Download this article  [pdf, 991 KB]
Issue 73: The Benefits of Ancient Logic
The Question:
What sort of logic circuitry should I use in (mostly) analog circuitry?
Download this article  [pdf, 707 KB]
Issue 72: Balancing Phase in High-Speed Converters
The Question:
Why is analog input phase balance so important to my high-speed converter design?
Download this article  [pdf, 920 KB]
Issue 71: Looking for savings in all the wrong places
The Question:
I'm trying to save cost and printed-circuit board space. Can I use an amplifier's internal ESD diodes as clamping diodes?
Download this article  [pdf, 1.19 MB]
Issue 70: Do Something with that Unused Pin!
The Question:
What should I do with the unused terminals of analog ICs?
Download this article  [pdf, 942 KB]
Issue 69: Considerations on High-Speed Converter PCB Design, Part 3: The E-Pad Low Down
The Question:
What are some important PCB layout rules when using a high-speed converter?
Download this article  [pdf, 942 KB]
Issue 68: A Foolish Question about Op Amps and Capacitors
The Question:
Can you help with the strange interaction between the op amp and capacitors in my circuit?
Download this article  [pdf, 1.41 MB]
Issue 67: Measuring the DC Parameters of Op Amps
The Question:
Modern op amps have very high open-loop gain and very low offsets. How can one possibly measure them? Or is that a closely held secret?
Download this article  [pdf, 1.11 MB]
Issue 66: Considerations on High-Speed Converter PCB Design, Part 2: Using Power and Ground Planes to Your Advantage.
The Question:
What are some important PCB layout rules when using a high-speed converter?
Download this article  [pdf, 1.06 MB]
Issue 65: Op Amp Dedicated Feedback Pin Helps Achieve New Levels of Performance. or This feedback pin is dedicated to all those op amps out there tonight!
The Question:
I've noticed a new pinout on some of your high-speed op amps. Why the change after all these years?
Download this article  [pdf, 1.73 MB]
Issue 64: Oversampling and Undersampling
The Question:
Why do many modern ADCs have a signal bandwidth much greater than their maximum sampling frequency? Doesn't sampling theory require the signal frequency to be limited to half the sampling frequency? Wouldn't it save power if their input stages had less bandwidth?
Download this article  [pdf, 1.63 MB]
Issue 63: Considerations on High-Speed Converter PCB Design, Part 1: Power and Ground Planes.
The Question:
What are some important PCB layout rules when using a high-speed converter?
Download this article  [pdf, 1.92 MB]
Issue 62: Is amplifier headroom cramping your style?
The Question:
When running on a single supply, my op amp's output is highly distorted. Could this be some kind of headroom issue?
Download this article  [pdf, 1.79 MB]
Issue 61: Make Do!
The Question:
What is the most important characteristic of a successful analog engineer?
Download this article  [pdf, 1.26 MB]
Issue 60: Watch for Those Multiple Clocking Edges!
The Question:
How can I improve system performance when using multiple clocks?
Download this article  [pdf, 1.20 MB]
Issue 59: Is Your Amplifier Disabled, or is it Merely Sleeping?
The Question:
What's the difference between, power-down, shutdown, standby, sleep mode or disable?
Download this article  [pdf, 1.07 MB]
Issue 58: How to Avoid Burning Cakes
The Question:
How does one measure temperature outside the silicon range?
Download this article  [pdf, 1.27 MB]
Issue 57: Keeping Your A/D Converter Clocks Jitter Free
The Question:
How do I make improvements to my system clock or clock circuit so as to reduce jitter?
Download this article  [pdf, 702 KB]
Issue 56: Amplifier, attenuator or both?
The Question:
Can I use an amplifier as an attenuator?
Download this article  [pdf, 1.34 MB]
Issue 55: Taming A/D Converter Power Supplies
The Question:
Will a switching power supply (dc-to-dc converter) really degrade the A/D converter's performance?
Download this article  [pdf, 890 KB]
Issue 54: Don't be Mean - be Root Mean Square!
The Question:
How do you measure a varying signal?
Download this article  [pdf, 1,195,069 bytes]
Issue 53: VFB or CFB that is the question!
The Question:
Why is it, that voltage feedback amplifiers seem to dominant the op amp landscape compared to current feedback amplifiers?
Download this article  [pdf, 101,978 bytes]
Issue 52: Bring on the Converter Noise! - Part 2
The Question:
How does resistor noise compare to A/D converter's noise?
Download this article  [pdf, 98,498 bytes]
Issue 51: Bring on the Converter Noise! - Part 1
The Question:
Is Noise Figure important from the A/D converter's perspective?
Download this article  [pdf, 96,879 bytes]
Issue 50: What's the big deal about ABSOLUTE MAXIMUM RATINGS?
The Question:
Is it alright to exceed just one of the parameters in the ABSOLUTE MAXIMUM RATINGS table, by just a little bit?
Download this article  [pdf, 102,089 bytes]
Issue 49: What's the (Converter) Frequency Kenneth?
The Question:
How Do I Design a Converter Front-end without Compromising the Performance?
Download this article  [pdf, 97,703 bytes]
Issue 48: Some Chips Have Moving Parts!
The Question:
Is it true that over the years electrical devices have evolved to where they have no moving parts at all?
Download this article  [pdf, 104,025 bytes]
Issue 47: Which carries more weight, a datasheet or SPICE macromodel?
The Question:
When running a SPICE (Simulation Program with Integrated Circuit Emphasis) simulation with one of your amplifier models, I get different results than those shown in the datasheet. Which should I believe?
Download this article  [pdf, 100,637 bytes]
Issue 46: What shall we do with an unused op-amp?
The Question:
In the RAQ on op-amps as comparators you mentioned an unused op-amp in a quad as being possible justification for using it as a comparator. What do you do if you've an unused op-amp and don't need a comparator?
Download this article  [pdf, 129,560 bytes]
Issue 45: Glass Diodes May See the Light – and Hum
The Question:
A few months ago you discussed how to prevent the inherent high frequency (HF) noise of a switching supply from escaping into sensitive analog circuits and corrupting them. My op-amp output hums and no amount of supply filtering helps. What should I do?
Download this article  [pdf, 127,971 bytes]
Issue 44: Keeping Common Modes Common
The Question:
How come my A/D converter is not able to achieve full scale as the datasheet specifies?
Download this article  [pdf, 95,957 bytes]
Issue 43: Impedance Measuring Chip Offers Little Resistance to Applications
The Question:
I came across your Impedance measuring chip (IMC) in a recent article. It sounds interesting, but how does it work and what can it be used for?
Download this article  [pdf, 125,661 bytes]
Issue 42: Are Your Filters Filtering?
The Question:
Why is my A/D converter's anti-aliasing filter showing inadequate spurious and noise rejection?
Download this article  [pdf, 731,084 bytes]
Issue 41: What's in Your Toolbox?
The Question:
Do you prefer power tools or hand tools for solving problems?
Download this article  [pdf, 103,363 bytes]
Issue 40: How Big Is It?
The Question:
How can I handle signals with huge variations in amplitude?
Download this article  [pdf, 102,537 bytes]
Issue 39: Off-amps, what the heck are off-amps? Op amps with no power applied, of course.
The Question:
What kind of performance (pick a parameter, it could be input impedance or any other op amp specification) can I expect from your amplifier ... when the power is off?
Download this article  [pdf, 126,118 bytes]
Issue 38: Single Supply Amplifiers – They Sound Simple ... are they?
The Question:
Running rail-to-rail op amps on a single supply sounds like a winning combination, but what drawbacks will I encounter when using amplifiers like this?
Download this article  [pdf, 119,519 bytes]
Issue 37: Lock Down That Noise - Don't Let It Escape
The Question:
How can I prevent switching-mode power supply noise from devastating my circuit performance?
Download this article  [pdf, 1,056,768 bytes]
Issue 36: The Difference Between Different Differential Amplifiers
The Question:
I'm perplexed. Why is the output voltage of my ADC differential amplifier driver not what it should be?
Download this article  [pdf, 737,280 bytes]
Issue 35: Stress Is Bad For You – and For Analog ICs
The Question:
Does too much mechanical stress when mounting precision analog ICs damage their accuracy?
Download this article  [pdf, 126,976 bytes]
Issue 34: A Capaciticectomy? Sounds Awfully Painful!
The Question:
One of your high-speed triple amplifiers is oscillating. What's wrong with it?
Download this article  [pdf, 118,784 bytes]
Issue 33: Hysteresis Blocks Magic
The Question:
Can I recalibrate my precision analog circuits to achieve higher accuracy?
Download this article  [pdf, 118,784 bytes]
Issue 32: Political Correctness and Datasheets
The Question:
Datasheet Specifications: Provoking or Confusing?
Download this article  [pdf, 114,688 bytes]
Issue 31: Too Hot or Too Cold May Be Just Right
The Question:
Are absolute maximum and minimum temperatures as absolute as voltage or current ratings?
Download this article  [pdf, 995,328 bytes]
Issue 30: CAPACITIVE SENSORS: Rugged Enough for the Real World?
The Question:
I've seen cap sensors used in consumer electronics, but are they rugged enough to function in a harsh industrial environment?
Download this article  [pdf, 688,128 byte]
Issue 29: Half Full or Half Empty? Thoughts on Capacity.
The Question:
Are all components with just two wires as complicated as the resistors we discussed recently?
Download this article  [pdf, 114,688 bytes]
Issue 28: What's Your Amplifier's Febricity Due to Electricity?
The Question:
What's the best way to measure the die temperature of a small package op amp or similar device?
Download this article  [pdf, 565,248 bytes]
Issue 27: Resistors (and old ladies) have hidden depths
The Question:
You recently discussed the thermal noise of resistors. They seem to be such straightforward components with just two connections and one simple property: resistance. Do they have more traps for the unwary?
Download this article  [pdf, 770,048 bytes]
Issue 26: Op-Amp Noise can be Deafening Too
The Question:
Last month you blamed op amp noise on external resistors. Surely this is not always the case?
Download this article  [pdf, 106,496 bytes]
Issue 25: Resistor Noise can be Deafening, and Hard to Reduce
The Question:
My low-noise amplifier is not low noise enough. What am I doing wrong?
Download this article  [pdf, 167,936 bytes]
Issue 24: Are leaky amps sinking your design?
The Question:
My amplifier "leaks" a low level signal at the output with the power supplies off. What gives...is my circuit sunk?
Download this article  [pdf, 602,112 bytes]
Issue 23: Should you butcher your circuit or your test gear? Capable engineers should be prepared to question, and modify, anything, not just their own designs.
The Question:
While probing a new high-speed amplifier board, I noticed the output had huge overshoot and was ringing like a bell. I had paid close attention to the layout, optimizing supply bypassing, grounding, parasitic reactances, and trace terminations. Where did I go wrong?
Download this article  [pdf, 71,688 bytes]
Issue 22: Things (animals and ADCs) are not always what they seem
The Question:
Must I consider high frequency issues when driving a slow ADC?
Download this article  [pdf, 65,536 bytes]
Issue 21: Breadboarding With Surface Mount ICs Too small, or not too small? That is the question...
The Question:
How do I build breadboards with tiny surface mount (SM) ICs?
Download this article  [pdf, 90,112 bytes]
Issue 20: Igor, pass me some supply sequencers!
The Question:
Last month you said that some absolute maximum ratings depended on the value on some other pin of an integrated circuit (IC). How do I ensure that they are not exceeded during power-up?
Download this article  [pdf, 94,208 bytes]
Issue 19: Observing Maximum Ratings or How to Avoid Forecasts of Doom
The Question:
How much safety margin is there in "absolute maximum" ratings?
Download this article  [pdf, 782,336 bytes]
Issue 18: Just How Accurate was William Tell, Anyway?
The Question:
How accurate is an ADC?
Download this article  [pdf, 1,036,288 bytes]
Issue 17: More Chips are Better than One (or the Benefits of Belgian Food)
The Question:
When are two chips better than one?
Download this article  [pdf, 1,314,816 bytes]
Issue 16: Replacing Optocouplers with Digital Isolators (or How a Midnight Ride Could Have Been Avoided)
The Question:
Can transformer-based isolators replace optocouplers? Won't there be problems with dc signals, magnetic fields and size?
Download this article  [pdf, 368,640 bytes]
Issue 15: The Long Term Stability of Precision Analog ICs, or How to Age Gracefully and Avoid Sudden Death
The Question:
The life expectancy of my product is 20 years. How well will its calibration survive?
Download this article  [pdf, 163,840 bytes]
Issue 14: Resourcefulness, or Why Good Engineers (and Good Cooks) Use the Best Available Components.
The Question:
Which (amplifier/switch/converter) is best in a (mobile phone/strain gauge/medical ultrasound system)?
Download this article  [pdf, 147,456 bytes]
Issue 13: It May be Greek to You, but Sigma Delta Converters are Not Really Hard to Understand
The Question:
Can you please explain, simply, as to a Bear of Little Brain, how sigma-delta converters work?
Download this article  [pdf, 110,592 bytes]
Issue 12: How Not to Design Active Filters
(or Not Only Sharks Distract Divers)

The Question:
Why does my carefully designed active filter not meet its specifications?
Download this article  [pdf, 106,496 bytes]
Issue 11: Comparators & Op Amps – May They Never Meet
(or Good Advice from Mr. Punch)

The Question:
Why buy an expensive comparator when an op amp is just as good?
Download this article  [pdf, 364,544 bytes]
Issue 10: Digital Processing Does Not Always Rule (or What Makes an Analog Engineer Laugh?)
The Question:
Why recommend analog signal processing when digital systems are so cheap and powerful?
Download this article  [pdf, 28,672 bytes]
Issue 9: Grounding Converters (or Philosophy to the Rescue!)
The Question:
I can't believe that you recently advised us to connect the digital ground pin of ADCs to the system analog ground. Was this a mistake?
Download this article  [pdf, 32,768 bytes]
Issue 8: Happy Groundhog Day!
The Question:
Why do I have an output when my analog input is grounded?
Download this article  [pdf, 1,787,527 bytes]
Issue 7: It's Easy to Drive An EE to Drink – but What Drives One to Poetry?
The Question:
My SPICE simulation worked perfectly, but now I've a large production batch which doesn't meet its specification.
Download this article  [pdf, 94,208 bytes]
Issue 6: Tick-Tock, Tick-Tock
The Question:
My clock is accurate to 1 ppm – what could possibly need improvement?
Download this article (pdf, 852,045 bytes)
Issue 5: Check the Bias Current (or How to Ensure that a Blessing is Heard)
The Question:
The bias current in modern amplifiers is very small. Do I need to bother about it?
Download this article (pdf, 1,571,506 bytes)
Issue 4: Caveat Emptor
The Question:
How do I make sense out of a data sheet ?
Download this article (pdf, 126,976 bytes)
Issue 3: Isolating the Key Detail (or Lunching With a Mermaid and Pickled Herring)
The Question:
There is a fault in my CMOS multiplexer...?
Download this article (pdf, 1,892,352 bytes)
Issue 2: Slow "Starting" ADCs (or the Beneficial Effects of Spirits in Diagnosing Converter Problems)
The Question:
To save power, my ADC is powered up only to make a measurement. The system is very accurate in continuous operation, but completely unpredictable when power is strobed.
Download this article (pdf, 28,672 bytes)
Issue 1: Radio Frequency Interference (or How to Get a Free Lunch at a Very Good Restaurant)
The Question:
I've heard that RF can make low-frequency circuits do strange things. What's that all about?
Download this article (pdf, 73,728 bytes)

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