Rarely Asked Question Page
What is the most common problem with precision analog circuits?
Probably grounding errors, but there are a number of frequently seen mistakes. They are mostly sins of omission; engineers are not perfect and can forget things
- Don’t forget to read the data sheet. (Application engineers routinely shout “RTFDS”1 as they hang up after an enquiry.) Extracting implicit information from a data sheet, not just the explicit details, is important.
- Don’t forget Ohm’s Law. The resistance of a wire or PC track is not zero, and leakage in “insulators” matters when measuring low currents.
- Don’t forget the bias current. Sometimes greasy fingerprints provide a current path in the prototype, leading to surprises in the clean(er) final version.
- Don’t forget the stray resistances, inductances, and capacitances of the final (crowded) PCB; don’t assume that all is well because the breadboard (or the SPICE model) worked.
- Don’t forget that EMI and RFI occur everywhere; filter your supplies and input/output leads.
- Don’t forget to consider the effects of temperature variation on components (including the effects of differing temperature coefficients in nominally identical components).
- Don’t forget to verify that the circuit can tolerate having its supplies (and signals) applied in any order (and with any value of dv/dt)—or to ensure that it cannot be exposed to unacceptable power/signal sequences and rates.
- Don’t forget that switching power supplies are not as noise free as a battery.
- Don’t forget that analog circuits, unlike microprocessors, often do not reset on power up and that you may need to ensure correct start-up.
- Don’t forget that circuits don’t start instantly: capacitors must charge and precision circuits must stabilize.
- Don’t forget that some circuits are unstable when driving a reactive load. An output stage that will drive a wide range of resistive loads may oscillate with capacitance, such as that of a cable.
- Don’t forget that noise, like death and taxes, is universal. Every ADC has quantization noise, every resistor has Johnson noise—you can’t avoid them.
- Don’t forget that IC designers may not be user friendly. Devices may not work as you expect. Again I say, “RTFDS!”
All these issues, and more, have been discussed in earlier columns. Read them all, and pin up this list where you can read it every day!
1 RTFDS = "Read The Friendly Data Sheet."
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For More InformationAnalog Dialogue:
- A Dozen Ways to Make a Circuit Fail
- The Contaminator of Signals: HF Common-Mode-Generated Errors
- Avoid Common Problems When Designing Amplifier Circuits (pdf)
- Power Supply Management–Principles, Problems, and Parts (pdf)
- ADC Input Noise: The Good, The Bad, and The Ugly. Is No Noise Good Noise? (pdf)
- A Practical Guide to High-Speed Printed-Circuit-Board Layout (pdf)
- Practical Techniques to Avoid Instability Due to Capacitive Loading (Ask the Applications Engineer-32) (pdf)
- Avoiding Op-Amp Instability Problems in Single-Supply Applications (pdf)
- AN-1120: Noise Sources in Low Dropout (LDO) Regulators (pdf)
- AN-345: Grounding for Low-and-High-Frequency Circuits (pdf)
- Do Something with that Unused Pin!
- Considerations on High-Speed Converter PCB Design, Part 4: Plane Coupling
- Considerations on High-Speed Converter PCB Design, Part 3: The E-Pad Low Down
- Considerations on High-Speed Converter PCB Design, Part 2: Using Power and Ground Planes to Your Advantage
- Considerations on High-Speed Converter PCB Design, Part 1: Power and Ground Planes
- Taming A/D Converter Power Supplies
- Bring on the Converter Noise!
- Lock Down That Noise - Don't Let It Escape
- Op-Amp Noise can be Deafening Too
- Igor, pass me some supply sequencers!
- Grounding Converters (or Philosophy to the Rescue!)
- Caveat Emptor
James Bryant holds a degree in Physics and Philosophy from the University of Leeds. He is also C.Eng., Eur. Eng., MIEE, and an FBIS.