QUESTION:
What is the most common problem with precision analog circuits?
Answer:
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!
Troubleshooting: A Dozen Ways to Make a Circuit Fail
1 RTFDS = "Read The Friendly Data Sheet."
