Electronics > Projects, Designs, and Technical Stuff
power decoupling myths
David Hess:
--- Quote from: exe on July 24, 2020, 09:21:09 am ---And another question that bothers me a lot. A common best practice is to put a decoupling capacitor for each power pin. I often have one LDO supplying power to multiple ICs. Question: can these caps start resonating with each other? Or should I use a separate LDO for each potentially noisy IC? To be more specific, my typical scenario: MCU, ADC (with two power rails: analog and digital), and digital isolator on the same board.
--- End quote ---
Yes, it is possible, and it can be calculated with transmission line length, loss, and impedance but this is not always easy. The impedance mismatch between the transmission line and decoupling capacitors causes reflections and if the Q is high enough, they become a problem. One solution is to properly terminate the transmission line. Lossy ferrite beads or RLC decoupling can be used to decrease Q which has saved me a few times before I understood what was happening.
exe:
I let myself ask another question or two about decoupling and noise :).
Is it a common practice to use, say, series resistors on digital lines? I once did an experiment: I breadboarded an spi digital isolator. When I probed the signal there was massive ringing (thanks to long jump wires). I installed 1K resistors in series and all ringing was gone. Why 1K? Values much bigger than that increased rise time too much for the data rate.
Strangely enough I almost never heard of this method of reducing digital noise. Only in one datasheet for an LT ADC there was a suggestion to put up to, afaik, 150 Ohm resistor. Values bigger than that were discouraged because digital inputs had some sort of protection from slowly rising signals, or something like that. I don't remember what part was that, but, e.g., datasheet for ltc2420 also mentions serial and parallel termination of digital lines.
I've also seen a small series resistor (1-2 Ohm) before ldo and its input capacitor. This, presumably, should reduce noise from fast-switching load back to the power rail. Is this a common practice? If not, why?
thm_w:
--- Quote from: exe on July 24, 2020, 08:43:36 pm ---Is it a common practice to use, say, series resistors on digital lines? I once did an experiment: I breadboarded an spi digital isolator. When I probed the signal there was massive ringing (thanks to long jump wires). I installed 1K resistors in series and all ringing was gone. Why 1K? Values much bigger than that increased rise time too much for the data rate.
Strangely enough I almost never heard of this method of reducing digital noise. Only in one datasheet for an LT ADC there was a suggestion to put up to, afaik, 150 Ohm resistor. Values bigger than that were discouraged because digital inputs had some sort of protection from slowly rising signals, or something like that. I don't remember what part was that, but, e.g., datasheet for ltc2420 also mentions serial and parallel termination of digital lines.
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Its not terribly common, because the device would work without them (unless we are talking about high speed signals 200MHz+), the benefit is for noise as you say in specific situations. eg TI suggests doing this for their 24-bit ADCs. Or if you were driving a chip thats a long distance on the board and worried about EMI, it would be worth adding. Otherwise, why add them? Build your circuit on a PCB and see if there is much ringing.
"TI recommends placing 47-Ω resistors in series with all digital input and output pins (CS, SCLK, DIN, DOUT/DRDY, DRDY, RESET and START). This resistance smooths sharp transitions, suppresses overshoot, and offers some overvoltage protection. Care must be taken to meet all SPI timing requirements because the additional resistors interact with the bus capacitances present on the digital signal lines"
--- Quote ---I've also seen a small series resistor (1-2 Ohm) before ldo and its input capacitor. This, presumably, should reduce noise from fast-switching load back to the power rail. Is this a common practice? If not, why?
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Sometimes this resistor is for additional power dissipation reasons. Normally you'd use a ferrite bead for noise suppression.
T3sl4co1l:
Jumper wires on breadboarding will be in the 150-300 ohm range; a comparable size resistor will control the ringing. Loose wires make poor transmission lines, expect lots of coupling between groupings.
The general term is "source termination resistor", for which you should find a lot of information I would think. :-+
Ferrite beads are useful for the same purposes, for similar reasons -- at ~100MHz they offer significant resistance (per the rating).
Tim
exe:
Can you advice me on which ferite beads to use? From what I read, there is always a danger of resonance between ferite beads and decoupling caps. Would it help to use the beads with the biggest dc resistance? (I bought some with 2 Ohm beads, but never tried them in action). I work with low-power stuff (dac, adc, mcu) and my concern is noise injection into analog circuitry.
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