On Tuesday, September 9, 2014 9:05:04 AM UTC-5, David Riley wrote:
They're usually intended as a drop-in for standard aluminum electrolytics, so the form factor will probably be all wrong if you're trying to replace tantalums, but they're a lot closer to tantalums in terms of nominal ESR without a lot of the associated problems (particularly regarding derating of the operating voltage).
I wouldn't worry too much about ESR for the stereotypical decoupling. I just toss in 0.1 uF ceramics as per tradition (not per engineering) and it always works anyway. Or electrolytics if its an RC for a one shot or something.
Digital components are a lot tougher than people give them credit. As an example where analog/RF types pull hair out, is the stereotypical 0.1 we use for decoupling has a self resonance below our system clocks (figure like 4 to 8 MHz depending on exact type) and WAY below the gate switching current spikes freqs. This drives the RF guys nuts because I spend like $3 each on exotic MLCC chip caps with crazy high self resonances for my microwave work, because I prefer my capacitors to act as capacitors in the ckt rather than little inductors or resonant cavities.
I have a nearly completely populated console IO board on my workbench and I'm tempted to hook it up with RF gear in place of the regulator and sweep the board and see where it resonates with no (or few) actives installed. Intuitively somewhere around 300 MHz it'll make a nice quarter wave antenna, and thats so far above the resonant freq of the decoupling caps that plus or minus their ESR who knows what it'll act like. I should bring a working board into work and sniff a working system with a spectrum analyzer sometime. I'll hook up a board to an antenna analyzer too, for some laughs. Maybe I could use a board with no actives populated as an ultra QRP 70 cm antenna LOL. If time permits etc etc I'll report back with some RF data.
This is tangentially related to the issue where the frequency aka reciprocal of duration of spike, depends solely on logic family and has nothing to do with system clocks (other than here comes a 100 MHz gate switching noise spike, although it is at a 4 MHZ pulse repetition rate).
Digital stuff is tough and although it doesn't have the power supply CMRR that an op amp can have, you can feed some nasty power in and it'll still work. Of course for when it doesn't, that's why they invented schmidt triggers and stuff like that.
I know the actual history and reasons, but I've often thought one of the best things about the S100 bus in on board regulation, with decent regs, means excellent power noise immunity between boards that bus designs with a large central regulator don't have. Given the cost and availability of TO-92 sized 7805 I've occasionally daydreamed about board designs where you ignore power noise because every chip has its own dedicated regulator... That would be interesting indeed. And a pain to solder up. Still, TO-92 regs are cheaper than the higher quality machine pin sockets, so its not entirely insane as far as daydreams go.