Check the curves of capacitance vs. DC bias - most X7R caps "squish up" under bias, and if you use them with high voltage (e.g. ±15V) rails, you'll find that many high K caps provide only a small fraction of their faceplate capacitance.
I've been using 3 Kemet C1206C105K1RACTU 1µF 50V X7R MLCCs in parallel to achieve a guaranteed 1.5µF capacitance to stabilize the ADP7142 and ADP7182 regulators. I'd like to use only one part per rail to achieve this guaranteed 1.5µF capacitance at 15V bias, but that is not possible given the actual parts we have available. If I'm wrong, please let me know and I'll send you a nice gift!
A number of vendors like Kemet and Murata have simulators online that will show you the expected behavior of their parts under DC bias. I think it's worthwhile to use these tools to see what the part will actually do when employed as a power supply bypass cap under DC bias, since some caps squish down to only 20-30% of their faceplate capacitance under bias.
I use these caps to stabilize low impedance regulators, so the bulk of the low rail impedance is from the regulators. However, above 10kHz or so, the bypass caps do 'take over' and dominate the rail impedance as the regulators lose gain bandwidth and feedback margin. Still, an 0402 1uF cap is gonna get destroyed by 15V rails, and will not provide much capacitance, regardless of its geometry.
Not to beat a dead horse, but I cannot find a single cap that will provide 1.5µF of guaranteed load, life, and tolerance capacitance at 15V+ bias. Please correct me if I'm wrong - it's tedious to have to place that many 1µF parts on a PCB!
I was just talking about replacing 100nF with the 1uF as an example. They will definitely have a much smaller effective capacitance due to the bias of course.
Are you actually sharing the regulator Cout and the load decoupling? Or do you mean the 1.5uF is just the regulator output capacitance.
The only capacitors that appear across the op amp power supply rails are the three 1µF X7R ceramics. these, with a 4 layer board using power supply planes, are sufficient to make it stable with any amplifier I've tested, including some extremely fast amplifiers such as the THS4011 and THS3001 used as buffers within an LM4562 or LME49724 feedback loop.
I'm not talking about any specific case. However, the AD has a higher current noise than the e.g. the LM. Is the AD used in Analog Presison?
Right, but it can be much quieter than the LM4562 if you are driving it from a very low impedance. OPA827 has much lower current noise than both, but it depends on your circuit as to what's important.
Since you're using 1206 aren't there bigger value caps you could use?
Capacitors | details for GRM31CR71H475KA12# | Murata
However the package inductance is of course determined by the package size, so the three gang has the benefits of overall lower esl. The terminal reversed parts are generally smaller in value, back to square one as they say.
This part seems interesting! I have considered using larger caps, but they tend to "squish up" more because the larger sizes have thinner dielectric layers. The last time I checked, it was pretty much a wash - larger parts didn't buy you much benefit, as some of the larger parts like the 2.2uF caps can squish down to 30% of their faceplate capacitance under bias.
However, the part you mentioned, the GRM31CR71H475KA12, seems to have about 3.5uF of capacitance under 15V bias, which is quite sufficient for my uses. It will definitely save PCB area and cost to place only one part and not 3, and it's not clear that I need the low ESL of 3x parts. It's certainly worth buying a few and swapping them into the current boards to see how they work. Thanks for the tip! If anything, the world is moving toward larger values for the 1206 package caps, and the original 1uF cap I was using became EOL. So, it may be best to use a more modern part.
However, the part you mentioned, the GRM31CR71H475KA12, seems to have about 3.5uF of capacitance under 15V bias, which is quite sufficient for my uses. It will definitely save PCB area and cost to place only one part and not 3, and it's not clear that I need the low ESL of 3x parts. It's certainly worth buying a few and swapping them into the current boards to see how they work. Thanks for the tip! If anything, the world is moving toward larger values for the 1206 package caps, and the original 1uF cap I was using became EOL. So, it may be best to use a more modern part.
Indeed, the 10...100 ohm//100pf...10nF from input ground to case or noisier ground might be the key to Nirvana as good pcb designs when using op-amps need no regulator at all...They don't show the PSRR and CMRR in their specs for nothing...Even better pcb design need no decoupling at all...Some of the best designs i ever saw only used electrolytic capacitors for filtering and/or decoupling and that was all no matter the op-amps slew rate!
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