SRF Tech
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Posts: 59
Arizona
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I am going to take a shot at this thread and try not to repeat what anyone has already stated as there are many good reponses here.
My thoughts or "rule of thumb", take them with a grain of salt:
There is no right # of domains or max # either, I have built padrings for large mixed-signal products that had upwards of 50 separate domains (don't ask..not my preference).
The trick is ensuring that everyone has an low impedance path for large ESD currents from any domain to any other domain.
It is best to avoid trying to tie all domains to each other using b2b diode, layout becomes a nightmare and it is not a good ESD approach.
Do not attempt to diode connect power supplies together, especially in a part with mixed voltages and different power down modes unless you are absolutely positve that you understand every potential user implementation your customers will dream up.
The solution to these SoC products is to pick the largest 'substrate' ground domain on the product and use that domian as the common ESD node.
All grounds must have b2b diode'S' to the large substrate ground. (layout and placement is critical here).
All powers must have excellent clamping to their respective grounds. if this is true than those respective grounds will pass ESD currents to the rest of the chip through their b2b diodes to the primary substrate ground and there will be no further need to clamp different power supplies to each other and complicate your power down modes.
This also helps with leakage as you will have no b2b diode with voltages on them, the only b2b will be between grounds where the voltage potential will likley be ~0v across the diode.
Regarding noise: I have seen many of loose-electrons comments on noise on these boards and I very much like his approach. Few people really understand noise and many people have an unhealthly paranoia in trying to avoid it. Loose_electrons previous comment is correct..study the parasitics and do the analysis.
...Now please forgive me this very crude hand analysis example, but I feel it makes the point correctly. If I have a b2b diode with 0.7pF of capacitance (this is a large conservative number mind you), it will ideally act as an 45 ohms impedance to a 5Ghz signal. however if my ground impedance for my supplies on either side of the diode is say 2 ohms at 5Ghz (bond wire, metal resistance etc etc), than any noise coupling across the diode is divided down between 45 ohms into 2 ohms (-27dB..of whatever noise is coupled), and this is at 5Ghz, its much lower as frequency lowers, I am not even taking into account the PSRR ratio of my individual blocks and so forth. Now if you are desiging an LNA on an integrated baseband RF chip, 27 dB may not be enough, but for most applications, including analog PLL and oscillators it very well maybe. Alot also depends on how much actual noise is generated on your noisy supplies. (25mV attentuated by 27dB is very different from 500mV attentuated by 27dB) Basically, do the analysis before you panick about ESD noise coupling is what I am saying.
If your noise anlysis shows that you may be better off with grounds shorted together..do it! Sometimes when you short multiple grounds together the overall noise actually lowers as the overall impedance for that larger ground domain is lowered; as it becomes larger and takes advantage or more pins/bondwire/decoupling, etc offchip.
On another note, do not muck around with trying to tailor the ESD connections for large inductive isolation. Remeber that CDM ESD currents have enegy high into the GHz range, any inductive isolation will isolate those enegrgies as well and nullify your ESD structures. It is a very bad practice to inductively isolate ESD structures if you want to pass CDM type events (which face it, are more reflective of todays manufacturing enviroment.)
I will stop here...
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