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To my understanding analog grounding is separated from the Digital grounding to suppress the latch up effect introduced by digital circuitry. On the other hand ground planes serves as substrate noise shield, for high frequency circuit application. If you observes in a conventional inductor layout there would be PGS(Patterned Ground Shileding) where it isolates the inductor's circulating current from the substrate. Hope this helps
You mean in PCB or IC design. The fact is that in PCB design both digital and analog ground can be isolated. So any switching noise of digital component hase no influence on digital ground. It is very important for RF and high-resolution ADC application where detected signal a few uV.
In IC design analog and digital grounds cannot be isolated completely, because all component use the same substrate.
I am doing pure analog IC design without any digital blocks.So, I want to know for pure analog circuits like limiting amplifier or transimpedance amplifier , is it better to have a ground plane or just ground lines which fulfill the current density requirements ??? Thanks.
Power/ground planes aren't present in the chip, but on the PCB; so your IC design ha snothing to do with ground plane.
IC power structure has usualy rings around the chip.
Grounding strongly depend on your application so specify for an accurate response. Usualy inside an IC you provide both VCC and GND lines as thick as possible, however for example in a micropower application this loose sense.
As far as I know is not a usual practice to provide a full gnd plane as you do in PCBs. Probably because you have a limited number of metal layers (normally 2-3 in analog technologies), and always result dificult to route if you assign a full layer for GND. Also you increase parasitic capacitances etc.
What is a usual practice is to provide as much as possible GND contacts to substrate, close to all your transistors (of course respecting same sorroundings in matched devices). I have seen layouts that once finished fill all empty spaces with a grounded metal layer plenty of substrate contacts where possible. These kind of layout ressembles a GND plane.
As pointed out by other messages, preserve separated digital and analog supply (and Gnd).
The importance of ground planes and several ground lines is highly dependant to the frequency of application, it could be observed in some layout ground planes sorrounds the circuitry in order to prevent stray charges, where it could highly degrade the dc biasing of the circuitry, hope this helps.
its better to have separate ground plane for digital and analog due to current injection introduce in digital circuit .Which can cause substrate noise where it would higly degrade performance analog circuitry .
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