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Actually I am using mixed signal device. It has Analog supply pin(5v) digital supply pin(5v), analog ground, digital ground. Also, i need to apply an ac signal between VP and VN...So i need 2 sources..one 5v supply(for both analog and digital supply) and an ac source( i either use a transformer connected to mains or signal generator)..can u please tell me how to do wiring
First, you should use 2 differents power source, one for analog and the other for digital.
You may use only one source, but you need to separate the power line for AC current/voltage generated by digital switching, by using decoupling cap parrallel decoupling cap and a serie inductor on the analog power line.
Do you have only one mixed device on your board, or several ???
If you have only one mixed device, you should separate the analog and digital ground planes, and connect it together under your mixed device.
By this, you will prevent current return path of one plane to go throught the other plane (you generate 2 path, one with a higher impedance than the other, so since the current follow the least impedance path,....)
The question is how to connect the ground plane to the ground of your power source. If the analog is the polluting part, you should connect the analog ground to the power source ground. If the digital is the polluting part, you should connect the digital ground to the power source ground. The return path of the least polluting part will be forced to go througth the ground plane of the most polluting part...
The AC source is your input signal source for your analog world ??? If so, connect he ground of your input to the analog plane.
Hi norteast1...i have attached a circuit...in that i have shown various sources i am using...the one having circle is ac source..i just shorted all the negative sides of all sources together analog and digital grounds of mixed signal device..but i am not getting expected results..i think the ac input across VAP and VN is getting single ended..i.e . it is considering negative cycle..My IC should detect zero crossing across VAP and VN