Question about splitting the power plane

I am designing a four layer board with layer 1 and 4 for signal and 2 is ground plane and 3 is power plane ( followed this from analog devices eval board which i am partly replicating in my design since I am using the same ADC chip)

I am having a single 6V power coming in to the board using an adapter and this i am splitting into the required analog an digital vdd's using appropriate voltage regulators. there are 2 analog vdd's and 2 digital vdd's in my system. say analog 2.5V and digital 3.5V for my sensor and analog 3V and digital 3V for my ADC.

Now i have to split the power plane into 4 islands intellegently such that I can place the components and directly put a via from their respective pins to the voltage plane. Here should I have the analog planes together and digital planes together ? (I am having only one ground plane )

ADC has to be positioned at the end of the board with the digital signals going to a connector which sits on another board. Sensor has all its outputs on one side. i am attatching the way i have to place the components

I will run the board at a max clock of 20 MHz

Re: power plane issue

The use of separate voltage regulators for Analog and Digital circuits is a good idea, however, voltage regulators are not very good with HF signals, so use a small inductors (usually 1 - 10µH is enough) from connector pin power in to each of voltage regulators. Also use combination of capacitors (100nF ceramic + 10µF tantalum) close to regulators Vin and Vout.
I would leave both AVDD and DVDD as separate as possible ..

As far as AGND and DGND are concerned,you can connect both together at one point (with a thin truck) somewhere undernith A/D converter, or if this is impossible just leave the hole layer as one big ground plane ..

Re: power plane issue

thanks. yeah i am leaving the 2nd layer entirely as ground plane and putting via at respective points from the pins to this ground plane ( i am replicating this design from A/D eval board and even an engineer in AD suggested me this )

yeah i am having the inductors and proper bypass capacitors at the regulator input and output. now the issue i am still not clear is I am havig AVDD and DVDD for ADC chip and AVDD and DVDD for sensor chip. here can i split my PCB 3rd layer as

-----------------------------------------------------------------------------
DVDD (Sensor) | AVDD (Sensor) | AVDD(ADC) | DVDD(ADC) |
-----------------------------------------------------------------------------

and the analog signals from Sensor go directly to the ADC.

Re: power plane issue

DVDD (Sensor) cap/inductor| AVDD (Sensor) cap/inductor| AVDD(ADC) | DVDD(ADC) | and DVDD(sensor) can be fed of the AVDD of sensor ..

Re: power plane issue

thanks ! so on the 1st layer where i keep all the components, i can route the power signal (say 50 mils wide) from the connector to all the regulators input and then at the output of the regulator, i just put a via to the island of that Vdd.

power plane issue

As I understand you plan to use only one ground plane (for both Digital and Analog GND). It is not a good idea, you should split the GND plane to AGND and DGND and try to put the AGND under analog signals.

Re: power plane issue

shurilo said:

As I understand you plan to use only one ground plane (for both Digital and Analog GND). It is not a good idea, you should split the GND plane to AGND and DGND and try to put the AGND under analog signals.

Click to expand...

Yes I agree with you. If use a whole plane as GND (both analog and digital), there must be some other VCC or VDD planes are in parallel to the GND copper, that's a big trouble of EMI. It's a good idea to seperate the analog part and digital part as seprated as possible.

mike

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