I am designing a PCB with 3 AD7853 A/Ds and 1 AD5323 D/A.
The board has a ground layer with separate analog and digital ground planes.
It also has to power layers.
I have the following questions. They get trickier as you go.
1) I understand from reading tech notes that GND pin 15 of the AD5323 D/A
should be connected to the analog ground plane. Is that correct?
2) It appears that GND pin 19 of the AD7853 A/D should be connected to the
analog ground plane as well as the AGND pins 6 & 12?
3) Should the other AD7853 A/D pins that are connected to resistors to ground
like POLARITY (14), SM1 (15), SM2 (16) also go to analog ground, rather than
the digital ground?
4) Should the DVDD bypass capacitor go to analog ground or digital ground?
I wonder because of 2).
5) Since I have four mixed digital/analog chips, how should I tie the analog ground
plane to the digital ground plane. I read in one tech note that I should tie the planes
together under each of the chips. Another place suggested I ties them in one place
with a 3.9 ohm resistor.
6) I have some space on my power planes where I could have additional analog ground planes
parallel to the one on the ground plane. Is this better? Worse (maybe create ground loops?)
Without knowing the details of this application, it is impossible to give clear advise. My experience using these types of devices is that a single ground plane works best and is the simplest to implement. This is especially the case where low currents are involved and the signal frequency is less than 10 MHz or so. What is very important is to partition the layout into the functional blocks and try to prevent current paths crossing over blocks. Implementing split ground planes is a complex task and can result is many more problems. There are many good books available on this topic and I could recommend ‘EMC for Product Designers by Tim Williams’.