Question about split ground plane

[danda821]

split ground plane

I am doing a four layer design. The second layer is the ground plane. Since there is ADC on the board, I split the plane into analog ground and digital ground. Since there is only one power supply for the whole board, the two gournd should connected at some point (power supply ground). Or can they be totally seperated?
But if they are seperated how the return signals (analog and digital) come back to the power gnd.

thanks.

 

[circuit]

split ground plane connector

I had looked at several ADI eval board incorporating A/D converters. they are four layer board with ground plane as second layer, they recommend having a single ground plane and just have vias at respective pins to ground. But yeah this is going cause interference of digital and analog signals.

i read somewhere that when you have separate analog and digital grounds, you connect them at the most critical point which is underneath the ADC, using a thin truck. I am not pretty clear in this, and hope that the experts in the forum can shed some light on this topic !

 

[sundance+dk]

split plane power supply return

You always have to connect the two ground place at some position, and the best position is around the most sensitive component.
The power supply ground must also be connected to the other grounds, but connect the power supply ground to digital ground. That way you minimize power supply interfering with you'r sensitive ADC.

I have previously made designs with an ADC/DAC (codec) for audio design, and used seperate digital and analog ground plane. I can recomend to place the connection under the adc component or at the bottom of it.
(see attatched note page 13) that shows well how to do that.

 

[btbass]

how to connect split planes using beads

The grounds should be connected at the power supply, (Star earth principal). It is the currents returning in the ground producing voltage noise that interfere with each other, connecting them at the power supply effectively isolates them from each other.

 

[jdhar]

split plane connection

No, the grounds should not be connected at the power supply. This is not practical, especially when doing split plane design in my opinion. Connect the planes under the critical component, as already mentioned. If the grounds are routed back to the power supply, this will create a large amount of noise due to current return paths. Also, watch out for digital traces crossing the break in the planes.

 

[chiara]

adc ground plane design

The best solution about this problem is, in my opinion, the experiment.

add pads to connect ground near sensitive points
and try to do it with measurements.

typically, most problem causing issue is the packed component layout
thus cannot make effective currnet loop avoiding noise.

I made 15 DAC/ADC combo board, it got terribly noisy analog part with normal
1 point GND connection.

if you have enough time to analyze the pcb,
lean to the capability of EM simulation ^^

good luck

 

[pvskt]

splitting digital and analogue ground planes

I do not agree that the planes should not be shorted at the power supply.

Infact its always the best practice to short the analog and digital grounds at the power supply entry point . Firstly, the analog and digital return currents do not mingle with one another and thereby preventing common mode noise. The question of long curernt return paths do not come into picture as jdhar has pointed out. The analog current is restricted to analog ground and the digital to the digital ground.

As a ,atter of practice both these grounds are shorted by means of a ferrite bead along with a MOV, near the power supply or the power supply connector i.e both analog and digital planes are shorted at the power supply ground. In case of any surges or inductive spikes, the MOV and ferrite jump into action thereby preventing them from propaghating inside the pcb.

 

[dearbabu]

analog and digital ground planes

when there is a split gnd plane , normally the digital noise should not propagate to the sensitive analog side ( in your case ADC).

If the card is big ( again roughly 6U) then there is no need for the split. Reason being you have a stable ground i.e less digital noise.

If your card is of size of 1U. Then think of connecting a Ferrite bead connecting the split gnd plane. Again look for the frequency of operation at the digital side and rough estimate of that will give good attenuation of Low pass filter. Look for DCR, to be minimum.

I hope this clears your doubts.

cheers.

 

[kimjin]

analog ground plane

what's MOV?

 

[cyberrat]

I join the grounds using ferrite beads to help get rid of any unwanted noise on them.