Best way(s) to isolate ground and power between Analog & Digital sections

[pavjayt]

Hello,

I have a PCB design that has both Analog and Digital sections that run on single +/-15VDC Linear regulated power supply. I have a 5V/1A LDO for digital section that runs off the 15VDC. The only link I have between the input ground (which acts as analog ground plane) and digital ground plane is a 1.27MM width trace (shown in the attachment).

My design is a 4 layer PCB design, with layer2 acting as 5VDC and layer3 acting as digital and analog ground plane. The analog and digital ground sections are 2MM apart.

Any suggestions to help reduce ground noise in digital section and virtually isolate it from analog section?

thanks

 

[MikeMl]

The proper place to put the jumper is not at the power supplies, but near the ADC, or where the analog signals meet the digital signals. I also would not put it in the PCB. I would use a zero Ohm chip so I could test the isolation with the chip removed.

 

[pavjayt]

The proper place to put the jumper is not at the power supplies, but near the ADC, or where the analog signals meet the digital signals. I also would not put it in the PCB. I would use a zero Ohm chip so I could test the isolation with the chip removed.

Thanks Mike for your suggestion. From what you said, its better to put a 0ohm resistor between the 2 ground planes at the place where I have the signal that goes from digital section to analog section (which is the place where my DAC is). But if I remove my chip how can I have a ground connection to my digital section (and also for my 5V LDO)?

 

[ronsimpson]

Some ADCs have differential inputs. That helps get low noise readings. This way ground bounce is abstracted form the readings.

 

[crutschow]

With the jumper connection you can also try a low-resistance ferrite bead to connect the analog to digital ground instead of a wire. Depending upon the type of noise that may be better than just a solid wire.

 

[MikeMl]

Thanks Mike for your suggestion. From what you said, its better to put a 0ohm resistor between the 2 ground planes at the place where I have the signal that goes from digital section to analog section (which is the place where my DAC is). But if I remove my chip how can I have a ground connection to my digital section (and also for my 5V LDO)?

My point is that it is convenient to remove the chip to see if (in fact) the grounds are isolated. Continuing with Zapper's suggestion, putting a few Ohms (~10) there in place of a short sometimes helps, too.

 

[pavjayt]

Some ADCs have differential inputs. That helps get low noise readings. This way ground bounce is abstracted form the readings.

There are 3 places where signals from digital section are connected to analog section and vice versa.
1) from my 556 timer in my digital section
2) from my AD7533 DAC
3) Reference voltage (Rfb i/p) to my AD7533 from AD5292 digital potentiometer.

They all are crossing the sections at the same place close to each other.

I would like to minimize the ground noise between them as much as I can without going crazy on the design so that the outputs will have less noise.

 

[pavjayt]

My point is that it is convenient to remove the chip to see if (in fact) the grounds are isolated. Continuing with Zapper's suggestion, putting a few Ohms (~10) there in place of a short sometimes helps, too.

Zapper's suggestions sounds like a good idea. I already have ferrite beads of 80 Ohm @ 100MHz impedance with 10 mOhm DCR connected to both my 15VDC and -15VDC each. I will try to add one more instead of a solid trace connecting the grounds. Is that a good idea?