Separating Analog and Digital Ground

[dreamyboy_999]

I am designing a circuit in which I have only one reference voltage (power supply). I have some digital stuff in the circuit. I have to measure some nodes in the circuit using and ADC and report it to the digital part. The digital part is creating cross talk on my analog nodes. I was wondering how can I separate the digital and analog grounds knowing that I only have one reference voltage in my design.

 

[crutschow]

A few recommendations:

Keep the analog and digital circuits separate from each other as much as possible, and use separate ground planes for the analog and digital circuits.

The two planes should be tied together at only one point, likely at the ADC since the ADC has both analog and digital signals. The digital side of the ADC should be over the digital plane, and the Analog side over the analog plane.

Tying the two planes together through a ferrite bead filter can help prevent noise from the digital getting into the analog.

Never run any digital signals over the analog ground plane or analog signals over the digital ground plane.

Use a ferrite bead fllter between the analog and digital power, and have separate decoupling capacitors on the digital and analog side of the power. 0.1µF ceramic decoupling caps should be connected directly from all IC power pins directly to their respective ground planes.

 

[frozenfire_981]

i don't think that digital and analog grounds should interfere with each other. can u post some circuit diagram ?

 

[dreamyboy_999]

A few recommendations:

Keep the analog and digital circuits separate from each other as much as possible, and use separate ground planes for the analog and digital circuits.

The two planes should be tied together at only one point, likely at the ADC since the ADC has both analog and digital signals. The digital side of the ADC should be over the digital plane, and the Analog side over the analog plane.

Tying the two planes together through a ferrite bead filter can help prevent noise from the digital getting into the analog.

Never run any digital signals over the analog ground plane or analog signals over the digital ground plane.

Use a ferrite bead fllter between the analog and digital power, and have separate decoupling capacitors on the digital and analog side of the power. 0.1µF ceramic decoupling caps should be connected directly from all IC power pins directly to their respective ground planes.

my problem is that I have only one power supply in my design, so how can I have two separate power sources for analog and digital part ?

 

[andre_luis]

my problem is that I have only one power supply in my design, so how can I have two separate power sources for analog and digital part ?

The tips crutschow gave concerns just to the placement scope, and mainly refers to reference Ground location.

Although not recommended, you can share the same Power supply, but taking care to ensure proper decoupling filtering for dump switching effects of digital circuitry.


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[dreamyboy_999]

The tips crutschow gave concerns just to the placement scope, and mainly refers to reference Ground location.

Although not recommended, you can share the same Power supply, but taking care to ensure proper decoupling filtering for dump switching effects of digital circuitry.


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So in my case, I use the same Supply node for analog and digital parts (VDD) and use seperate ground connections for digital and analog parts and connect them only at ADC using a ferrite bead ?

 

[andre_luis]

So in my case, I use the same Supply node for analog and digital parts (VDD) and use seperate ground connections for digital and analog parts and connect them only at ADC using a ferrite bead ?

I don´t know more details about your design concerning to required precision, but another approach could be taking a separate reference for A/D, such as a 2,5v LM431 device, which probably could also yield some additional rejection for disturbance becoming from power supply.


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[aashishsharma]

You should use separate power supply for analog and digital part then only there is any use of different ground planes. While designing circuit board make sure to use multiple layers( I would recommend 4 layer would be sufficient).
If you dont want to go into complexity of using different layers at-least make sure your board layout is done properly with separate analog and digital section/ proper track widths/lengths and tracks surrounded by ground plane
thanks

 

[dreamyboy_999]

I don´t know more details about your design concerning to required precision, but another approach could be taking a separate reference for A/D, such as a 2,5v LM431 device, which probably could also yield some additional rejection for disturbance becoming from power supply.


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U know what my problem is .... I am trying to power everything up using a solar panel, I also need to sample some nodes at my circuit. So I just have one power supply! (reference voltage)... some nodes I am trying to measure have variations of order mili volts..but the cross talk from digital parts on these nodes are so high.....I was just wondering is it possible to create two isolated power supplies out of one ? for example using some kinda transformers or something ?

- - - Updated - - -

You should use separate power supply for analog and digital part then only there is any use of different ground planes. While designing circuit board make sure to use multiple layers( I would recommend 4 layer would be sufficient).
If you dont want to go into complexity of using different layers at-least make sure your board layout is done properly with separate analog and digital section/ proper track widths/lengths and tracks surrounded by ground plane
thanks

Thanks for ur reply..my problem is I have only one power supply entering my circuit...(it is just a solar panel providing energy to the whole circuit..) so in this case having a common vdd but different ground connections for digital and analog parts won't help ? (of course connecting them at ADC)

 

[andre_luis]

...I was just wondering is it possible to create two isolated power supplies out of one ? for example using some kinda transformers or something ?

As said before, you don´t need properly another power supply, but a stable A/D reference could suffice for most applications.


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[aashishsharma]

No you need to have common ground now. Try to isolate analog digital and rf circuit(if any) on your circuit board. Playing with track width might be helpful.
also crosstalk and coupling between adjacent signal can be reduced by surrounding them with ground layer.

 

[crutschow]

my problem is that I have only one power supply in my design, so how can I have two separate power sources for analog and digital part ?

My last paragraph covers that.

Use a ferrite bead fllter between the analog and digital power, and have separate decoupling capacitors on the digital and analog side of the power. 0.1µF ceramic decoupling caps should be connected directly from all IC power pins directly to their respective ground planes.

Decoupling the analog and digital power as described should help minimize the noise coupled between the two even if they come from the same supply. Run the power first to the analog side and then, through the ferrite bead, to the digital side.