Isolation between digital 3.3V and Analog 3.3V supply

[sreejithrnair]

Hi @Peter BENSCH 

Greetings again.

I am using STM32H5 and STM32G4 series MCU in my project. As per the datasheets and application notes, it is recommended to isolate analog and digital supply voltages with a proper ferrite bead or a zero ohm resistor. But I had faced issues in EMC and EMI testing when isolated with the same. Can you confirm whether it is mandatory to isolate both analog and digital voltages? What is the proper design that you would recommend?

Regards,
Sreejith

[Andrew Neil]

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@sreejithrnair wrote:

it is recommended to isolate analog and digital supply voltages with a proper ferrite bead or a zero ohm resistor.

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A zero-ohm resistor wouldn't provide any isolation, surely?

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@sreejithrnair wrote:

I had faced issues in EMC and EMI testing when isolated with the same.

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So what issues, exactly, did you face?

What did you do to track down the source(s) of those issues?

Was that with a ferrite bead, or a zero-ohm resistor?

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@sreejithrnair wrote:

is mandatory to isolate both analog and digital voltages? Sreejith

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That very much depends on your application - what level of digital noise can your application tolerate on the analogue supply?

eg, if you're not doing precision, low-noise analogue measurements then it may not be needed at all.

[Uwe Bonnes]

The ferrite bead will smooth out high frequency ripply from the supply. Ripply on the analog supply will disturb analog measurements. If you can bear with this disturbance, you do not need the "isolation" But I wonder how this "isolation" give additional issues with EMC/EMI. Are you shure. the verrite bead is the culprit?

[Christian N]

Hello sreejithrnair,

Thank you for contacting STMicroelectronics.

Your post was escalated to ST Online Support Team for additional assistance.

Kind Regards,

Christian

ST Support