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Electronics => Beginners => Topic started by: lk.dgironi on January 17, 2023, 10:24:34 am

Title: Question about mixed signal board with multiple power and ground
Post by: lk.dgironi on January 17, 2023, 10:24:34 am

Hi all,

I'm designing a mixed signal board with DC-DC converters and so different power GND references.
I've 4
- GND_power (+24V), goes in to DC/DC and LDO regulators and is also used by the low current/low speed mosfet digital output (opto-isolated from MCU)
- GND_MCU (+3.3V), used for digital logic
- GND_analogout (+-12V and +24V), low speed output, used for voltage output (+-12V low current) and current output (+24V 0..20mA) (opto-isolated from MCU)
- GND_digitalinput (opto-isolated from MCU), low voltage low current encoder input

I would like to use 2 or 4 layer stack.
If I'm going to use 2, the stack will be 1:SIG/PWR, 2:GNDs
If I'm going to use 4, the stack will be 1:SIG, 2:GNDs, 3:GNDs, 4:SIG/PWR

Is it a good practice to split the GND layers in big polygons area containing the used GNDs?

Thanks!

Edited: find example at post below https://www.eevblog.com/forum/beginners/question-about-mixed-signal-board-with-multiple-power-and-ground/msg4645156/#msg4645156 (https://www.eevblog.com/forum/beginners/question-about-mixed-signal-board-with-multiple-power-and-ground/msg4645156/#msg4645156)

Title: Re: Question about mixed signal board with multiple power and ground
Post by: jonpaul on January 17, 2023, 11:01:11 am

Mixed sig is vague: exactly what is the device/board?

SA, DSP, ADC? DAC? Audio? Video? Data acq?

Freq band eg Audio, DC, RF, microwave?

schematic? Draft PCB image?

DC-DC is your design? Module? China unk ?

Jon

Title: Re: Question about mixed signal board with multiple power and ground
Post by: Siwastaja on January 17, 2023, 11:58:36 am

Single ground, full ground plane with no cuts or other bullshit tricks.

Physical separation (even just a few mm) of sensitive traces from noisy parts.

Ground splits are used for DC (or low frequency) small signal (millivolts) accuracy in presence of larger currents, but make EMC more difficult.

Title: Re: Question about mixed signal board with multiple power and ground
Post by: lk.dgironi on January 17, 2023, 01:04:08 pm

Thank for reply.

It's a encoder reader to voltage/current/CAN/SPI converter with configurable UART interface.
Unluckily I can not share the schematic for this board. The high level block to convert Encoder input to Voltage is the following chain: IN->Optocoupler->MCU->Isolator->DAC->Op Amp->OUT
Encoder reading freq max will be 100kHz, but usually will work at 5kHz.
DC-DC are Traco Power.

Title: Re: Question about mixed signal board with multiple power and ground
Post by: Siwastaja on January 17, 2023, 01:09:34 pm

Measure current differentially over the shunt. Measure voltage differentially using sense wires, from separate sense plug posts. As a result, ground DC currents do not matter. You can use one contiguous ground plane over the whole thing, for best EMC and ease of design.

Title: Re: Question about mixed signal board with multiple power and ground
Post by: lk.dgironi on January 17, 2023, 01:11:56 pm

Thanks @Siwastaja
So I should bring GND_MCU, GND_analogout and GND_digitalinput as wires. Then use GND_power as layer 2 ground plane. Right?

Title: Re: Question about mixed signal board with multiple power and ground
Post by: Siwastaja on January 17, 2023, 01:17:23 pm

I would route signals that drive optocoupled LEDs as pseudo-differential, i.e., do not use planes, planes only introduce capacitive coupling between the grounds, which is something one usually tries to minimize when they actually need isolated interfaces.

If on wire, use twisted pair for example connecting from MCU and adjacent GND pin to the optocoupler LED + and -. If on traces, just two closely routed tracks.

And of course, do this only for something you are actually going to isolate. The grounds never connect within your product, right?

Title: Re: Question about mixed signal board with multiple power and ground
Post by: lk.dgironi on January 17, 2023, 01:22:24 pm

Optocoupled LED signals as pseudo-differential, get it!

It's not on wire, it's on trace, all in the same PCB.

Yes, GND signal are never connected within the board, I mean I keep GND_power, GND_MCU, GND_analogout, GND_digitalinput separated on PCB and outside the PCB. To isolate it I use
- DC-DC converters (for power stages)
- optoisolators (digital in/out)
= digital isolator (SPI to DAC for analog out)

Title: Re: Question about mixed signal board with multiple power and ground
Post by: lk.dgironi on January 18, 2023, 09:24:10 am

I make an example with 3 different ground. This is of course an example, you have to think as something a little more complex, not a very high switching freq (max 100kHZ on some input), neither high voltage or current.

GND1: main power +24V ground
GND2: +12V ground
GND3: +5V ground

example schematic attached

2 Layer stack (images with prefix 2-1)
L1 : signal / power
L2 : GND1 + GND2 + GND3 splitted

4 Layer stack version1 (images with prefix 4-1)
L1 : signal
L2 : GND1 + GND2 + GND3 splitted
L3 : GND1 + GND2 + GND3 splitted
L4 : signal / power (only power is traced for this simple example)

4 Layer stack version2 (images with prefix 4-2), same as 4-1 except for layer 3
L1 : same as 4-1
L2 : same as 4-1
L3 : GND1
L4 : same as 4-1

Of course there are other stack and plane option. What do you think? I'm using the 2 layer one, with doubt to change it to 4 layer version 1.