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I am working on a PCB design with STM32 MCU. On the PCB there will be 2 MAX490 transceivers and one bluetooth module. Communication speed will be 921.6 Kbit/s for all communication elements. Bluetooth module uses 3.3V as MCU and transceivers use 5V supply voltage. Board will be powered by a 12V battery.

So i need 2 buck converters for to step-down 12V to 3.3V and 5V. But i am concerned about their EMI performances. Would these converters make any interference on the comm lines?

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  • \$\begingroup\$ Whats stopping you running everything on 3v3? At least you'll remove 1 further switcher. Alternatively you could switch 12v to 5v then linear regulate 5v to 3v3. As for interference that entirely depends on your filtering design and pcb implementation \$\endgroup\$ – theguitarfreq May 24 at 12:36
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    \$\begingroup\$ Sure they will and so it's the engineers job to design the PCB and schematic to minimize the interference to acceptable levels for decent comms integrity. \$\endgroup\$ – Andy aka May 24 at 12:36
  • \$\begingroup\$ @AndyWest Transceiver input supply voltage range is 4.5V to 5.5V. I don't they would work decently on 3.3V. \$\endgroup\$ – Teo Laferla May 24 at 12:41
  • \$\begingroup\$ @TeoLaferla Yes but could you use an alternative such as: mouser.co.uk/Maxim-Integrated/RS-422-RS-485-Interface-IC/… \$\endgroup\$ – theguitarfreq May 24 at 12:43
  • \$\begingroup\$ @AndyWest The sensors connected to these MAX490s are using full-duplex comm. The ICs from the link you've shared are half-duplex. \$\endgroup\$ – Teo Laferla May 24 at 12:48
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It can cause communication issues if one side is earth grounded with an EMI filter to ground and the other has a loop antenna for cabling on a floating supply without earth grounding.

Even with differential drivers, receivers and a balanced cable, CMRR is compromised due to the leakage capacitance in the ungrounded side.

A workaround is to use two floating SMPS and a CM choke that operates >> 200 Ohms in the bandwidth of the Rx and CM noise or earth grounding both sides to re-direct leakage capacitance across the SMPS transformer.

I have proven this to PhD post-grad workers in their EE lab with 10yrs industry experience in SMPS using USB on a 1m cable at 1.5Mb/s USB1. This is similar to your application near 1Mbps.

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  • \$\begingroup\$ Would you recommend using a different board for step down circuits? \$\endgroup\$ – Teo Laferla May 24 at 14:25
  • \$\begingroup\$ Different from what? This issue is Common mode noise between two terminals. One is battery powered, what is the other side using? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 24 at 14:49
  • \$\begingroup\$ I mean another board for buck converters. Board and battery will be in a casing so i can choose the buy one commercial buck converter and just place it somewhere in casing and take power supply from this commercial buck converter. It wouldn't be a professional thing to do of course but i think a power switcher would make drastic noise on the MCU board. \$\endgroup\$ – Teo Laferla May 24 at 15:28
  • \$\begingroup\$ You have not defined current or power specs. OKI makes a 3 terminal SMPS 1A at digikey. but I asked what about the other end. Is it known>? or the Logic level interface ground? where does it come from? You need to show a block diagram of all the interfaces with DC , data and earth ground. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 24 at 15:44

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