Analog ground on P1? (VSSA)

For a project, I need an isolated ground reference. Looking at the P1 documentation, there is only one type of Ground connections. Looking at the STM32F205 datasheet there is an Analogue Ground (VSSA) available. Could it be that this pin is broken out to one of the GND pins without being documented?

Hello,

Let me ping someone that might be able to help, @rickkas7 are you able to help?

Kyle

Hi @jenschr

The Vssa pin on the STM32F205 is brought out separately just so you can add decoupling caps to it separate from the main Vss (GND) pins. You could not connect the Vssa pin to any other voltage reference other than Vss (GND) and have the device work.

Can you tell us what kind of isolated ground reference you are looking for to use in your project? I don’t think you are thinking about this pin the right way–it is just so the A2D sees less power supply noise with separate bypass caps.

Hi @bko,
I have a machine that has a 20kV spark igniter that also needs to read precise analoge voltages (0-2.048V) using a 24-bit ADC. The igniter sparks to the common ground and causes disturbance, so a decoupled ground would be preferred for use with the ADC.

That is a very challenging problem. I think you will need a great deal of shielding and signal conditioning in order to get anywhere near your goal of 24-bit ADC readings.

Even if you can isolate the 20kV spark from the rest of the system, it is still broadcasting across essentially the entire radio band and has the potential to interfere with the WiFi connection and all nearby sensitive electronics every time it fires.

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Is the “igniter” read before the spark is fired? Reading the igniter BEFORE the spark would alleviate your noise problems. You still need to make sure you properly protect your electronics (Faraday shield?) to minimize electrical noise though.

Yeah. I realise that this is a problem. I think I’ll just try to work around it with code actually since it produces such deviations in the signal that I should be able to just filter it out there.

Thanks guys!