Please any advice for connecting shield for my project ?

Thread Starter

rdpdo

Joined Nov 2, 2020
11
Hello everyone,

I have a project consisting of making an ADC main board with a raspeberry PI4B which will allow me to measure differential voltages from a MEMS sensor on remote micro-boards.

These boards are connected to my main ADC board with 4-wire AWG32 cables + shield. The wires transport differentials analog outputs from MEMS IC.

I wonder how to connect the shield on the side of the ADC card and on the side of my MEMS micro-cards.

Could you give me your opinion please?

Thank you !
CarteADC.png
 

MisterBill2

Joined Jan 23, 2018
18,176
What I think I see in the drawing is that on that small PCB there are connections of V+, signal + and signal -, and an "analog ground." so no clue as to where to connect the cable shield or the power (V+) return.
If that small pCB has any polarized capacitor, an examination might reveal where the negative side is connected. If not, I suggest that the "analog ground" is also the power common.
And I am guessing also that the supplier of those small PCBs knows nothing at all about them except for the shipping weight and the price.
 

MisterBill2

Joined Jan 23, 2018
18,176
I read the reference and it's summary. What I can tell you, based on many years of designing industrial test machinery, is to start with the shield connected as described, only at the source. If noise is still a problem, then add connections to a solid system ground experimentally, and usually one connection will provide much better noise blocking than the others.

In this particular application there is no choice because there must be a return connection for the device power.
What I do not see at all is any power source return connection on either end of the cable.
Does the TS have any information on either of the items in the drawing?? Specifically, power to external devices information.
 

Thread Starter

rdpdo

Joined Nov 2, 2020
11
Hello,

Thanks you for your advice. I will read the application note.

Do you think it would be better if I enclose my main board in a metal case and connect shield to the case ? But then do I connect it to GND too ?

The small pcbs will be designed by me and the schematic is this one.

Sans titre 1.jpg

The datasheet say to design it in 4-layers . It say that even if the mems (https://vespermems.com/products/va1210/) is not vulnerable to FR pickup, the traces VOUTP, VOUTN and VDD can be if they are close to RF sources like bluetooth or Wifi module.

In my case I will not use any bluetooth nor wifi but I hesitate to still use 4-layers... Here is the example of the 4-layer pcb in the design application note.

Sans titre 2.jpgSans titre 3.jpg

Sans titre 4.jpgSans titre 5.jpg
 

MisterBill2

Joined Jan 23, 2018
18,176
I see that the PCB has a connection for Vdd, and ""ground" connection. (pin3) which really should be marked as common or V- or something other that "ground", which on the 4 layer board corresponds to the layers that form the shield for the two circuit layers.
The reality is that there needs to be a return path for the Vdd supply that powers the active components. That can easily be the shield, or much less easily be an extra conductor.

If this project is for a "one-off" home project, I suggest dispensing with the 4 layers and instead fabricating a small shield box. But if it is a commercial venture with a production run of hundreds then go with the 4 layer PCB. But you still need to provide a return path for the +Vdd. The ground symbol in the schematic is rather misleading, because there are no other connections
 

StefanZe

Joined Nov 6, 2019
191
Hi,
Connect the shield on your PCB to your AGND with jumper pads. This way you can test your design with and without a connection to shield.

The return path is AGND (this is a little bit misleading). PIN 4 is connected to the ground symbol.
 

Thread Starter

rdpdo

Joined Nov 2, 2020
11
Good morning,

I slowly continue my reflections on my project... Even if I am an amateur I would like to make a device as clean as possible. After some reading I made, could you give me your opinion on these ideas:

I specify that in my main card I will have 20MHz for the CLK of the SPI bus. I only have WiFI and FM/AM radio signals in my home. My sensor only sends a few 10ene of mV on the differential signals. The currents are of the order of 0.1mA.

- I plan to use an onboard battery (1.8V) in my sensors to power them rather than deporting the power supply from the main board. This way there would only be the differential signals to be transmitted which I could do on a 1 pair twisted shielded cable.

- I also plan to put my sensors in a metal shield and use an EMC metal gland to connect the cable to the shield. I plan to connect the mass of the battery of the sensor to the shield of the cable and also to the shield of the box.

- I also plan to use a battery for my main board so as not to pollute with the 50 Hz and the DC/DC choppers.

- I plan to connect the ground of the battery of my main board to the shield of the cable from the sensors but only with a capacitor so that there is no low frequency connection (and therefore avoid the ground loop) but an HF connection.

What do you think ?
 

Thread Starter

rdpdo

Joined Nov 2, 2020
11
My sensor is to sense acceleration.

My ADC have differential inputs so my idea is this but I dont know if it is correct ? It multiply the number of GND and so perhaps it is a bad idea ? (I have 32 sensors)

Sans titre 1.jpg

Else I can do that wich is less expensive by taking power from main board. In this case there is only one GND so perhaps it is better ?
Sans titre 2.jpg

Thanks you very much !
 

MisterBill2

Joined Jan 23, 2018
18,176
Differential signals not referenced to "ground" are still, usually, referenced to some common point, at some voltage level.So even true differential signals going to an isolated input must be constrained in their offset voltage. BUT that does not mean that THEY do not need shielding. The shield needs to be connected to a point of the circuit where no signal voltage is present. That point is often called "ground".
For the arrangement in Post #12, the shield would, at each end, be coupled to the supply negative terminal at each end. If there is an obvious voltage difference during correct operation, then connect to that point with a capacitor with a value presenting a low impedance at the signal frequency.
 

MrChips

Joined Oct 2, 2009
30,712
The sensor world has gone digital. Analog information is digitized in the sensor and the output is digital.
Move up to the world of digital sensors and you don't have to mess around with ADCs, cable problems and analog noise.
 

Thread Starter

rdpdo

Joined Nov 2, 2020
11
The sensor world has gone digital. Analog information is digitized in the sensor and the output is digital.
Move up to the world of digital sensors and you don't have to mess around with ADCs, cable problems and analog noise.
Yes I know but I cannot synchronize multiple digital accelerometer ... There will be lag between them. The only way is to use analog accelerometer and syncronizing ADC conversion on them...
 

MisterBill2

Joined Jan 23, 2018
18,176
There is a time and place for both varieties, and they are seldom the same. And there is always the delay with digital, and always the frequency response concern with digital.
Consider that automotive crash testing is done with ANALOG accelerometers, at least at one major US auto company. Digital is fine for monitoring machine vibration. Not so fine for real time phase relationships.
 
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