Spongebobgulasch
Posts: 3
Joined: Tue Nov 01, 2016 7:06 am

MCP3208 works fine at home but not at work

Tue Nov 01, 2016 7:36 am

Hey,

I just created a PCB like this one:
Image.
There's no capacitor plugged in right now, the holes on the PCB have been made for "backup-reasons" as i might need them someday (maybe now?).
Works fine testing with a battery at home, had this running for a few days. But it doesn't when I connect it to the frequency inverter which runs a Motor I want to watch over (The frequenzy inverter has an analog output on it, using an older version of this one here). In fact it works well for a couple of time but then the Raspberry Pi doesn't get any feedback from the MCP3208. The ADC is quite hot when it stops working. When I replug the cobbler it's working great again for some time - no need of restarting the Pi.
Just took a look on the analog output with an oscilloscope and it looks like this:
Image Image
Isn't that stable, might that be the reason for my problem? There are two more motors near to the Raspberry Pi B+ and the connected PCB (AC motor). Some said it looks like these two would get the system stop working - don't know. I'm using a screened cable to connect the PCB to the frequenzcy inverter. The Raspberry Pi is getting Power from the USB-Port of the screen which get's it's power from the same socket as the motors and the frequency inverter does.
Can attach my Code too if you want too.

Already tried to change the Raspberry Pi including the microSD, changed the PCB including the MCP3208 and the cobbler - don't know what to do getting it running stable for a couple of hours.

Very thankful for your answers and sorry for my bad english - I'm a german student and I can't get help on german websites :oops:

ait
Posts: 127
Joined: Sun Oct 30, 2011 6:51 pm

Re: MCP3208 works fine at home but not at work

Tue Nov 01, 2016 8:50 am

AGND and DGND must be connected
If no ground plane is utilized, both grounds must be
connected to VSS on the board. If a ground plane is
available, both digital and analog ground pins should
be connected to the analog ground plane. If both an
analog and a digital ground plane are available, both
the digital and the analog ground pins should be
connected to the analog ground plane. Following these
steps will reduce the amount of digital noise from the
rest of the board being coupled into the A/D converter.
Last edited by ait on Tue Nov 01, 2016 8:58 am, edited 1 time in total.

Spongebobgulasch
Posts: 3
Joined: Tue Nov 01, 2016 7:06 am

Re: MCP3208 works fine at home but not at work

Tue Nov 01, 2016 8:57 am

AGND and DGND must be connected
Thank you for your answer. I remember having these two connected when I startet testing around on a breadboard. But I'm not using the ADC measuring something connected to the Pi (like a Potentiometer). I want to measure a current from another device, therefore I connected the AGND (analogue ground) to the ground-potential of this device.
It's working fine that way with a battery, so this might not be the issue :/

ait
Posts: 127
Joined: Sun Oct 30, 2011 6:51 pm

Re: MCP3208 works fine at home but not at work

Tue Nov 01, 2016 9:07 am

sorry, i misunderstood your Post.

Spongebobgulasch
Posts: 3
Joined: Tue Nov 01, 2016 7:06 am

Re: MCP3208 works fine at home but not at work

Tue Nov 01, 2016 9:21 am

ait wrote:sorry, i misunderstood your Post.
But I tried to connect them - still works fine... Should i give it a try and connect them at work too?

ait
Posts: 127
Joined: Sun Oct 30, 2011 6:51 pm

Re: MCP3208 works fine at home but not at work

Tue Nov 01, 2016 10:43 am

No,i Just read the Datasheet again and found:
The
two grounds are connected internally through the sub-
strate, which has a resistance of 5 -10Ω.
maybe thats why the ADC gets hot.
I would try galvanically isolation with
http://www.ebay.de/itm/Stromsensor-20A- ... SwZJBX~ejU

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