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Newbie Question: Ground Plans

campos.rr

New Member
Hi all,

I'm new designing PCB's and I have a problem with my board.

I used a CNC machine (http://www.accuratecnc.com/A362.html). The prototype is two layers where vias were connected with wire. All the components are SMT. I never used something like "ground plane" in the design. I just connected the circuit in Multisim and then transfer it to Ultiboard. Then routting connected pins and exporting to the CNC-362software. The machine do the job. Routes were 10milis wide and separation also 10milis.

The PCB includes an amplification analog stage (with opam's) and a digital processing stage. A PIC24f reads analog signals from the ADC, then it apply digital filters, and finally it sends the data to PC by USB link). The PC software plots in real time the read signals. The design also includes a switching regulator (MCP1727) reducing the 5V in the USB port to 3.3v required by the PIC24F. The PCB looks like a memory stick. Image attached.

Apparently everything works well on desktops. However, when I use a signal generator (TEK-AWG5000) to test the design on a laptop, the signal is a mess. Then, I used an oscilloscope (TEK-TDS1000B) to see the signal at the input pin on the microcontroller. Surprisingly, the signal becomes completely clean when I put the negative of the oscilloscope tip on the ground of my design (specifically, on the metal USB connector).

I read something in the web about ground plans and impedance couplers, which could be the solution to my problem.

I would appreciate any help you can give me.

Best regards
 

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peufeu

New Member
Mixed signal design is a lot easier if you have a good ground plane.

I think you have some EMC problem here. Basically your desktop chassis, the signal generator's chassis, and the scope's ground, are all grounded (to safety ground), but the laptop is not grounded.

Some common mode current will always want to flow between whatever is plugged at the input of your circuit and the computer case. This current can come from transformer interwinding capacitance, antenna effects, etc.

If both ends are grounded (desktop chassis and signal generator), no problem, the current will flow in the ground wires.

With an ungrounded source (laptop), common mode current must flow in your device's ground.

A big fat ground plane has an extremely small impedance, so there would be no problem except at RF, but the common mode ferrites take care of this.
But you have skinny long ground traces (I can't even locate which one is ground) which are high impedance, thus you'll get nasty parasitic voltages in your ground. Basically all the "GND" voltages in your circuit become random, which is not so cool.

When you connect the scope's chassis to the thing, the scope ground (via a fat coax shield) becomes the path of least impedance, and it sinks the common mode current. So it works.

The fact that it works when you put the ground clip on the USB connector makes me think that the laptop emits the offending current, probably due to the interwinding capacitance in the laptop's power brick.

Advice :

As a quick fix to this design, just take a fat piece of wire (lampcord will do) and connect your circuit's INPUT GND to the USB shield. Make the wire as short as possible (ie, straight).
The USB spec says you're not supposed to do that, but noone's going to sue you anyway. Also consider clipping a ferrite on the USB cable, so your circuit doesn't become a radio receiver.

Next time, read up on EMI/EMC, use a nice ground plane, put some ferrites, etc. If you want input and laptop grounds to be separate, use optical isolation, or at least a differential input amplifier with good CMRR.
 
Last edited:

campos.rr

New Member
Thank you for your time peufeu,

I find your response very comprehensive and instructive. I will try to expand my area of ground plans in the design by a thick cable and/or other means, as you suggest and post them the result.

Again thank you very much. ...

By the way, the GND line is the one in the down side of the USB connector at the right (4th pin from up to down) and yes, it is like a long Spaghetti

Best Regards ...
 

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