PCB layout problems.
PCB layout problems.
(OP)
Hello Everyone,
I am fairly new to circuit layout. I've designed a simple strain gage amplifier with a 0 to 10 vdc output. The circuit operates as design with one problem. When put my hand near the sensor wires I get a shift in voltage.
1. What causes this?
2. How can I fix it?
3. What kind I do for the next revision to prevent it?
4. Any resources I can look up?
Circuit is a two layer, two sided SMT with poured ground plane on the bottom side.
Any help would be greatly appreciated.
Mario
I am fairly new to circuit layout. I've designed a simple strain gage amplifier with a 0 to 10 vdc output. The circuit operates as design with one problem. When put my hand near the sensor wires I get a shift in voltage.
1. What causes this?
2. How can I fix it?
3. What kind I do for the next revision to prevent it?
4. Any resources I can look up?
Circuit is a two layer, two sided SMT with poured ground plane on the bottom side.
Any help would be greatly appreciated.
Mario





RE: PCB layout problems.
RE: PCB layout problems.
Just a thought!
RE: PCB layout problems.
RE: PCB layout problems.
RE: PCB layout problems.
If you make a 6 layer board you better be prepared to shell out some righteous bucks and do not be surprised if you have additional problems. Have you ever tried to diagnose or troubleshoot a 6 layer board for EMI problems? Its a nightmare.
Good luck.
RE: PCB layout problems.
I don't disagree with anything that BUZZP has said, because he is correct. However, all of these suggestions whould be taken with a grain of salt because they might not work in you application! If you are operating in the GHz range, than 2 layers can NEVER work. You need to have all of the ground isolation that you can get!!!! But, if you are running a simple 555 timer that oscilates at 2 Hz, 6 layers is over-kill. I guess, what I am trying to say, is that we will all try to give you suggestions on what could "fix" your problem. Don't be alarmed if some of our suggestions seem to contradict. We all have prejudices based on our past experience.
Good Luck and keep us posted!!!
RE: PCB layout problems.
Just a thought!
RE: PCB layout problems.
In response to the first reply.
Yes, I am in a room with Flourescent lighting.
The wire to the sensor is a 6 conductor shielded cable, (about 2.5 feet)
I am using a differential instrumentation amplifier (Burr-Brown INA118)
Output is tied to a voltage divider for trimming, and then tied to a unity gain buffer.
For my next revision I will probably go with a 4 layer board. 2 power planes, and 2 trace layers (top/bottom)
What things can I do to improve my circuit.
Basically I have a 12 volt regulator and a charge pump to provide a -12 volts. These voltages power my op-amps and instrumentation amplifier. A 10 volt precision source provides a clean excitation to the strain gage sensor. Two pots adjust zero and span.
Thanks, Mario
RE: PCB layout problems.
RE: PCB layout problems.
to my mind you are on the right path with your bugfix descirbed in your post dated Apr 2nd. After reading your initial post I guessed that your circuit picks up some Hf and rectifies it (e.g. in the input circuit of the amplifier). That adding the capacitors was helpful shows that this guess might be true.
To my mind your application should work well with a double sided board with a ground plane on one side, if the ground plane is only interrupted for short jumps or small components. Anything else would only add unnecessary cost.
Additionaly try to separate the charge pump from the rest of your design as good as possible and use additional LC-filters for your power supply to the amplifier.
Try to limit bandwith to the frequency range you can expect in a signal coming from a strain gauge by a passive filter before feeding the signal to the amplifier.
The ground connections might als be important: avoid to run currents from the shield through the ground plane of your circuit.
A good resource on the web can be found in application notes from analog devices: http://www.analog.com
RE: PCB layout problems.
I know of a colleague in ran into something similar with a microcontroller resetting when his hand was within an inch or so. As a short term solution, I suggested a tantalum cap across the supply on the micro and the problem went away. They had already ordered 500 through hole boards so the kluge was easier than smt. I think your boss messed up by not ordering a proto, especially with smt for the reason mentioned above. I am sure your boss was trying to save a buck at your expense. As a quick fix, try kluging a tant 10uf cap right on the supply to the chip with minimal solder lengths to the ground plane. Drill a new hole in the board if you have too but make sure the runs are short. Good luck
RE: PCB layout problems.
I managed to put a ceramic capacitor across the output leads and this took care of the problem.
The change in output occured when I got about 2 inches from the cable, when I grabbed the cable the change was more significant. The circuit I design was based on an existing amplifier. One potential problem is that I have pin jumpers that allows for polarity reversal. Meaning the signal from the wheatstone bridge will have through traverse through the jumpers until finally reaching the instrumentation amplifier. On my next revision I will be putting the ins. amp. closer to the signal, including a capacitor and making a 4 layer board. Basically keeping the voltage supply/regulators on one side and amps on the other. Would this be a correct strategy. Also I did include a LP filter on the output of the ins. amp.
Mario
RE: PCB layout problems.
Good luck and keep us posted!!!