Tunelabguy
Full Member level 5

While debugging a little noise problem in an amplifier, I noticed that the circuit is acting like a microphone. Vibrations in the PCB are showing up as noise in the final output.
The circuit is all surface mount, 4-layer, using CMOS MCP604 rail-to-rail op-amps with a very well-regulated and filtered +5v supply. There are three cascaded AC-coupled non-inverting stages with a combined voltage gain of about 3000. The input is currently disconnected, but returned to ground through a 1 Meg resistor. AC coupling is through a 0.002 uF ceramic cap. But I see about half a volt of dirty line-frequency noise in the output.
If I hard-short the input to ground, the noise goes away. If I lift the board off the workbench, the noise mostly goes away. If I play a loud tone generator a few inches from the board, I can see the audio waveform in the output. If I tap the board with something hard, a spike appears with every tap. Softer taps produce smaller spikes. It is linear.
What could be causing this piezo-sensitivity? A bad solder joint? A bad MCP604 amp? A bad ceramic cap or resistor? Whatever it is, I need to get rid of it.
-Robert Scott
Hopkins, MN
The circuit is all surface mount, 4-layer, using CMOS MCP604 rail-to-rail op-amps with a very well-regulated and filtered +5v supply. There are three cascaded AC-coupled non-inverting stages with a combined voltage gain of about 3000. The input is currently disconnected, but returned to ground through a 1 Meg resistor. AC coupling is through a 0.002 uF ceramic cap. But I see about half a volt of dirty line-frequency noise in the output.
If I hard-short the input to ground, the noise goes away. If I lift the board off the workbench, the noise mostly goes away. If I play a loud tone generator a few inches from the board, I can see the audio waveform in the output. If I tap the board with something hard, a spike appears with every tap. Softer taps produce smaller spikes. It is linear.
What could be causing this piezo-sensitivity? A bad solder joint? A bad MCP604 amp? A bad ceramic cap or resistor? Whatever it is, I need to get rid of it.
-Robert Scott
Hopkins, MN