I have about 72 of the following circuits on my board. The MOSFET and R2 comprise of the output stage and the pull up resistor is my input stage. The long length of wire in-between is the wire under test.
I've had this working for quite some time now and it works well. However, today I hooked up a wire harness of about 15 m in length - I wanted to see how much ringing was present. I noticed something else though - there was 1.3 MHz sinusoidal waveform present at the "Output Voltage" node. The peak to peak voltage was approximately 400mV(!). Here's the waveform in all it's glory:
When I measure the voltage at the MOSFET's drain, I get this:
The induced voltage seems to be gone when the MOSFET drives the line low. I then got another harness of about 4 m in length - the amplitude of the waveform was much reduced now - approximately 150mV but the frequency was the same. Twisting the wires reduced the amplitude (I forgot by how much. I will do the test again when I go back to work and post here) however, for us, twisting the wires isn't a solution. The harness is manufactured according to our customer's specs and if they dont' specify twisted wires, we can't have twisted wires!
My questions are:
- What could be causing this? Are these voltages being induced from somewhere? The environment isn't noisy.
- How can I ensure these voltages don't cause issues when my CPLD reads the voltage at the "Output Voltage" node.
NOTE: I know the o-scope screens don't show the frequency, I'm not sure why that is, but the frequency was a stable 1.3MHz. I remember this vividly.
How did I measure the signals? I connected my probe ground clip to a ground point near the pull up resistor, R1 (it's not visible in the schematic). I then I connected my probe to the "Output Voltage" node. This is the measurement at the input side. To see what was happening at the MOSFET side, I again connected my probe near a ground point that was near the MOSFET and measured at the MOSFET's drain. As the rising/falling times aren't excessively high, I don't think this is a measurement error - ESPECIALLY because the waveform's amplitude was reduced when I got a shorter wire.