I have a friend that is working on a project and has run into some weird behavior. He has a PCB that has the control logic for an external relay as well as an AD converter that is taking some external inputs. The entire thing is powered by a battery. The entire thing worked great until a recent redesign. Now, the AD converter stops working, usually right after the external relay is fired. There are a couple of changes that I'm focusing on as being the cause of the problem.
First, the AD input and Analog ground lines have been moved to exit the same port as the control lines of the solenoid. Also, the ad converter is now monitoring the battery voltage as well.
I'm wondering if, since the ADC input lines are now in close proximity to the relay control lines, that when the solenoid is powered, there is kickback current inducted on th ADC input lines due to mutual coupling.
Also, some of the measurements are very sensitive and he found that tying the battery ground to the ADC analog ground caused the measurements to be inaccurate after the layout change, but not before. The analog ground for the inputs comes from the measured devices through the port that runs close to the relay control lines so I'm wondering if the effects of mutual inductance are causing the measurements to be inaccurate when the battery ground is tied in but not when only using analog ground (the mutual inductance is affecting the high and ground lines going to the ADC equally so it cancels them out)? Can anyone explain this to me a little better?
I suggested he put TVS diodes on all of the inputs to the ADC that tied them to analog ground (not battery ground) and set the clamp voltage to 5V. Any other ideas on what to do or troubleshooting experiments to try?
Thanks for your input!
First, the AD input and Analog ground lines have been moved to exit the same port as the control lines of the solenoid. Also, the ad converter is now monitoring the battery voltage as well.
I'm wondering if, since the ADC input lines are now in close proximity to the relay control lines, that when the solenoid is powered, there is kickback current inducted on th ADC input lines due to mutual coupling.
Also, some of the measurements are very sensitive and he found that tying the battery ground to the ADC analog ground caused the measurements to be inaccurate after the layout change, but not before. The analog ground for the inputs comes from the measured devices through the port that runs close to the relay control lines so I'm wondering if the effects of mutual inductance are causing the measurements to be inaccurate when the battery ground is tied in but not when only using analog ground (the mutual inductance is affecting the high and ground lines going to the ADC equally so it cancels them out)? Can anyone explain this to me a little better?
I suggested he put TVS diodes on all of the inputs to the ADC that tied them to analog ground (not battery ground) and set the clamp voltage to 5V. Any other ideas on what to do or troubleshooting experiments to try?
Thanks for your input!