brain-dead
Junior Member level 3
Hi Everyone,
I have looked through many threads and researched on the net and have not found a clear answer to the following question.
How to ground a shielded cable when the PCB is enclosed in a plastic case.
My design is a high gain microphone amplifier, the board has a solid ground plane underneath and signal traces on the top. The circuit basically contains two quad op amps, a filter then multiple gain stages. The circuit works great until I put a long length of cable for the microphone > 1m. For testing I soldered an electret mic to the board with a short length of twisted pair, all was fine. I need to keepp the signal free from EMI/RF as the gain is so high
Alot of answers say ground the cable to the case at the point of entry, but I have a plastic case.
I am wondering which option to use:
1. Leave the cable unshielded
2. Connect the shield of the cable to 0V on the pcb side and not the microphone
3. Connect the shield to the microphone 0v and not the PCB side
4. Connect the shield to the PCB 0v, however place an inductor in series with it
5. Have a nervous breakdown and write a letter to someone requesting that RF transmissions be reduced
The problem is RF pickup, mostly intermittent, I performed two experiments:
1. Stuck a spec analyser round the board with the short twisted pair mic < 6' length
No RF noise anywhere
2. Stuck a long cabled mic > 1.5m and then tested with the analyser
Intermittent RF of all things weird and wonderful. Noise from 100MHZ upto about 900. I have one of them weather stations outside that transmits weather data to a console in doors, it is on 868MHz, that got picked up very well
Hopefully someone can shed some much needed light on this cold, dark and damp situation for me.
Thanks in advance
Brain-Dead
I have looked through many threads and researched on the net and have not found a clear answer to the following question.
How to ground a shielded cable when the PCB is enclosed in a plastic case.
My design is a high gain microphone amplifier, the board has a solid ground plane underneath and signal traces on the top. The circuit basically contains two quad op amps, a filter then multiple gain stages. The circuit works great until I put a long length of cable for the microphone > 1m. For testing I soldered an electret mic to the board with a short length of twisted pair, all was fine. I need to keepp the signal free from EMI/RF as the gain is so high
Alot of answers say ground the cable to the case at the point of entry, but I have a plastic case.
I am wondering which option to use:
1. Leave the cable unshielded
2. Connect the shield of the cable to 0V on the pcb side and not the microphone
3. Connect the shield to the microphone 0v and not the PCB side
4. Connect the shield to the PCB 0v, however place an inductor in series with it
5. Have a nervous breakdown and write a letter to someone requesting that RF transmissions be reduced
The problem is RF pickup, mostly intermittent, I performed two experiments:
1. Stuck a spec analyser round the board with the short twisted pair mic < 6' length
No RF noise anywhere
2. Stuck a long cabled mic > 1.5m and then tested with the analyser
Intermittent RF of all things weird and wonderful. Noise from 100MHZ upto about 900. I have one of them weather stations outside that transmits weather data to a console in doors, it is on 868MHz, that got picked up very well
Hopefully someone can shed some much needed light on this cold, dark and damp situation for me.
Thanks in advance
Brain-Dead