HamRadioAddict
18-01-2006, 14:03
Hello All,
Really enjoyed reading the thread “28X ReadADC10 Problem – AGAIN! I hate to kick a dead dog but there was one thing that instantly caught my attention.......somewhere around reply 27(ish). This was the comment about the temperature sensor instrument cable shielding earthed or grounded at BOTH ends.
In my line of instrumentation work, where we work with the usual industry standard small signal sources such as 4–20 mA, 0-5VDC & 0-10VDC, earthing the screen or shield of the instrument cable is only done at ONE end. And that is at the source or supply end. This would be at the PICAXE or OpAmp end not out at the temperature sensor end. Earthing at both ends can cause unwanted current loops. These are unwanted currents flowing along the instrument cables shield. These currents could be anything from pulsed DC or 50 Hz etc. etc. (60Hz elsewhere;-) what ever decides to hitch a ride and use the shield as return path. When and if these currents are strong enough, they will cause noise on the very signal wire and circuit you are trying to shield.
I’m not sure if shielding at one end only was practical in the temperature sensor, OpAmp
PICAXE arrangement but thought I’d throw my 2 cents in just for the heck of it. Sorry I didn’t respond earlier but I didn’t read the post until the problem was solved.
73’s
Steve VK6HV
Really enjoyed reading the thread “28X ReadADC10 Problem – AGAIN! I hate to kick a dead dog but there was one thing that instantly caught my attention.......somewhere around reply 27(ish). This was the comment about the temperature sensor instrument cable shielding earthed or grounded at BOTH ends.
In my line of instrumentation work, where we work with the usual industry standard small signal sources such as 4–20 mA, 0-5VDC & 0-10VDC, earthing the screen or shield of the instrument cable is only done at ONE end. And that is at the source or supply end. This would be at the PICAXE or OpAmp end not out at the temperature sensor end. Earthing at both ends can cause unwanted current loops. These are unwanted currents flowing along the instrument cables shield. These currents could be anything from pulsed DC or 50 Hz etc. etc. (60Hz elsewhere;-) what ever decides to hitch a ride and use the shield as return path. When and if these currents are strong enough, they will cause noise on the very signal wire and circuit you are trying to shield.
I’m not sure if shielding at one end only was practical in the temperature sensor, OpAmp
PICAXE arrangement but thought I’d throw my 2 cents in just for the heck of it. Sorry I didn’t respond earlier but I didn’t read the post until the problem was solved.
73’s
Steve VK6HV