Grounding Instrumentation project

[dreaquil]

So I've designed and implemented a precision instrument. One issue is that when the oscilloscope probe ground is connected to the board ground it reads perfectly. When I disconnect it, it fluctuates a lot. Does this mean I have to connect the board ground to mains earth for good readings? Whats causing this? I'm very interested in the reason behind it. I think its relevant that I specify I'm using an external SMPS by CUI Inc. P/N ETMA360166UD-P5P-IC (Medical grade PSU).

[DTJ]

Is your power supply output isolated or is the -ve grounded?

Do you have sensors or other instruments feeding into the circuit? Grounded?

[Rerouter]

If the 2 devices do not share a common reference, your measurements are meaningless, it would be like trying to measure a battery with only 1 wire,

[dreaquil]

The output is isolated according to the datasheet. I do not have external sensors. The instrument measure the insulation resistance of cables. so I simply connect one end of a cable to my instrument.
http://www.mouser.com/ds/2/670/etma-60w-534969.pdf

[Rerouter]

That is an isolated supply,

Now at this point i would recommend sketching up a crude diagram of what your measurement setup is like, (Power goes in here, Device under test connects here, measurement connects here), you can keep your device a black box if you wish, but an image would help clarify,

[dreaquil]

Something like this? The PSU is a linear one on the PCB and the barrel connector connects to it. All linear regulators are referenced to the SMPS ground terminal.

[dreaquil]

I can confirm that the oscilloscope probe is connected to earth (checked on my tektronix oscilloscope from probe ground to earth ground pin at the back). So it seems this solves the issue.. just not sure why.

[tautech]

I can confirm that the oscilloscope probe is connected to earth (checked on my tektronix oscilloscope from probe ground to earth ground pin at the back). So it seems this solves the issue.. just not sure why.

Probe ground has unfortunately been called by most, just that: ground.
While it technically is, as with most scopes the BNC outers are at mains ground.

However it is the reference to which the signal is referenced, just like in the analogy Rerouter provided.

So to avoid future confusion think of the probe lead as the reference lead for it's a more accurate description.
Just be aware it's grounded of course.