how to determine floating signal?

[andy257]

Hi All,

When dealing with analogue voltages how does everyone decide if the signal they want to measure is floating or ground referenced?

Is a quick multimeter test good enough to determine floating or ground referenced?

e.g power off, measure between common and earth ground. If so what typical values are we looking for, a dead short?, high resistance?

It might sound easy if your talking about batteries etc but what about power supplies?

Even if you know your PSU is floating or grounded, the device it is powering can have its signal output isolated from its power connections.

So when trying to make measurements of a sensor do we only care about the actual sensors relationship to ground or the whole system>?

Thanks

 

[crutschow]

Generally the first question is whether the sensor ground is the same as the measurement circuit ground. If you measure no continuity (a few ohms or less) between the two grounds with an ohmmeter, then they are isolated. The next question is whether the two grounds (commons) can be tied together. That can generally only be determined by knowing the circuit connections to the system for both the sensor and the measurement circuit. Often, to eliminate ground loop noise, a differential input is used by the measurement circuit so the sensor common can remain floating from the measurement circuit common.

 

[Mikebits]

One would need a schematic to be sure. If one were to measure a high resistance between two points, without a schematic how would one know if this were correct or a problem ?

 

[andy257]

Generally the first question is whether the sensor ground is the same as the measurement circuit ground. If you measure no continuity (a few ohms or less) between the two grounds with an ohmmeter, then they are isolated. The next question is whether the two grounds (commons) can be tied together. That can generally only be determined by knowing the circuit connections to the system for both the sensor and the measurement circuit. Often, to eliminate ground loop noise, a differential input is used by the measurement circuit so the sensor common can remain floating from the measurement circuit common.

so you dont care about the PSU?, only the sensor common. It makes sense i guess since its the output of the sensor which we want to measure.