singel ended inputs

[Tompa]

We have a 6218 ADC and use 16 singel ended inputs and LabView to collect data. Problem is that we see signals on all 32 inputs. Why? Unused inputs need to be grounded?

[KRAZE4LV]

The single-ended inputs you connect one wire from each signal source to the data acquisition interface. The measurement is the difference between the signal and the ground. This method relies on

1. the signal source being grounded (earthed), and
2. the signal source's ground .

The ground as a constant 0V, but in reality the ground, or earth, is at a different level in different places. The closer together the places, the more likely the ground level will be the same.

generally make a connection between two grounds and the difference in levels can drive large currents, known as earth or ground loops. This can lead to errors when using single-ended inputs.

Single-ended inputs are sensitive to noises. It is added because signal wires act as aerials, picking up environmental electrical activity. With single-ended inputs you have no way of distinguishing between the signal and the noise.

The ground and noise problems can be solved by differential inputs if you want!

Dont forget to give a Kudo!

KRAZE4LV

[ABCPrograms]

it's normal that if you read all 32 inputs, that you read data on all 32.  it doesn't mean that if you don't put a signal on the other 16, that the readout=0.  Has to do with noise etc.

Why aren't you reading differential?  this uses 2 inputs for 1 analog signal and you have a lot less hazard of external factors and noise.

If you're not interested in reading the second half of the inputs you can simple ignore them, or start measuring differential.  More info in difference can be found here

[Tompa]

Thanks for the prompt reply. We use 16 inputs because out detector system have 16 outputs (between 0 and 5 volts) and a common ground. But also because, in the future, we will use 32 inputs; just waiting for more sensor devices. The thing is that the inputs of the NC ones look very much like "real signals", or in any case not just like noise signals as could be expected. If you have a slowly varying DC signal, why should I use diff input.

Ciao // Thomas

[ABCPrograms]

you need to use differential to have less hazard from noise.  but if you have no noise, it's ok for me 😉

just remember the answer to your question,  if 16 channels aren't connected to a signal it's only normal you receive values because of the noise.

Normally if you don't use these 16 extra inputs, then it shouldn't matter for you what is on it.  If you really want to clear these values, connect them to the GND of the module.

[Tompa]

Thanks for the reply. I see the problem but my signals come from 16 sensors with pre-amps in a common chip. This means that I have 16 output signals 0-5 volts but only ONE ground. So I connect the 16 inputs in single ended mode and the one common ground to the analog input ground.

Furthermore, the signals on the not connected inputs look like real signals, not noise signals.

I do not quite understan why I have this situation.

Ciao // Thomas

[ABCPrograms]

they are called "floating signals"  it's because they are not grounded, they do follow the other inputs.  

I don't know the exact theory behind it, cause I'm not an electronics expert, but this gives you an explanation of what I mean