I am using a NI 9205 to measure differential Voltage.
I am a bit confused on the wiring. It appears that the wiring diagram shows that all low side signals ( -) should be tied to common which is tied to ground.
It seems if I do this, then I am not truly measuring a differential voltage, but RSE voltage.
I have read the manual and the NI field wiring tutorials, so please don't point me there, or post a picture, I am fully aware of what they look like, and still don't understand
them as well as I want.
Thanks for any help.
You don't necessarily need to connect the negative terminal to COM directly, though you should definitely make sure that COM is connected to something that isn't floating (if you external signals will stay withing about 10V of ground, you can just connect the COM to the chassis ground). The reason that you need this connection is that, without it, the COM terminal may float outside the specced allowance from Earth Ground.
One thing you may need to do with a differential measurement is bias both + and - to COM using resistors if you are measuring a floating source. This would be the Floating Signal Source portion of Table 1 in Field Wiring and Noise Considerations for Analog Signals. In this situation, it's still a true differential measurement as your resistors will be large enough to simply provide a path for bias currents to return to ground. This has the effect of cleaning up the signal without affecting the differential nature of the measurement. If you are measuring grounded sources, this won't be necessary.
One last queston.
On the 9205, there is a "port" on the module for single wire labeled COM, should this be tied to ground; i.e. earth, or floated with a resistor off of ground?
Where COM should be tied depends on the voltages that you plan on measuring.
There are several relevant voltage limits for the NI 9205 to consider:
Measurement Range:±10V max
The module can only measure a signal that is a maximum of 10V between the negative terminal and the positive terminal. The negative terminal in this case is either AI - for differential measurements, AI Sense for NRSE measurements or COM for RSE measurements. This is the range that the device can actually measure signals
Maximum Channel to Channel Voltage±30V max between any channel and COM
No channel (AI, DI, or PFI) should be more than 30V from the COM terminal. Any more than this and the module may be damaged.
Maximum Channel-to-earth Voltage±60VDC (±250Vrms for the Spring Terminal version)
This is the maximum potential that can be sustained between the COM terminal (or any other terminal) and chassis ground before the isolation barrier is damaged. The isolation barrier can withstand higher for short periods of time, but nothing over this value should be sustained.
So, the answer to your question depends on the voltages you are measuring. If all of the signals that you are measuring will be withing ±10V of earth ground, the easiest solution is just to tie COM to the chassis ground lug. If the voltages need to be between 10V and 30V from earth ground, you are certain they won't float out of this range, and you are measuring differentially then you can again tie COM to chassis ground as well. If the voltages must exceed 30V from ground, you'll need to tie COM to something that ensures that all of the three above conditions are met.
For example, if you are measuring signals that vary from 40V to 60V, you'd tie COM to a 30V power supply to ensure that it stays withing 60V of earth ground and within 30V of the highest signal (if you are measuring differentially or NRSE). If you were measuring in RSE mode, you'd tie COM to a 50V power supply to keep it within 10V of each AI measurement.
Does this make sense? For most people, the signals will remain within 10V of earth ground and so just time COM to earth ground.