NI 9237 full bridge (connect excitation sensing or not?)

[kjrust]

The 9237 manual says the excitation sensing (RS+ and RS-) must be connected when connecting a full bridge transducer. However, the 9237 appears to function the same whether or not I use a 4-Wire connection (no sensing) or a preferred 6-Wire connection with excitation sensing connected at the transducer end. Minus the excitation drop along the cable, I would have expected functionality problems if the excitation sensing was not properly connected (such as floating, noise or overload problems). It's confusing since my full bridge transducer appears to function properly if I use either a 4-Wire or 6-Wire connection. But the manual clearly shows excitation sensing must be connected and makes no reference to not connecting sensing.

Questions:

Voltage drop aside, is the 9237 fully functional if excitation sensing is not connected? 

Does the 9237 detect if excitation sensing is connected or not?

If you don't externally connect sensing, does the 9237 switch to internal sensing?

Or, is the sensing circuit floating around and thus bridge excitation is not uncontrolled?

If sensing is not connected, will this cause functionality, noise or other problems?

[Logan_K]

Check out figure 1 in this user guide: https://www.ni.com/pdf/manuals/374186a_02.pdf

The remote sense is connected internally through a resistor, so if you leave it disconnected it will still work, just with less accuracy relative to your lead wire resistance. Other than gain error, you shouldn't see any other functional issues when leaving remote sense disconnected.

[kjrust]

Thanks. Well that's a weird circuit configuration. So, if I connect the excitation sense leads at the transducer end, how does the 9237 sense the lower excitation voltage at the transducer versus the higher voltage at the internal sense resistor? Just trying to understand how the 9237 compensates for the voltage drop along the cable when it's already sensing the excitation voltage internally.   

[Logan_K]

When the remote sense lines are disconnected, and those inputs are floating, there is no current flowing across the internal resistor that is internally connected between the remote sense and the excitation lines. So, the internal amp that compensates will simply read the same value as the excitation and not perform any compensation due to lead resistance.

However, when a remote sense wire is connected, then there will be a voltage drop between the remote sense and excitation inside the module across the internal resistor. As long as that resistor is a high enough value, the amount of current flowing should be negligible. But the voltage different between those two signals is used to compensate for the lead wire resistance.

Hope that helps in explaining the internal workings of the remote sense and why it can be left disconnected... although just to be clear, when it is disconnected, it is not compensating for lead resistance.

[kjrust]

OK. Now I understand how the circuit works with either a 4-Wire or 6-Wire connection.

Thank you for the detailed explanation.

A similar explanation in the NI documentation would have been helpful.