Cabelling, Connectors, and Excitation Voltage Questions

[hecmar.arreola]

Hi!

I connect a lot of strain gage pressure transducers to NI Multifunctional DAQ devices.  I have been the first and only EE at my company, and have had to learn the "hard way" on a lot of these issues.

I'm hoping someone with a good background might be able to help me in component selection.

When it comes to these Pressure Transducers, I deal with Honnywell FP series and a variety of Omega transducers.  My preference is the 4-20mA output ones.  The cable I use is shielded, but I tend to attach lengths of cable together using quickconnects.  I've noticed there are a lot of different types of 4-conductor connectors, and I would like to get away from using Quickconnects (if I can).  I would like to know what kind of connectos I should use for attaching lengths of wire together.

I typically terminate the signal to my DAQmx device in differential mode.  I measure the voltage across a 500ohm resistor which converts the 4-20mA to 2-10V signal.

I use component power supplies (typically 24Vdc) from MPJA.  Mostly because they always have the best deal.

Sometimes I find that I have a lot of noise issues that I can't overcome.  Other times my signal look fine.  I think I need to get away from using these quickconnects because they don't look like they would be good in a high noise situation.

Any advice I would very much appreciate.  Good reference material is also very helpful.  I feel like I'm a good programmer and can solve things in software very easily.  When it comes to a noise problem I'm often left scratching my head :P.

-Nic

[AnalogKid2DigitalMan]

Nic:

I use Amphenol circular connector quite often. You mentioned you are using shielded cable, are you connecting the shields together along the whole chain of quickconnects?

Also, are you connecting the shield to earth ground at one end? Keeping signal cables away from AC mains cables?

I did not look up the supplies you are using, but in general, linear supplies typically have less noise than switchers.

Can you characterize the noise with an oscilloscope or a DAQ card at a high sampling rate? If you know the nature of it you can sometimes track down the source of the noise.

Ground loops can cause issues too. On one job I had to galvanically isolate all my 4-20mA signals from one another, without isolation it drove the PID controllers out of their minds and my sampled signals were useless.

Hope this helps

-AK2DM