Pressure Transducer Connected to SCB-68

[RossP]

I have a Kulite miniature pressure transducer that I supply with a good, solid, steady 10V from a digital power supply. The output of the transducer is in the range of 0-200mv. It is a differential pressure transducer so that if the pressure increases from my reference the signal is positive and negative if it decreases. I am trying to figure out the best way to connect the signal leads to the SCB-68 connector block. I first thought to just connect the positive lead to one channel like ACH7 (terminal 57) and the negative lead to the AIGND terminal. But now I am reading in the manual for the transducer and the manual for the SCB-68 that I should have a larger resistance inline to the data recording equipment, which for me is the SCB-68 with Labview 7 express running.

Does this mean I should:
- put a resistor inline with the neg/pos lead on its way to the AIGND?

Or

- connect the neg lead to ACH15 (ACH I + 😎 and also connect it to the AIGND with a large resistor?

I see these configurations in the manual, but can't decide if I have a NRSE,RSE or DIFF input mode set up on my DAQ device and if I have a Floating Signal Source or a grounded Signal Source input.

Any help would be great. There is a lot of terms I am not familiar with and I am worried about damaging the pressure transducer.

[Otis]

I've done some work with pressure transducers and the best way to set up your measurement is to set the measurement as being differential. Then connect the positive sense lead to the AICH (i) and the negative sense lead to AICH (i+8). If your transducer is grounded, then you don't need to worry about connecting any resistors; however, if your transducer is floating, then you might want to connect a bias resistor of 10k - 100kOhms between the AICH (i+8) and AIGND. This might help to keep your signal from floating. If you really don't know where your signal is coming from, then just try both with and without the resistor and see what works better for you.

Ideally you would also have some preamplification so that the signal utilizes the full range of the DAQ board. If you have a -200 to 200mV signal, then you should ideally amplify the signal 50 times. However, you should still see the pressure change even without this, you will just lose some resolution on the signal.

[WM_John_Weeks]

I once used a Kulite miniature pressure transducer. They are really cool!

My recollection (this was like 25 years ago!) is that the one I used consisted of a silicon diaphragm with semiconductor strain gages integrated directly into the diaphragm. The gages were arranged to form a full bridge.

In that case, you would want to connect AI Sense to AI Ground, connect the power supply negative to AI Ground (assuming there isn't anything else that's grounded- be *very* careful). Use Differential input and connect the plus to channel aiN and negative to aiN+8.

The bridge and power supply form a low-impedance path for the bias currents (the reason for the resistor to ground) so you don't need the resistor. The output of the bridge is not at ground, since the power supply at the other corners is grounded, so you have to use differential input.

Now, remember- this was a long time ago 🙂

[RossP]

Ok thanks.
It seems I do need to do things in DIFF mode. I will try the resistor. But that begs a question: Is there any input resistance built in to the SCB-68. I have not found anything explicitly saying so in the manual/specs but me and my lab group were under the impression that modern DAQ systems all had like 1megaOhm resistance?

Also Otis: What exactly does floating mean? Bias? My transducer is not grounded unless you count the power supply's ground. It just has four leads: power +, power -, signal + and signal -. If you could point me to any tutorial or docs on this stuff would be helpful.

Thanks

[Otis]

The reason for the resistor is that it helps to make sure that your signals are not floating. You want a large resistor because if it is small you could have some current leakage to ground.

As you said you have four lines. Power + should be connected to your voltage source's positive lead. Power - should be connected to the negative lead of your voltage source (or ground if that's your case). Signal + goes to AICH n. Signal - goes to AICH n+8.

The SCB-68 does not have any input impedance, but the DAQ boards do have input impedances and you can find all of the specs for your board by going to our website and looking for your device.