two channels with large voltage diffence

[Bill Wang]

Hello,

Several days ago, I had noise problems with two pressure transducers. I solved this problem by using DIFF mode with bias resistors. Almost no noise now (0.1 mV). Thanks for help from this forum. I really appreaciate that.

However, I need to add another DCDT channel to my USB 6211 DAQ card.  After add DCDT channel to my card, I get not only 30mV noise from my DCDT, also get 5mV from pressure transducers. The max output ranges for these two pressure transducers are +/- 200mV, and the max out range for DCDT is +/- 10V. I wonder if there is any compatitable problem within the channels in the card. I mean, does the high voltage DCDT influence low voltage pressure transducers? Could anyone give any sugguestions on this please?

Thanks.

Bill

[Bill Wang]

My two pressure transducers: PDCR 610 and Omega PX302-5KGV

DCDT: Trans-Tek 0244-0000

Thanks. 

[Jason_D]

Bill,

I would advice you to check out this article.  A very good start for any AI signal problem you are having.  I think the most likely culprit is ghosting which is talked about here.  Let me know if trying any of the suggestions here helps.

Additionally could you try moving the new hardware to a different channel (especially important if they are adjacent)?  Does this help either measurement? 

[Bill Wang]

Thanks, Jason.

It seems my DCDT has high source impedance, around 5600 Ohm. I want to make unity-gain buffer. Do you know how to make it? 

If I use INA 105, do I need +/- 15V voltage as shown in the figure   ? And what is the R0 value?

Thanks very much.

Bill

[Jason_D]

Bill,

I don't quite understand what you are confused on about making it?  Can you identify which parts you do not understand? 

This is what the document recommends:

(Optional) R_o = 50 W; R_o must be large enough to keep op-amp stable when connected to a capacitive load like a cable.

[Bill Wang]

I'm sorry, Jason.  I don't know what is the "W" in "R_o = 50 W? Is R_{o resistor? If yes, does W stand for ohm? If no, what is that?}

Thanks.

Bill

[AnalogKid2DigitalMan]

The W should be the greek letter omega, denoting ohms, thus Ro should be 50 ohms. It is a common typo in a lot of NI's documentation. There is even a comment at the bottom of that document dating back to 2003 to fix the unit symbology.

-AK2DM

[Jason_D]

AnalogKid is absolutely correct.  It should be Ω.  I will change this documentation.  I am not sure this resistor will be required for your application but it may help.