How to set bias-resistor for multi-channel pressure signal collection

[hanwei]

Hi all:

I am a beginner of NI DAQ.

Now i meet some problems about pressure signal collection (power supplier for pressure transducer is 24v).

firstly, I use 500ohm resistor to transfer the current signal(4mA-20mA) to voltage signal, and then i want to use SCXI1002 and PCI6040E to test these voltage signals.

because the voltage signal cross the resisors are floating signal, so I use two bias-resistors.

But the problem is :

  when I collect one channel pressure signal, everthing is fine,

  but when I want test 2 or more channel pressure signal, the voltage is wrong! (too high).    (I build another 2 bias-resistor for 2nd channel)

 does everybody knows how to set-up multi-channel bias-resistor ?? or is there any problem in my system??

thanks a lot!!

[hanwei]

the electriccal schematic drawing is attached here, anybody knows how big of bias resistor that I should use for multi-channel??

thanks a lot

[Wallace_F]

Hello hanwei,

The voltages you are seeing might very well be the result of improper biasing of your analog input channels.  Since you are using an E series DAQ board, you should use biasing for DC coupled signals.  This would only require a single bias resistor (10k - 100kOhm) to be connected between the negative terminal of your analog input and analog ground. There is no need for a bias resistor between the positive terminal of your analog input channel and analog ground. Repeat this configuration for each input channel individually. Please refer to the knowledge base article at the link below for more information.

http://digital.ni.com/public.nsf/allkb/7B99932232E8C10B86256297005EA780?OpenDocument

Best wishes,

Wallace F.

[RT4CY]

Hello again hanwei,

Keep in mind that a single bias resistor is sufficient for low-impedance DC-coupled sources such as thermocouples.  The main point to take a way is low-impedance.  If the source impedance is high, then using one bias resistor will lead to an unbalanced system.  In the Field Wiring developer zone, click on Measuring Floating (Nonreferenced) Sources in the Table of Contents.  This goes into detail as to why you would want to use two bias resistors and how to wire them.

Couple questions for you

-Could you give a little more detail as to what voltage values you are seeing? 
-What resistor values are you using for the bias resistors?

-Are you including multiple channels in your Custom Voltage with Excitation task in MAX? 

-How many channels are you trying to read from?

-What happens if you simply read from a single channel and try each ai channel?

Message Edited by RT4CY on 08-27-2008 12:22 PM