NI-PCI6221 AND scB-68 anolog input issue

[jinwei]

Hi, two issues thanks. (1) I use ni-pci-6221 DAQmx  and scb-68 to input voltage( 0.001~0.00001v) and temperature(thermeocouple sensor), I use AI0 and AI8 as voltage channel, and Al1 and Al9 as temperature senser channel, I adjust the swiths in scb-68 box in  "Fcatory default setting", I can get good voltage signal, and then i swith to " Temperature sensor enable( differential)" mode, I just only get temperature signal, the voltage teminal do not work, yes, i set my voltage channel in differantial mode, still do not work, why?(2) Can i let the two channel work together? i want to probe the temperature change when i measure the voltage.Thank you

[S_Hong]

Hi jinwei,
 
(1) The reason why you are unable to get any correct voltage readings from AI0 and AI8 is because by setting the your SCB-68 to the Differential Switch Configuration, you are effectively wiring the Cold-Junction Compensation (<a href="http://digital.ni.com/public.nsf/allkb/E332E1D829059FB5862570AE005066B3" target="_blank">CJC</a>) to differential channel 0 (AI0-AI8) of your DAQ board. Therefore, nothing should be connected to thes channels and this is the reason why you&nbsp;may be&nbsp;getting faulty readings. You can work around this by simply using other channels on your SCB-68 to take voltage measurements.
&nbsp;
(2) Yes, you can take both voltage and temperature measurements on the SCB-68 as long as you do not use differential channel 0 (AI0-AI8).
&nbsp;
I hope this helps,
S_Hong

[jinwei]

thank you very much, your mean the channel al0 : al8 is can not be used,&nbsp; except the al0: al8 channel, I can use any channels as my voltage channel and temperature channel, so the which swich I should use, use" Temperature sensor (differantial):? Tomorrow i will try this, wish get your further help, thank you very much.Jinwei GaoMIT

[S_Hong]

Hi jinwei,
Yes, you should keep your SCB-68 in Differential Switch Configuration as indicated in Figure 4-2 of your user <a href="http://www.ni.com/pdf/manuals/320745b.pdf" target="_blank">manual</a>.
S_Hong

[jinwei]

Thank you very much, this issue is ok, I got it. Another question, today, I try to measure resistance with PCI-6221 and PCI-6704, I provided constant current by 6704, I get resistance, but, the error is very big, the fluctuation is larger, when I mease 1 ohms precision reisistor, I use 150uA current, I GOT the 1.3~1.5 ohms value, i detect the constant current, the error is so small( about 0.3%), input voltage is about 10 microvoltage(fluctuation), I DO NOT KNOW WHERE THE FLUCTUATION AND ERROR IS FORM? please check? thenk you very much.Jinwei

[S_Hong]

Hi jinwei,
&nbsp;
The reason for the fluctuations may be because you are trying to measure very small resistance values which lends&nbsp;to unwanted lead resistances. I believe that for measurements in this range, you will probably be better off using a DMM rather than a Multifunction DAQ. I also just wanted to let you know that you can perform a resistance measurement task in Measurement &amp; Automation Explorer. This way, you will only need to use&nbsp;a single&nbsp;board (PCI-6221) rather than have one&nbsp;to generate current and the other to measure the voltage.

[jinwei]

Thank you very much, your mean I just need pci- 6221 to measure reistance in MAX panel? firstly I build a resistance task in MAX, and then I measure it? but when i do this task, I chose iex soure as "internal",the follow error happen: Error -200161 occurred at DAQ AssistantPossible
Reason(s):Measurements: Device to which the sensor is attached does
not have an available internal excitation source.Select another device
with an available internal excitation source or supply external
excitation.Channel Name: ResistanceDevice: Dev2no external excitation current? but when I chose "external" as iex soure, yes, it works, but the resistance is abviously wrong, I am comfused that.

[S_Hong]

HI jinwei,
I'm sorry for the confusion. It appears that the PCI-6221 does not generate output currents and I failed to notice this. It&nbsp;seems that your method was the way to go on this one. If you have any questions on how the system can be connected, you can always use the Connection Diagram tab in you Resistance task in MAX to help you.

[jinwei]

Thank you very much, sorry to let you confusion. I really did it according to the diagram of resistance in MAX, yes, it says I need to connect extenal current, I&nbsp; proviede current with NIPCI6704, yes, I get resistance value, but the question is why the value is wrong, such as, when I measure 25 ohms precision resistor, I get 26~27 resistance, this show my measure has bigger error, but my current is very precision( 0.02%), my input voltage is good too( fluctuation is microvoltage), SO i donot know where the error from?You said that I just use pci6221 to measure reisistance in MAX, i am confusion, as to above, the 6221 two analog output channels, can i use the the internal output channel and input voltage channel to get reisitance?Jinwei

[S_Hong]

Hi jinwei,
It seems to me that this is expected behavior as you are trying to measure very small voltages. From my calculations, if you are using a current of 150 µ A and putting it through a 25 ohm resistor, you should be expecting to measure 3.75 mV. The accuracy of your measurement will depend on many factors such as&nbsp;when the device was last calibrated, temperature,&nbsp;the number of readings you are taking, etc...&nbsp;By using the <a href="http://www.ni.com/advisor/accuracy/" target="_blank">Accuracy Calculator</a> on our website, you can have a good idea of how accurate your readings will be. I calculated that at best, even with a 100 point average, you will achieve an absolute accuracy of approximately +/- 0.0709 mV which translates to approximately 0.472 ohms.

[jinwei]

Yes, you are right, I just get about 0.5 resisteance, when i measure in short circuit, but question is the vules fluctuates up and down,&nbsp; i am confusion. if just&nbsp; offset , it is ok, but&nbsp; I try to figure out.. tired... Thank you.

[Ana P]

Hi jinwei,

As S_Hong said before the accuracy of your measurement will depend on many factors. The accuracy is not an offset value the signal will be shifted to, but a certain amount that the signal can vary within. Have a look at <a href="http://www.ni.com/support/calibrat/accuracy.htm" target="_blank">this</a> Developer Zone for more information about Accuracy and the System Environment. I hope this is helpful,

[jinwei]

I GOT IT!