Help !!!!!!!!!!!!!!!! offset problem when channel isn't red in first position !!!!

[marc france]

Hi everyone,

I am using Labview 8.0, a 6221 PCI card with a SCB-68 connector block.

I am using the 8 AIs in the differential mode (0 to 5V).

Channel 5 to 7 : connected to sensors (T°C, Humidity rate, pressure) no problem

Channel 1, 2 : no sensors. AIs all connected to their respective AIground

However I have problems with channel 0 and 3 where I am measuring the voltage of 2 resistances (rougly 0.160 mV and 0.320 mV on each channel). I have attached the very simple circuit where I need to measure the 2 points : signal 1 and signal 2. I have chosen the differential mode because it's a noisy envirement.

I am  using the Daq assistant to measure the first 4 voltages. I have set the Daq assistant to take 1000 pts on each channel from 0 to  3 at 1 Khz, and then I make an average.

AI8->signal 1

AI0->ground circuit

AI11->signal 2

AI3->ground circuit

Here is my pb : I can get the correct value of any channel (0 or 3) only when I set (in the daq assistant) that particular channel in first position ( where you are supposed the choose the order).

 For instance : If channel 3 is set to be red in first position, the voltage will be correct(+/-0.5mV) ,and channel 0 will have an offset of about 2 mV (the prcision of the card is much better than this). However if channel 0 is set to be red in first place, it’ll have no offset, but channel 1 will (because not red in first position).

To test the PCI card, I have connected  2 voltage supplies to chan. 0 and 1, and I don’t have that problem at all, evrything is perfect what ever order I choose.

I believe I have a problem with my wiring…..? I am using the diff mode, and connected 10 Kohms resistances between V+ and Aigd and between V6 and Aigd for both signals. I  get worse results if Idon’t use them. I have also wireed the ground one of the Aigds to the ground of the supplie voltage : I get much less noise doing this.

I have checked with the assistant DAQ graph, and the problem isexactly the same. With Max, I get the right value for any channel, but that is because I can only check one channel at a time : so what ever is being measured is always in first position....

 I believe I have a problem with my wiring...because the measure is correct when red in first position. I am wondering  if the card doesn't just take into consideration the V- of the channel in first position... and then makes the difference between the V+ of the other channels... I have also tried pulling only one V- (ground of the circuit ) and connecting the V- together, but same offset....

Could someone tell me how I to solve this offset problem....? Or tell me how everything should be wired in my situation ( taking in consideration my circuit) ? use another mode than diff? And how to wire then?

Thank you very much, I really can't find any solution ....

Regards,

Marc

[marc france]

I forgot to say that if I wire signal 1 for instance to channel 0 and channel 3 at the same time, the  value is correct what ever order it is.....

[Ben]

A quick read (may be wrong) makes me think this is related to the input impeadance of the DAQ device.

Quick follow-up Q to confirm;

If you always read from a low impeadance source (ie jumper to ground) prior to a high impeadance device, do your readings look better?

If, so then the devices you are rading from are not supplying enough charge fast enough to change the input capacitance.

If not, please igonre this post!

Trying to help,

Ben

[marc france]

Hello Ben,

Thanks a lot for your help. I guess you were right because when I set the assistant daq to acquire AI0, AI1,AI1,and AI1 again, the correct value for AI1 starts with the second reading of AI1 (the first one has the off set). I suppose the multiplex is too slow when changing impedance from AI0 to AI1.....

Thanks again !

Regards,

Marc