Voltage drop while reading analog input of cDAQ with extremely low current.

[5thGen]

I am trying to read an analog signal using the cDAQ-9172 with module 9205.  The signal has a current in the low uA range and ranges from 0VDC to 5VDC.  I have grounded the COM port.  An ammeter reads the correct voltage at the input of the 9205 but the VI displays something significantly lower.  I am using the "DAQmx Create Channel" and "DAQmx Read" to monitor the signal.  I am also finding that the values are jumping all over the place - within +/-200mV at around 1.5V.  Why am I getting a voltage drop?

Thanks,

-Stephen

[Hillman]

Hi Stephen,

I have researched into this and have found a few interesting bits.

In the manual for the 9205 - page 19, states the input bias current of +/- 100pA. So it would seem uA range is well within the spec of the device.

What device are you using to generate this signal, and what other inputs do you have to the chassis (such as resistors, or low resistance devices which may draw current away from the input of the DADS) - all these factors can affect the performance of the DAQ device? Also, is it easy to change the voltage of the input signal - if so, maybe try high voltages and see if you still have the same issues. 

If it is the current, I would suggest you need to alter or add some signal conditioning to the input of the module. You could use an opamp for example to boost the current. (but like i said, it would seem to be in spec). 

The jumping's most liekly cause is noise - again using signal conditioning to 'clean up' the signal from at its source would help. Do a search for reducing noise with DAQ and you should find some useful sources. 

Please post back - this is an interesting issue,

Regards

[5thGen]

Hillman, thank you for taking a look into this issue.  I have been troubleshooting this as well.  I am trying to read the potential created by nine individual 2" x 3" solar cells.  I just updated to Signal Express 3.0 so I used that program to monitor the channels.  The readings were accurate but I noticed that each signal had a sinusoidal shape to it.  I did a frequency analysis and found several harmonics and realized I was testing the cells in artificial light and that the variation in the signal was related to the frequencies of the fluorescent bulbs (whoops!).  My ammeter was unable to catch the voltage changing so rapidly while the DAQ was much more sensitive. I have not been able to retest under sunlight yet but I am almost certain that it will resolve my problem.  Thank you again for your help and I will repost if this is not the solution.

-Stephen

[Hillman]

Hi stephen,

I am glad your troubleshooting was successful!

Please do let me know how you get on,

Regards