09-29-2008 10:25 AM
I have a USB-6009 and all the Anologe Inputs read 1.4 volts in RSE mode without any connections to it what so ever. Is this normal? I am using the Test Panel of Measurement and Automation Explorer to troubleshoot this but I can't figure it out. When I set the voltage on the AO channel and connect it to the AI channel it reads the correct voltage. So what do I need to do in order to measure a reading of 0 when the input impedance is high or the channel is open input?
09-30-2008 09:59 AM - edited 09-30-2008 10:09 AM
Hi bdbrown, welcome to the NI forums!
This is actually expected behavior. From the USB 6009 manual (bottom of page 18):
When no signals are connected to the analog input terminal, the internal
resistor divider may cause the terminal to float to approximately 1.4 V
when the analog input terminal is configured as RSE. This behavior is
normal and does not affect the measurement when a signal is connected.
The 1.4 volts that you are measuring comes from the internal 2.5V reference which is divided down. Figure 7 of the manual (p.16) has a circuit diagram that should make this more clear. If you want to use RSE mode, you will have to ground your analog input channels to measure a 0. To measure a 0 on an open channel, you could use differential mode (which measures the difference between the voltages on two ai channels). Thanks for posting!
09-30-2008 10:11 AM
Thank you John P. for the explaination.
My concern is that I am doing battery life testing and having another voltage source may have some adverse effect on the results, i.e. the batteriy will run longer under test condition than it would in real life. How can I be sure that this is not the case?
Bruce
09-30-2008 01:12 PM
This should be fine as long as you are using a reasonably small shunt resistor (relative to ~160 kohms). This would prevent the internal resistor network from having a significant effect on current draw and would also minimize the effect of the 2.5V internal reference voltage.
If you use a differential measurement as opposed to RSE, the 2.5V internal reference voltage would essentially be cancelled out, so you would achieve better accuracy. Differential measurements are also more resistant to noise. The drawback is that you would only have half as many input channels (4 instead of 8).
The circuitry of the USB 6008 and 6009 is very different from our other DAQ boards as they are designed to be our low-cost solution for data acquisition. If you provide more detail about how you are setting up your battery testing (voltage/resistor values, wiring, etc.) I can verify if the 6009 will work properly, or possibly recommend another board that will meet your needs. Thanks!
09-30-2008 01:51 PM
Thanks John,
I feel a little more confident now that I understand the resistor network.
Any effect should be negligible. I am just used to working with the USB-6251.
Thanks again,
Bruce