PCI-6221 Erroneous readings

[doliver]

I have the PCI-6221 DAQ with SCB68 set to continuous acquire at 1 sample at 1hz. I am acquiring voltage and current over time to monitor a battery charger, charging a battery. The system works fine on chargers referenced to ground. On floating chargers I get erroneous readings. I've tried adding a 10K bias resistor from negative terminal to AIGND as described in the manual. I've tried adding a 1M resistor from both positive and negative inputs to AIGND. 

 Any help I could get would be greatly appreciated

Darren

[McKala[DE]]

Hello Darren,

What kind of erroneous readings are you getting - noise or just incorrect voltages? 

How are you measuring the signal - in Measurement and Automation Explorer or in LV?  What input configuration are you using - NRSE, RSE, or Differential?  Are you attaching the bias resistors to positions B and D as mentioned in the manual?  You need to configure you measurement to be either non-referenced single-ended (NRSE) or differential when you create the NI-DAQmx task.  The Field Wiring and Noise Considerations for Analog Signals is a good reference as you get started.

Good luck!

[doliver]

Micaela,

Thanks for the reply!

We have identified the problem to be with the voltage scale. I have a 10:1 divider on the voltage so I scale it as follows:

Pre-scaled value

Max            Min

5.0 0

Scaled Value

Max Min

50.0 0

When I try and read 27 volts the value displayed is 10.62 volts. When I read 16.9 volts the value displayed is 10.62 volts. When I read anything below 10 volts the value displayed is correct. For now I'm just not scaling the voltage. What should I do to correct this?

Thanks

Darren

[devchander]

If you read the specifications of analog Input of your hardware,

you will find that the maximum and minimum Voltage that the ADC of your card can measure is +10 V and - 10Volts respectively.

Anything above that, will show saturated +10 V reading on that analog input channel

You are speaking of an external  'voltage scale'??, are you not using this to scale down your analog input voltage to fall into the +/- 10V ADC range?

Message Edited by devchander on 03-22-2006 06:32 AM

[doliver]

 I have a 10:1 voltage divider to measure the voltage. For example:

Battery voltage = 24V

Divided down = 2.4V

Now I apply my voltage scale to Map ranges

Pre-scaled value

Max                  Min

5.0V                  0V

Scaled Value

Max                  Min

50V                     0V

[McKala[DE]]

Hello Doliver,

Look at this KB and let me know if this is not the problem.

Message Edited by McKala[DE] on 03-22-2006 08:46 AM

[doliver]

I can't seem to open you're shortcut

Thx,

Darren

[McKala[DE]]

Sorry, I placed the wrong link, here's the correct one.

[doliver]

Thanks Micaela!

I was using the "Pre-scaled value" in the "Signal Input Rage"