PXI-4220 measured voltage decreases, why?

[KarstenJ]

Hello,

we use the "PXI-4220" ADC to measure voltage at a full-bridge DMS. We use the on board excitation power supply

and use the remote sense wire. After a while we repeat the measurement. The measured values are lower than the

first ones. We didn't change the force. Closing and restarting our application hasn't any effect. If we change

the settings of the task e.g. AutoZeroMode and recompile the project, we will get the origin values after

restart our application. However without changing the task, the values decrease. The differences depends on the

time between the measurements. It doesn't depend on the number of the measurements.

Why do the values decrease?
What could we do to avoid this?

Thanks,
Karsten

To create and start the task, we use c# and the libraries of the Measurement Studio:

 // declare a task
 public class VoltageTask : NationalInstruments.DAQmx.Task {
  public VoltageTask(string taskName) : base (taskName) {
   this.Configure();
  }
       
  public virtual void Configure() {
    AIChannels.CreateVoltageChannelWithExcitation( "PXI1Slot2/ai0", "ai0", AITerminalConfiguration.Differential, -0.06, 0.06,  

         AIBridgeConfiguration.FullBridge,  AIExcitationSource.Internal,    10, true,AIVoltageUnits.Volts);

    AIChannels["ai0"].AutoZeroMode  = AIAutoZeroMode.None;
    AIChannels["ai0"].LowpassEnable  = true;
    AIChannels["ai0"].LowpassCutoffFrequency = 100;

   Timing.ConfigureSampleClock("", 1024, SampleClockActiveEdge.Rising, SampleQuantityMode.FiniteSamples,  1024);
  } 
 }

 // create the task

   force01Task = new VoltageTask("force01");
   force01Reader = new AnalogMultiChannelReader(force01Task.Stream); 

 // start and stop the task, get data
   force01Task.WaitUntilDone(-1);
   force01Task.Start()

   double[,] force01Data = force01Reader

   lateral01Task.Stop();

[KarstenJ]

Hello,

it is not a software problem. We had connected the DMS to a different ADC. The signal was also drifting there. It seems, that the drift depends on the strain. A smaller strain causes a smaller drift.

Any ideas?

Thanks,
Karsten