DAQmx Offset Nulling on SCXI-1520, NI 9237 and PXIe-433X

Updated Apr 11, 2019

Reported In

Hardware

  • NI-9237
  • PXIe-4331
  • PXIe-4330
  • PXIe-4339
  • SCXI-1520

Issue Details

  
  • What is offset nulling and how is it implemented on SCXI-1520, NI 9237, PXIe-4330 and PXIe-4331?
  • How to implement offset nulling on SCXI-1520, NI 9237, PXIe-4330 and PXIe-4331?

Solution

It is very unlikely that the bridge outputs exactly 0 V when no strain is applied. Slight variations in resistance among the bridge arms and the lead resistance generate some nonzero initial offset voltages at the terminals of the bridge. We can implement offset nulling to account for this initial voltages. 
  • SCXI-1520: NI SCXI-1520 has an internal hardware offset nulling circuitry that is used to null out the initial bridge offset. When using very large gains on the SCXI-1520, this hardware nulling circuitry is valuable because it prevents small offsets from saturating the input circuit and allow us to maximize the full input range. The nulling circuitry consists of a course potentiometer and a fine potentiometer. The coarse potentiometer is adjusted first, then the fine potentiometer is adjusted until the reading is nearly 0 V. If there is still a voltage offset, the value can be stored in the Initial Bridge Voltage property shown in the image below, allowing the DAQmx to subtract this value out before applying any scales. Refer to the SCXI-1520 User Manual for further details on offset nulling
  • NI 9237/PXIe-433X: The NI 9237 and PXIe-433X do not have internal hardware nulling circuitry. Offset nulling is performed in DAQmx where the value of offset is stored in the Initial Bridge Voltage property. Both NI 9237 and PXIe-433X utilize a 24-bit Delta Sigma ADC. Offset nulling can be completed in software because the input range is sufficiently wide so that the offsets combined with the measurements will not saturate the input range even with a very large initial bridge offset. The 24-bit resolution provides accurate measurements over the wide dynamic range