Calibration

Calibration and Temperature Variation

When a system is composed of multiple integrated instruments, the system is subject to temperature rise caused by inherent compromises in air circulation and other factors. Self-heating from surrounding equipment, uncontrolled manufacturing floor environment, and dirty fan filters are among these factors.

Refer to the PXIe-4139 Specifications for the following information for your instrument:

• Recommended operating temperature range
• Calibration interval

Refer to Best Practices for Building and Maintaining PXI Systems for the definition of ambient temperature.

If the ambient temperature is outside of the specified range, you may need to know the measurement accuracy to account for temperature variation. One way to calculate the specified accuracy outside of the temperature range is to externally calibrate the system at the desired temperature. External calibration, though inconvenient, should allow the device to attain its full rated accuracy at the calibration temperature. You can learn more about external calibration at ni.com/calibration.

Another way to calculate the specified accuracy outside of the temperature range is to add the temperature coefficient accuracy for each additional degree outside the calibration range.

The following equation represents the temperature coefficient (tempco).

Tempco = X% of accuracy specification/°C

For example, consider an instrument outputting 5 V with voltage accuracy specified at 0.05% of output + 100 µV in the range 18 °C to 28 °C, and tempco specified as 10% of accuracy specification per °C. If the last external calibration was performed at 23 °C, the following equation represents the 1-year accuracy of the instrument in the 18 °C to 28 °C range:

0.05% of 5 V + 100 µV = 2.6 mV

If the ambient temperature changes to 38 °C, the device is operating 10 degrees outside the specified range, the accuracy is calculated as follows:

±(2.6 mV + ((10% of 2.6 mV)/°C) * 10 °C) = ±5.2 mV

The total error is twice the specified error (5.2 mV in the example above, versus 2.6 mV if temperature effect is ignored) due to the 38 °C ambient temperature. If the additional error term due to temperature drift is unacceptable, some devices support self-calibration at the desired measurement temperature to improve accuracy.

Refer to the PXIe-4139 Calibration Procedure for the external calibration procedure for your instrument.

When you run self-calibration, the output terminal is disconnected. Low-amplitude, low-energy glitches may appear at the output, but in most circumstances, these glitches are not noticeable.

When to Self-Calibrate

For optimum performance, use self-calibration when the following conditions have been met:

• After first installing the PXIe-4139 in a chassis
• After any module that is in the same chassis as the PXIe-4139 is installed, uninstalled, or moved
• When the PXIe-4139 is in an environment where the ambient temperature changes. Refer to the PXIe-4139 Specifications to find the allowable ambient temperature for your instrument.
• When the PXIe-4139 temperature has drifted outside of the specified T_cal since the last self-calibration. Refer to the PXIe-4139 Specifications to find the allowable difference from T_cal for this instrument.
• Once 24 hours elapse after a previous self-calibration

The instrument incorporates a temperature sensor that is used to determine when the temperature changes outside the specified conditions from the previous calibration. When the most recent self-calibration time and temperature are queried using niDCPower Get Self Cal Last Date And Time (niDCPower_GetSelfCalLastDateAndTime) or niDCPower Get Self Cal Last Temp (niDCPower_GetSelfCalLastTemp), the value returned is from the most recent self-calibration. When the one-year calibration interval expires, an external calibration is required.

The result is an instrument that yields full performance over its operating temperature range and recommended calibration cycle. When the recommended calibration interval expires, an external calibration is required to ensure that the device operates within specifications. Some devices, particularly those that provide self-calibration as an alternative to auto-zero, have been designed to minimize the time of self-calibration. Therefore, self-calibration can be run often to reduce offset and gain error with minimal performance penalties.

Accuracy represents the uncertainty of a given measurement or output level and can be defined in terms of the deviation from an ideal transfer function, as follows:

y = mx + b

where m is the ideal gain of the system

x is the input to the system

b is the offset of the system

Applying this example to a power supply or SMU signal measurement, y is the reading obtained from the device with x as the input, and b is an offset error that you may be able to null before the measurement is performed. If m is 1 and b is 0, the output measurement is equal to the input. If m is 1.0001, the error from the ideal is 0.01%.

Parts per million (ppm) is another common unit used to represent accuracy. The following table shows ppm to percent conversions.

Most high-resolution, high-accuracy power supplies and SMUs describe accuracy as a combination of an offset error and a gain error. These two error terms are added to determine the total accuracy specification for a given measurement. NI power supplies and SMUs typically specify offset errors with absolute units (for example, mV or μA), while gain errors are specified as a percentage of the reading or the requested value.

Determining Accuracy

The following example illustrates how to calculate the accuracy of a 1 mA current measurement in the 2 mA range of an instrument with an accuracy specification of 0.03% + 0.4 μA:

Accuracy = (0.0003 × 1 mA) + 0.4 μA = 0.7 μA

Therefore, the reading of 1 mA should be within ±0.7 μA of the actual current.