|NI-SCOPE (English | Japanese)|
Every measurement instrument performs within its specifications over some finite temperature range and time period. If the temperature changes and time exceed those specified, and your application requires tight specifications, calibration is required.
For example, if the accuracy of a digitizer is specified as ±(1% of input + 10 mV), and you apply 5 V to the input, the error is:
1% of 5 V + 10 mV = 60 mV for temperature range 18-28 °C
This example demonstrates the traditional method of specifying accuracy. The problem with the traditional method is that in a system environment, temperature is not easily controlled. 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.
If the ambient temperature is outside of the 18-28 °C range, you may need to know exactly what the measurement accuracy is to compensate for this temperature variation. With the traditional method, the only way to get the specified accuracy outside of the 18-28 °C range is to externally calibrate the system at the desired temperature. However, an external calibration is time-consuming and expensive and is infrequently done, so the specified accuracy is rarely obtained. You can learn more about external calibration at ni.com/calibration. In the example, if the ambient temperature of the digitizer is 48 °C, assuming the Tempco (TC) error is specified as
TC = (0.1% of input +1 mV)/ °C (a typical number is 10% of accuracy/ °C)
The additional error is
20 °C x TC = ±(2% of input + 20 mV) or 120 mV
The total error is three times the specified error (180 mV in the example above, versus 60 mV if temperature effect is ignored) due to the 48 °C ambient temperature.
To eliminate errors caused by changing temperatures, NI-SCOPE provides a highly repeatable self-calibration function.
For the NI 5122/5124/5142, this self-calibration capability yields the following benefits:
Using the example as a comparison, the NI 5122/5124/5142, using self-calibration and a 5 V input, will have an error at 48 °C of
5 V(0.65% of input + 8 mV) = 40.5 mV error
This result compares favorably to the 180 mV error for a typical digitizer as described in the example without self-calibration.
For optimum performance, use self-calibration when the digitizer is placed in a new system, any time the temperature changes more than 5 °C from the previous self-calibration, or 90 days after the previous self-calibration. The result is a product that yields full performance over its operating temperature range and two-year calibration cycle for DC accuracy, AC response, and trigger level/timing. When the two-year calibration interval expires, an external calibration is required.
The NI 5122/5124/5142 has a temperature sensor that monitors temperature variations. The previous self-calibration time and date can also be read. Unless temperature variations are a serious problem, self-calibration is not recommended more than once per day.
The NI 5122/5124/5142 internal circuitry is automatically isolated from the input during self-calibration. However, problems may occur if high-voltage, high-frequency signals (in excess of 500 V/µs slew rate) are present during self-calibration.
When in doubt, disconnect the inputs as directed. If you are absolutely certain that the maximum slew rate of the input signal is below 500 V/µs, then it is acceptable to leave the input signal connected during self-calibration.
The following diagram shows the typical programming flow for self-calibration.
NI-SCOPE provides the Calibrate example, which you can find by using the shortcut at Start»All Programs»National Instruments»NI-SCOPE»Examples.
A summary of the calibration options available and when to use them is shown in the following table.
|External calibration||Calibrate time drift of onboard reference||Every 2 years||Calibrates and verifies to full specifications|
|Self-calibration||Offset and gain|
|90 days, or when temperature changes |
|Ensures range to range matching|
Ensures trigger accuracy
Optimizes performance with external 10:1 probes
|No calibration||None, within 2 year calibration cycle or if temperature stays within ±5 C||High accuracy not required outside of 5 °C||If self-calibration is not used, derate the accuracy using the specified Tempco|