Self-Calibration

Every measurement instrument performs within its specifications over a finite temperature range and time. If the excursions in temperature and time exceed those specified, and you need performance close to the specifications, then you must recalibrate the instrument at the new temperature or after a certain amount of time. The NI 4070/4071/4072 has a self-calibration function that allows you to perform a calibration at the temperature you are making your measurements.

Self-calibration fully recalibrates all functions for DC volts and resistance.

Note  niDMM Self Cal and niDMM_SelfCal reconfigure the DMM without caching the previous configuration. When you attempt to read after self-calibration completes, you receive an error for an invalid function for the current operation mode for reading a waveform, a timeout error for multiple point acquisitions, and a read error for using the incorrect mode for single point acquisitions.

Self-calibration makes the NI 4070/4071/4072 highly accurate and very stable at any operating temperature—well outside of the traditional 18 °C to 28 °C range.

The typical way of expressing accuracy is:

Accuracy = ±(X ppm of reading + Y ppm of range)

For example, if the last calibration (self or external) was performed at 23 °C, and you are using the 10 V range on the NI 4070/4072 and applying a 5 V signal within 18 °C to 28 °C, the 2–year accuracy is:

2–year Accuracy = ±(25 ppm of 5 V + 6 ppm of 10 V) = ±185 µV (uncertainty)

This traditional calculation method can cause errors if the temperature is not tightly controlled. When a system is built of multiple traditional instrument components integrated together, the instruments are subject to temperature rises caused by inherent compromises in air circulation and other factors.

If the ambient temperature is outside of the 18 °C to 28 °C range, to calculate the measurement accuracy, you need to calibrate your instrument at the ambient temperature, or add temperature coefficient accuracy for each additional degree outside the calibration range.

Using the NI 4070/4072, assume the ambient temperature is 38 °C. The temperature coefficient (tempco) without self-calibration is:

Tempco = (1 ppm of reading +1 ppm of range)/°C

The 2–year accuracy at 38 °C becomes:

±[25  ppm of 5 V + 6 ppm of 10 V] + [(1 ppm of 5 V + 1 ppm of 10 V) x 10] = ±335 µV (uncertainty)

Externally recalibrating a system can be difficult and expensive and as a result generally is performed infrequently.

The NI 4070/4071/4072 incorporates a proprietary self-calibration function for DCV and resistance. This function is unique to the NI 4070/4071/4072 in the following ways:

• Self-calibration corrects for all DCV gain and offset errors within the DMM using a precision, high-stability internal voltage reference, which has an outstanding temperature coefficient and time drift.
• Self-calibration also accounts for all resistance, source, and gain errors.
• In resistance, all errors are corrected to a single internal 10 kΩ aerospace grade, high-stability foil resistor, which is stable to within 0.8 ppm/°C over the full operating range.

After performing self-calibration at 38 °C, you do not need to add any uncertainty to account for temperature change.

The 2–year accuracy after self-calibration at 38 °C becomes:

±(25  ppm of 5 V + 6 ppm of 10 V) = ±185 µV (uncertainty)

Note  ADC calibration adjusts the drift of the ADC only. While the ADC is the largest drift component of a DMM, other minor, second-order factors exist, such as resistance current sources and signal conditioning gain stages. Self-calibration fully corrects these factors, including the ADC. The following table lists your calibration choices and when to use them:

Calibration Option	Application	When
Factory calibration	All functions Recalibrates for time drift of onboard references Corrects for AC flatness	Every two years
Self-calibration	Recalibrates the measurement path and ADC for DC volts and resistance	Every 90 days or when the temperature changes more than 5 °C from last self-calibration
ADC calibration	Recalibrates ADC drift for all functions	Every reading when selected
NONE	Add to all specification 50 ppm of reading every 90 days and 3 ppm of reading/°C typical	Extended high-speed acquisitions

The NI 4070/4071/4072 incorporates a temperature sensor that can be read to determine when the temperature exceeds 5 °C from the previous calibration. In addition, the previous self-calibration time and date can also be read. For maximum accuracy, NI recommends self-calibrating once every 24 hours or when the temperature changes by more than ±1 °C on the NI 4071 and more than ±5 °C on the NI 4070/4072. Otherwise, NI recommends running self-calibration every 90 days. NI does not recommend running self-calibration more than once a day.

During self-calibration, the internal circuitry is automatically disconnected from the input. Therefore, for the majority of applications, users do not need to disconnect the input signals during self-calibration. However, excessive signal levels (>30 VDC, >30 VAC_rms, >20 kHz) at the input terminals of the NI 4070/4071/4072 generate a self-calibration error. Disconnecting the input signals during a self-calibration prevents the error from occurring. If you are using the NI 4070/4071/4072 as part of a switching system, you can open (disconnect) the connections to the NI 4070/4071/4072 input terminals, switch to a non-connected path, or switch to a low voltage, low frequency path. For optimum reliability, avoid application of current to the Amps terminals or signals of >30 V to the Volts Input terminals during self-calibration.