Eliminate Traditional Digital Multimeter Accuracy Errors

This tutorial goes through accuracy calculations and demonstrates how and why the NI 4070 outperforms traditional 6 ½ digit digital multimeters with regards to accuracy and stability. You can also view an interactive presentation that takes you through this tutorial's material at your own pace.

For additional information and more interactive tutorials on the NI 4070, visit the Digital Multimeter Measurement Tutorial.

Challenges in Maintaining Digital Multimeter Accuracy

Keeping the environmental temperature of a precision instrument within its operating temperature range can be challenging in a production environment or in a test system. This challenge can arise because systems are normally composed of multiple instruments and sources. Instruments in such a system are subject to temperature rise caused by inherent compromises in air circulation. These environmental changes along with normal climate changes can affect sensitive measurements because circuit components will drift and the result is a degradation of system accuracy.

Whenever a traditional digital multimeter is used outside of its 23 ± 5ºC temperature range, the accuracy specifications must be derated by a temperature coefficient, usually on the order of 10 percent of the accuracy specification per ºC. Therefore, 10 ºC outside of the specified range, you may have twice the specified measurement error, which can be a serious concern when absolute accuracy is important.

This tutorial takes you through accuracy calculations and demonstrates how and why the NI 4070 outperforms traditional 6 ½ half digit digital multimeters.

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Digital Mulitmeter Accuracy Calculation - Operating Within Specified Temperature Range

Accuracy under normal temperature conditions is defined as:

• Accuracy = Reading ± Measurement Uncertainty

The measurement uncertainty is the sum of two terms.

• Measurement Uncertainty = ppm* of reading + ppm of range

A typical one year accuracy specification for a traditional digital multimeter could be equal to:

• 35 ppm of the reading + 5 ppm of the range (for 23 ±5ºC temperature range).

You can now calculate the accuracy of the digital multimeter (the calculations assume you are taking a 5 V reading while using a 10 V range).

• Measurement Uncertainty = 35 ppm of 5 V + 5 ppm of 10 V
• Measurement Uncertainty = [(35/1E6) * 5 V] + [(5/1E6) * 10 V] = 225 µV
• Accuracy = 5 V ± 225 µV (for measurements within the 23 ± 5 ºC temperature range)

* ppm (part per million) = 1/1E6 = 0.0001%

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Digital Multimeter Accuracy Calculation - Operating Outside Specified Temperature Range

Let’s continue our accuracy calculation to consider an accuracy of our traditional digital multimeter outside the 23 ± 5ºC temperature range. To calculate the amount of increase in uncertainty you can use the tempco specification (short for temperature coefficient) of the device. Tempco is defined as the error introduced by a change in temperature. With regards to ppm, we can define tempco as:

• Tempco = (ppm of reading + the ppm of the range) / ºC.

This example uses the same 5 volt reading on a 10 V range but at 50 ºC. To determine the additional uncertainty at 50 ºC, you take the temperature difference multiplied by the tempco of the digital multimeter.

A typical tempco for a traditional digital multimeter is:

• Tempco = (5 ppm of reading + 1 ppm of range)/ ºC = (5 ppm of 5 V reading + 1 ppm of 10 V range)/ºC = 35 µV / ºC

Continuing, you can now calculate the additional certainty and accuracy of the digital multimeter at 50 ºC.

• Additional Measurement Uncertainty = (50 ºC - 28 ºC) * Tempco = 22 ºC * 35 µV / ºC = 770 µV
• Total Measurement Uncertainty = 225 µV + 770 µV = 995 µV
• Accuracy at 50 ºC = 5 V ± 995 µV

This measurement at 50 ºC ambient temperature is nearly five times worse than the specified one year accuracy.

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Accuracy Benefits of the NI 4070 FlexDMM's Self-Calibration Functionality

To help minimize the errors due to temperature drift, the NI 4070 employs some of the most stable onboard references available in the market. These precise, stable references provide much higher accuracy and much larger operating temperature range for the NI 4070 FlexDMM.

The FlexDMM uses a well-known voltage reference that is thermally stabilized and provides unmatched performance. This voltage reference is thermally shielded for optimum performance. The result is a typical reference temperature coefficient of 0.05 ppm/ºC. Time stability of this device is on the order of 8 ppm/year.

Resistance functions are referenced to a highly stabilized metal-foil resistor originally designed for demanding aerospace applications. This component has a typical temperature coefficient of 0.05 ppm/ºC and a time stability of less than 25 ppm/year.

NI 4070 Self-Calibration

By utilizing the previously mentioned stable on-board references, the NI 4070 FlexDMM incorporates a proprietary self-calibration function for DC volts, resistance, and digitizer readings. Self-calibration performs two main tasks:

1. Corrects for all signal-path gain and offset errors within the DMM back to the precision, high-stability internal voltage reference.
2. Accounts for all resistance current source, gain, and offset errors. In resistance mode, all errors are adjusted back to the single internal precision resistor.

The result is a highly accurate, ultra-stable digital multimeter that has accuracy specifications guaranteed over the entire operating ranges of

• 0 ºC - 55 ºC for the PXI-4070
• 0 ºC - 40 ºC for the PCI-4070.

Self-calibration fully recalibrates all ranges of voltage and resistance functions and takes only one minute. No other 6 ½ digit digital multimeter has this capability. Additionally, self-calibration allows the NI 4070's accuracy specifications to be guaranteed for two years, were as traditional digital multimeters have a 1 year calibration cycle. This improves overall accuracy and reduces system’s downtime. Performing self-calibration is quick and simple when using LabVIEW and other software applications with NI DMM driver software. A single VI or function call that is installed with the NI-DMM driver software can be configured to perform a self calibration on the NI 4070.


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Comparison of NI 4070 FlexDMM to Traditional Digital Multimeters

By combining the stable on-board references and the proprietary self-calibration functionality, the NI 4070 FlexDMM delivers excellent accuracy and stability. The table shows the remarkable difference in accuracy between the NI 4070 and traditional digital multimeters. This table illustrates the measurement uncertainty that we calculated for traditional digital multimeters when measuring a 5 V reading on a 10 V range.

As shown in figure 2, the NI 4070 FlexDMM introduces no increase in measurement error at 50 ºC. The NI 4070 accuracy far exceeds common digital multimeters and when measuring at 50 ºC with self-calibration, the NI 4070 is almost 8 times more accurate than traditional digital multimeters.


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