Replacing analog null detectors with precision multimeters

Historically, analog null detectors or null meters have been used in electrical metrology to measure small voltage differences between two points or to detect a zero current condition where the voltages at two points are the same. These might include comparing and  measuring the voltage difference between a standard and a device under test such as in the comparison of of two primary level standards or between a primary standard and a secondary calibration standard by interfacing with the instruments directly or with the aid of a voltage divider. The small differences in voltage that are measured by a null detector  allow voltages to be adjusted on standards so that there is effectively no voltage difference between two points or two instruments. The ability to zero voltage, or a null condition, is essential for electrical metrology.

When precision digital multimeters (DMMs) became available with resolution and sensitivities comparable to null detectors, they were quickly adopted, and null detectors were set aside. However, depending on the measurement circuitry characteristics and the unique characteristics of the DMM, significant measurement errors can be created. This application note discusses what to consider when using a DMM as a null detector.

This is especially important as improvements in new DMMs lessen some of the effects of these issues. However, in all cases they need to be understood and appropriately considered.

Since the 1960’s, commercial voltage and ratio calibration systems have been available to calibrate dc voltage from very low  levels–on  the order of millivolts–to relatively high values up to    1 kilovolt. A critical component of these systems was the analog high impedance voltmeter/null meter. These instruments were designed with extremely high input impedance (10 to 100 MΩ), excellent sensitivity (0.1 µv per division) and high isolation (on the order of 1012 Ω).

One series instrument, the Fluke 845 Series of High Impedance Voltmeter Null Detectors, was designed so that source loading through leak- age was virtually eliminated regardless of power line, chassis ground, or guard connections. Input voltages were applied through an input divider and filter circuit to a photo-chopper-stabilized amplifier. The input filter minimized the effects of source noise, and the photo-chopper-stabilized amplifier reduced the input current to a few picoamps. The 845AB could be battery operated, so that it was isolated from line power, had an analog input for nulling operations, provided a good measurement response time (5 seconds on the 1 µv range) and in general was easy to use.

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