App-Note :: AC Voltage Measurement Errors in Digital Multimeters.
When you make measurements with a digital multimeters (DMM), common errors will crop up. The following discussion will help you eliminate potential measurement errors and achieve the greatest accuracy with a DMM. This paper covers ac voltage measurement errors. For an overview of system cabling errors and dc voltage measurement errors, see Application Note 5988-5511EN. For a discussion of resistance; dc current; ac current; and frequency and period measurement errors, see Application Note 5988-5512EN.
Common Mode Errors
Errors are generated when the DMM’s input LO terminal is driven with an ac voltage relative to earth. The most common situation where unnecessary common mode voltages are created is when the output of an ac calibrator is connected to the DMM "backwards." Ideally, a DMM reads the same regardless of how the source is connected. However, both source and DMM effects can degrade this ideal situation.
Because of the capacitance between the input LO terminal and earth (approximately 200 pF for the Keysight 34461A), the source will experience different loading, depending on how the input is applied. The magnitude of the error is dependent on the source’s response to this loading.
The DMM’s measurement circuitry, while extensively shielded, responds differently in the backward input case due to slight differences in stray capacitance to earth. The DMM’s errors are greatest for high-voltage, high-frequency inputs. Typically, the DMM will exhibit about 0.06% additional error for a 100 V, 100 kHz reverse input. You can use the grounding techniques described for dc common mode problems to minimize ac common mode voltages (see Application Note 5988-5511EN).
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