Accurate determination of the input impedance of digital voltmeters

A good knowledge of the input impedance of digital voltmeters is sometimes needed in precision measurements, for example in some resistance measurements where the input impedance of the voltmeter shunts the resistors that are being measured. How to determine this input impedance is described. It appears that an accurate determination is only possible when the input bias current generated by the voltmeter input circuitry is taken into account. In addition to a direct current, large current peaks are generated by the input circuitry of the voltmeter due to the switching of this circuitry between the high and low input terminals of the voltmeter. These so-called ´pump-out´ current peaks can significantly affect the measurement, requiring an adjustment of the measuring method.