HP WaterSkis E1412A manual Low-Level Measurement Errors, Voltage Measured = Vin2+ Noise2

Low-Level

Measurement

Errors

When measuring ac voltages less than 100mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. Exposed (unshielded) cabling will act as an antenna and a properly functioning multimeter will measure the signals received. The entire measurement path, including the power line, acts as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the multimeter’s input. For this reason, you should apply low-level ac voltages to the multimeter through shielded cables. You should connect the shield to the input LO terminal.

Make sure the multimeter and the ac source are connected to the same electrical outlet whenever possible. You should also minimize the area of any ground loops that cannot be avoided. Measurements of high-impedance sources are more susceptible to noise pickup than measurements of low- impedance sources. You can reduce the noise pick-up by placing a capacitor in parallel with the multimeter’s input terminals. You may have to experiment to determine the correct capacitor value for your application since this capacitance will contribute some loading error.

Most extraneous noise is not correlated with the input signal. You can determine the error as shown below.

Voltage Measured = Vin2+ Noise2

Correlated noise, while rare, is especially detrimental because it will always add directly to the input signal. Measuring a low-level signal with the same frequency as the local power line is a common situation prone to this error.

AC Turnover Errors Errors are generated when the multimeter’s input LO terminal is driven with an ac voltage relative to earth. The most common situation where unnecessary turnover errors are created is when the output of an ac calibrator is connected to the multimeter “backwards.” Ideally, a multimeter reads the same regardless of how the source is connected. Both source and multimeter effects can degrade this ideal situation.

Because of the capacitance between the input LO terminal and earth (approximately 200 pF for the HP E1312A and HP E1412A), the source will experience different loading depending on how the input is applied. The magnitude of the error is dependent upon the source's response to this loading. The multimeter's measurement circuitry, while extensively shielded, responds differently in the backward input case due to slight differences in stray capacitance to earth. Because of this, the 100Vac and 300Vac ranges may latch up for high voltage, high frequency “backward” inputs. Therefore, only drive the high terminal when measuring ac voltages. You can use the grounding techniques described for dc common mode problems to minimize ac common mode voltages (see Common Mode Rejection (CMR) on page 27).

Chapter 2 HP E1312A/E1412A Multimeter Application Information 35