3. Influence of supply voltage

The power supply circuit can produce several effects on generated or measured signals. The most common power sources are the mains at 50 Hz frequency and batteries. Battery-powered devices, when not equipped with switch-mode voltage conversion, are by far the preferred choice when there is any doubt about the possible effect of mains power. Unfortunately, these devices often lack sufficient power and autonomy. It is common practice, for devices with a mixed power supply, to carry out a preliminary battery test and then connect the mains supply, checking that the result remains unchanged.

Devices powered by the 50 Hz grid produce common-mode and series-mode voltages. These voltages may include DC, AC and pulse components of varying spectra. The user must ensure that the effects of these signals are negligible in relation to the desired result. We will see later in...