Noise from external
sources may cause problems when powering a circuit that operates at a
very low voltage or a circuit that uses or measures very low currents.
The supply itself is one source of noise, which can be broken into two
components: normal-mode noise and common-mode noise. Normal-mode noise
is generated across the supply’s output terminals due to the supply’s
internal circuitry. Common-mode noise is earth-referenced noise
originating from the power line and stray capacitance across the main
transformer. For sensitive circuits, linear power supplies provide much
lower normal-mode output noise than supplies designed using switching
technology but have lower power-conversion efficiency and can be bulkier
and heavier. Switching supplies typically offer more output power in a
smaller enclosure. For noise-sensitive circuits, a linear supply can
have just one-fifth to one-tenth of the noise (5mVp-p vs. >50mVp-p)
of a switching supply. Whenever normal-mode noise is a crucial
consideration, use a linear supply, such as Keithley’s Series 2200
single- and multi-channel power supplies, if possible.
Assess common-mode noise current
Linear power supplies generally have lower common-mode noise than switching supplies. Common-mode noise is generated whenever changing voltages, such as AC voltages and transients (dv/dt) on either the primary or the secondary windings of an isolation transformer, couple current across the barrier. Whenever this current flows through an impedance, the noise voltage generated can degrade load (or DUT) performance or cause load-monitoring measurement inaccuracies. Sources of common-mode noise include voltage transients from rectifier diodes (on the secondary) turning on and off and either the 60Hz line movement or the abrupt voltage transient common with a switching power supply’s primary circuit.
Figure 2 shows a simplified block diagram of a power supply. The quality of the transformer’s construction, including sufficient shielding between the primary and secondary windings, can minimize the stray capacitance between primary and secondary. With minimal coupling capacitance, the noise current flowing through the load won’t generally affect the load’s operation or impact measurements on the load. If the transformer’s primary and secondary aren’t sufficiently shielded from each other, then the coupling capacitance can be large and milliamps of current can flow into the load, creating performance problems and load current measurement errors. For low power and sensitive components, modules, or end products, evaluate the power supply for low common-mode performance. Keithley’s Series 2200 power supplies have common-mode currents of less than 10µA.
Figure 2: Normal-mode and common-mode noise currents