AN3011 datasheet(24/37 Pages) STMICROELECTRONICS

24 / 37 page

Efficiency

AN3011

24/37

Doc ID 16043 Rev 1

input voltage is measured directly to the UUT input terminal, and the input current does not

affect the measured input voltage.

The voltage across the voltmeter causes a leakage current inside the voltmeter itself (which

is not ideal and which doesn't have infinite input resistance). If the switch in Figure 23 is in

position 2 (see simplified scheme of Figure 25) the voltmeter leakage current is measured

by the ammeter, together with the UUT input current, causing a measurement error. The

error is negligible in a case where the UUT input current is much higher than the voltmeter

leakage. If the UUT input current is low, and not much higher than the voltmeter leakage

current, it is probably better to set the switch (in Figure 23) to position 1.

In a case where it is not certain which measurement scheme least affects the results, it is

possible to try with both and register the input power lower value.

Figure 23.

Wattmeter possible connections with the U.U.T. (unit under test)

Figure 24.

Wattmeter connection scheme for low input current

AN3011 Datasheet(PDF) 24 Page - STMicroelectronics