High Amplitude Arbitrary/Function Generator Simplifies Measurement in Automotive, Semiconductor, Scientific and Industrial Applications Application Note
Signal Amplitude and Load Impedance
The output voltage delivered by a signal generator depends on the impedance of the connected load or device under test. The reason for this lies in the output impedance of the generator. As an example, Figure x shows the equivalent output circuit of the AFG3011. Depending on the amplitude setting, the instrument delivers a certain current I. If a load
of 50 Ω is connected to the generator output, half of I
flows through the generator's output impedance ZOUT and the other half through ZDUT. If ZDUT has an impedance that is significantly larger than ZOUT, then almost all of I flows
Figure x. Equivalent Output Circuit of AFG3011.
, resulting in almost twice the output voltage
compared to 50 Ω loads.
Specification sheets for arbitrary/function generators typically state the maximum output amplitudes for 50 Ω and for high impedance loads. The output amplitude of the AFG3011, for example, is specified as 20 Vpp for 50 Ω loads, and 40 Vpp into open circuits. For other load impedance values, the maximum output voltage can be calculated with the following formula:
In their standard setting, arbitrary/function generators are commonly configured for a load impedance of 50 Ω. For other load impedances, the impedance value can be config- ured into the instrument to enable the display of the correct amplitude and offset values. In the AFG3000 Series, the load impedance setting is made in the Output Menu, which becomes accessible after pressing the desired function button, e.g "Sine".
Figure y. Load Impedance Selection on the AFG3000 Series.
Please note that the load impedance setting does neither change the generator's output impedance nor the load impedance itself. It merely impacts the amplitude and offset display, and ensures that the instrument displays the correct values of the effective amplitude across the connected load.