How to Add DC Offset to Waveform/Function Generator Output

The waveform and function generator offset functions can be used with any waveform to generate any combination of amplitude and offset within the -5 Vdc to +5 Vdc window into a 50 ohm load. For applications requiring more offset, an external power supply can be used to generate additional offset. To generate DC offset using an external power supply, the following three steps can be used:

1. The function generator must be isolated from ground, i.e. "floating". The 33220A can float up to 42V from ground (output + DC bias). The maximum DC bias can be calculated by subtracting 42V from the peak output.

2. The DC power supplies must be connected in series. BNC-T cannot be used because it will connect the power supplies in parallel.

3. The voltage at the function generator output should be less than 42V. Connect the power supply to ground and calculate the voltage at the function generator output. The output voltage will be the DC voltage plus the peak voltage of the generator.

The figure below shows a model of a 33220A waveform and function generator connected to an external power source and load.

An easy way to make connections on the 33220A and 33250A Waveform and Function Generators is to access the floating ground through the Modulation Input BNC, which is located on the rear panel. You can use the BNC connector minus the center conductor; connect this wire to the high power side of the power supply. Using only one conductor, connect the center conductor of the Signal Output, Function Generator Output BNC, to the high side of the load (do not use the shield of the Output BNC). The low side of the load connects to the low power side of the power supply. Finally, the low power side of the power supply connects to ground.

Other series of function generators can also be operated similarly to increase the DC bias.