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Adding a dc offset to the output of your function generator

Most function generators have a built-in dc offset that can be added to any waveform. The 33220A's built-in dc offset function can be used with any waveform to generate any combination of amplitude and offset within a -5Vdc to +5Vdc window into a 50-Ohm load. For applications which require a larger offset, you may be able to use an external power supply to generate the additional offset. In order to use an external supply to create the DC offset - follow these three steps:

  1. The Function Generator needs to be isolated from ground, "floats". The 33220A can float up to 42 Volts (output plus dc offset) from earth ground. The maximum dc offset can be calculated by subtracting the peak output from 42 Volts.
  2. The dc power supply needs to be connected in series. A BNC-T can NOT be used as this would connect the supply in parallel.
  3. The voltage at the output of the function generator should be less than 42V. Connecting the power supply to ground makes it possible to calculate the voltage at the output of the function generator. The output voltage will be the dc supply voltage plus the peak voltage of the generator.

The diagram below shows a model of the 33220A, the connections to an external supply and the load.

33220A model with connections to supply

33220A, 33250A and 33120A Tip

An easy way to make these connections on the 33220A and 33250A is to access the floating ground through the Modulation In BNC, which is located on the rear panel. You can use a BNC connector minus the center conductor; connect this wire to the high of the power supply. Use just a wire to connect the signal output, center conductor of the output BNC of the function generator, to the high side of the load (do not use the shield of the Output BNC). The low side of the load is connected to the low of the power supply. Finally, the power supply low is connected to earth ground.

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