If I set my amplitude on my AFG3000 series generator+ the actual amplitude as measured by my scope or meter is double the voltage that I have set on the generator. Why does this happen?
The AFG3000 series instruments will output some amount of voltage, specified in V p-p into a 50 ohm load, which is a properly terminated configuration. This also results in the best system performance.
The output circuit has a 50 ohm series termination, so when the output is terminated into 50 ohms, forms a voltage divider that drops half of the output voltage across the 50 ohm load and the other half across the internal series resistor.
If you are working at lower frequencies and are not impacted by the very high frequency ringing that can occur on the fast transitions of a square wave, then using a load that is higher than the recommended 50 ohms can give you as much as double the output voltage.
Examples of various loads:
If we set the output to 2V p-p, we now know that it is producing 4 Volts internally, so:
(50 ohm load / 50 ohm load + 50 ohm series termination) = (50 / 100) = 0.5 * 4 V = 2 V
(1,000 ohm load / 1,000 ohm load + 50 ohm series termination) = (1,000 / 1,050) = 0.9523 * 4 V = 3.8095 V
(1,000,000 ohm load / 1,000,000 ohm load + 50 ohm series termination) = (1,000,000 / 1,000,050) = 0.99995 * 4 V = 3.9998 V
How to get the Amplitude setting of the AFG3000 to read correctly.
- If the scope has a 50 Ohm impedance setting, set the scope to his setting, or use an external 50 ohm termination.
- If the Load is not 50 ohms then you need to tell the AFG3000 what the load value you are using. This may be set for each channel. To access these menus press Function key for the selected channel (i.e. Sine wave) then press Output Menu > Load Impedance. Here you may select 50 ohm, Load (any value from 0 - 10k ohm), or High Z (any value above 10 K ohm). Now the AFG will calculate the output voltage being applied based on the load impedance connected.
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