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Tip 4: Measuring High Frequency Signals - Loading Errors
In the ac voltage function, the input of a multimeter will look like a large resistor in parallel with a capacitor. As an example, the 34401A appears as a 1 MΩ resistance in parallel with 100 pF of capacitance. The cabling that you use to connect signals to the multimeter will also add additional capacitance and loading. The table below shows the multimeter´s approximate input resistance at various frequencies.
| Input Frequency | Input Resistance |
|---|---|
| 100 Hz | 1 MΩ |
| 1 kHz | 850 kΩ |
| 10 kHz | 160 kΩ |
| 100 kHz | 16 kΩ |
The loading error can be determined using the formulas in the figure. The loading error is predicable and always lowers the reading. The results from the calculation will be negative to indicate that the measurement will be low.