Test Signal
The NI 4072 uses an AC current source as excitation for capacitance and inductance measurements. The current waveform is a very stable, harmonically limited square wave. The measurement method extracts multiple-tone information contained in the test signal to find the capacitance or inductance of the device under test. The frequency and level of the test signal and the tones extracted from it are shown in the following tables:
| Capacitance | ||||||
|---|---|---|---|---|---|---|
| Range | Fundamental | Third Harmonic | Effective Test Signal | |||
| Frequency | Current | Frequency | Current | Frequency | Current | |
| 300 pF | 1 kHz | 0.5 µA | 3 kHz | 0.16 µA | 3 kHz | 0.16 µA |
| 1 nF | 1 kHz | 1 µA | 3 kHz | 0.33 µA | 3 kHz | 0.33 µA |
| 10 nF | ||||||
| 100 nF | 1 kHz | 10 µA | 3 kHz | 3.3 µA | 3 kHz | 3.3 µA |
| 1 uF | 1 kHz | 100 µA | 3 kHz | 33 µA | 1 kHz | 100 µA |
| 10 uF | 1 kHz | 1 mA | 3 kHz | 330 µA | 1 kHz | 1 mA |
| 100 uF | 91 Hz | 1 mA | 273 Hz | 330 µA | 91 Hz | 1 mA |
| 1,000 uF | ||||||
| 10,000 uF | ||||||
| Inductance | ||||||
|---|---|---|---|---|---|---|
| Range | Fundamental | Third Harmonic | Effective Test Signal | |||
| Frequency | Current | Frequency | Current | Frequency | Current | |
| 10 uH | 10 kHz | 1 mA | 30 kHz | 330 µA | 30 kHz | 330 µA |
| 100 uH | ||||||
| 1 mH | 1 kHz | 1 mA | 3 kHz | 330 µA | 3 kHz | 330 µA |
| 10 mH | 1 kHz | 10 µA | 3 kHz | 3.3 µA | 3 kHz | 3.3 µA |
| 100 mH | 91 Hz | 100 µA | 273 Hz | 33 µA | 273 Hz | 33 µA |
| 1 H | 91 Hz | 10 µA | 273 Hz | 3.3 µA | 273 Hz | 3.3 µA |
| 5 H | 91 Hz | 1 µA | 273 Hz | 0.33 µA | 273 Hz | 0.33 µA |
The digitizer measures the DUT impedance at two frequencies (tones). From these two measurements, the losses are calculated (front-end, cabling, and DUT). Using the calculated losses, the software computes the capacitance or inductance at one of the two frequencies (effective frequency).
The effective test signal is included as a reference. It is the signal that would yield a comparable capacitance or inductance value if measured with the single tone measurement technique.