Agilent Technologies 4294A Specifications download pdf (Page 24)

2-4-3. Ranging function

To measure impedance from low values to high values, impedance measurement instruments have

several measurement ranges. Generally, 7 to 10 measurement ranges are available and the instru-

ment can automatically select the appropriate measurement range according to the DUT’s imped-

ance. Range changes are generally accomplished by changing gain multiplier of the vector ratio

detector, and by switching the range resistor (Figure 2-8 (a)). This insures that the maximum signal

level is fed into the A to D converter to give the highest S/N ratio for maximum measurement accu-

racy.

The range boundary is generally specified at two points to give an overlap between adjacent ranges.

Range changes occurs with hysteresis as shown in Figure 2-8 (b), to prevent frequent range changes

due to noise.

On any measurement range, the maximum accuracy is obtained when the measured impedance is

close to the full-scale value of the range being used. Conversely, if the measured impedance is much

lower than the full-scale value of the range being used, the measurement accuracy will be degraded.

This sometimes causes a discontinuity in the measurement values at the range boundary. When the

range change occurs, the impedance curve will skip. To prevent this, the impedance range should

be set manually to the range which measures higher impedance.

Figure 2-8. Ranging function

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Agilent Technologies 4294A Specifications Page 24