OPERATION
2-6
capacitors (test voltage = 0.5 Vrms). In these cases, use the vernier setting to
obtain the exact voltage required. In general use the largest voltage possible for
the best signal to noise ratio and accuracy.
RANGE The SR715/720 has 4 measurement ranges (0-3). The range may be selected
manually or the unit can autorange. The table below specifies the impedance
ranges for each of the measurement ranges. Each of the four ranges has source
impedance of approximately the mid-scale impedance. Note that the
measurement ranges determine an impedance range (not a value range), so the
ranges of inductance and capacitance depend upon the test frequency. The
parameter f is the test frequency. In addition, the impedance of capacitors is
inversely proportional to its capacitance, so larger capacitors are measured in the
lower impedance ranges.
Measurement Range and Impedance Range
Meas
Range Source R Resistance Inductance Capacitance
3
25.0 Ω 10 µΩ - 0.0001 µH - 99999 µF -
100 Ω
15.9/f H 1.59/f mF
2
400 Ω 100.0 µΩ -
15.9/f H - 1.59/f mF -
1.6 kΩ
256/f H
99.5/f µF
1
6.4 K Ω 1.6 kΩ -
256/f H -
99.5/f µF -
25.6 kΩ
4074/f H
6.22/f µF
0
100 K Ω 25.6 kΩ -
4074/f H -
6.22/f µF -
2000 MΩ
99999H 0.00001 pF
F is test
frequency
AUTORANGING During normal operations the unit automatically changes to the most accurate
range for the device under test. When the unit measures an impedance that is
out of its current range, it goes up or down one range, and makes another
measurement. If this measurement is within the current range it displays it; If not,
it changes ranges (if available), and repeats this process. There is built in
hysteresis to avoid repeated range changes when a component is on a range
boundary. Up range changes occur when the impedance measured exceeds
450% of the midscale impedance (i.e. the source impedance), or 12.5% over the
nominal range limit. Down range changes occur when the measured impedance
drops below 22% of the midscale impedance, or 12.5% below the nominal range.
See the table below for the actual values where the range changes occur.
Range Change Points When Autoranging
Ranging To Lower Impedances Ranging To Higher Impedances
Range Change
Impledance Range Change Impledance
2 to 3
Z < 88 Ω
3 to 2
Z > 115 Ω
1 to 2
Z <1.4 kΩ
2 to 1
Z > 1.8 kΩ
0 to 1
Z < 22.4 kΩ
1 to 0
Z > 29.9 kΩ
RANGE HOLD It is sometimes desirable to disable autoranging since it takes nearly a complete
measurement cycle each time a range changes occurs. This can be annoying if
there are no parts in the fixture and the unit is continuously triggering, since an
empty fixture appears as a very large (nearly infinite) impedance. The unit will
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