OPERATION
2-8
Range Change Points When Autoranging
Ranging To Lower Impedances
Ranging To Higher Impedances
Range Change
Impledance
Range Change
Impledance
2 to 3
Z < 315
Ω
3 to 2
Z > 400
Ω
1 to 2
Z <5.04 k
Ω
2 to 1
Z > 6.4 k
Ω
0 to 1
Z < 78.8 k
Ω
1 to 0
Z > 100 k
Ω
Under certain conditions the 'ovEr LoAd' message will be displayed. This normally
occurs when the unit is in constant voltage with the range hold on. To correct this,
simply change to a higher impedance range, or autorange.
BIAS
Internal or external DC bias voltages can be applied to capacitors. Electrolytic and
tantalum capacitors need a positive bias for accurate measurements, although
the SR715/720 bipolar test voltage will not typically be enough to damage them.
The 2 VDC internal or up to 40 VDC external bias voltage allows measurements
to be made while approximating actual operating conditions. The external bias
capability also allows C-V measurements to be made on semiconductor devices.
Always make certain that the capacitor being tested is inserted with the correct
polarity.
The bias voltage is always positive to the right
and is marked on the
SR715/720.
Failing to observe the correct polarity can result in the
destruction of the DUT and possible injury.
Be aware that charged capacitors
can hold their charge for a long time if removed from the fixture without
discharging the DC bias voltage. Care must be taken to discharge the capacitors
after making measurements,
ESPECIALLY WHEN USING EXTERNAL BIAS
WITH LARGE VOLTAGES!!
Failing to discharge the device can result in damage
to the device, damage to the SR715/720, or possible injury!! The SR715/720 is
protected internally against discharging capacitors with up to 1 Joule of stored
energy (C•V2 /2).
The internal and external bias circuitry will work only if the unit is in the C+D or
C+R mode. Bias cannot be applied if the unit is in the R+Q, L+Q or the AUTO
mode. The error message of "biAS for c" will appear if the bias key is pressed in
the inappropriate mode. Whenever the bias circuitry is active the unit is in the
constant voltage mode so that the voltage across the capacitor under test will
stabilize in a reasonable amount of time. See the constant voltage section for
effects on ranges and accuracy.
It will take a short while for the internal circuitry and the DUT to stabilize after
applying the bias voltage. The time is primarily determined by the RC time
constant of the source resistance and the capacitor under test plus the internal
AC coupling capacitor of .47
µ
F. During this time the reading on the SR715/720
may change if it is continuously triggered. In this case, the initial readings should
be disregarded. If this is a problem, use the triggered mode (simply wait a short
time before triggering) or increase the settling time. See the section on
measurement rate for information on how to set the settling time. In addition, the
capacitance of some capacitors will slowly drift after a change in DC voltage.
INTERNAL BIAS
To apply the 2.0 VDC internal bias voltage, first verify that the capacitor is
installed with the correct polarity in the test fixture (positive to the right). Press the
[Internal] bias key to apply the bias voltage. After a second or two the reading
should be stable. Press the [Internal] bias key a second time to switch off the bias
to discharge the capacitor before removing it from the fixture.
