OPERATION
2-9
EXTERNAL BIAS The SR715/720 has rear panel connections to allow an external bias voltage of
up to +40VDC to be applied across the DUT. This supply must be floating (neither
side connected to ground) and must be current limited to less than 250 mA. A
linear supply is recommended (vs. switching supply). The supply should be well
filtered, to remove ripple from the bias voltage. Connections to the bias supply are
made on the rear panel with two banana jacks. The red jack is the positive side of
the supply and the black, the negative. There is an internal diode within the unit to
prevent a negative voltage from being applied. The bias supply lines are fused
with a 250 mA fuse on the rear panel, next to the banana jacks. If the SR715/720
doesn't yield stable readings when the external bias switched is on, check this
fuse. If the bias supply is connected backwards, there is a good chance that the
bias supply fuse will blow. Provisions need to be made to discharge the bias
voltage from the capacitor under test before removing it from the fixture. In
general, external switches or discharging resistors need to be provided with the
bias supply to ensure safe operation.
To apply an external bias voltage, ensure that the bias supply is connected
correctly. Then verify that the capacitor is installed with the correct polarity in the
test fixture (positive to the right). Next press the [External] bias key to apply the
bias voltage. After a second or two, the reading should be stable. After the
measurement is complete, discharge the DUT before removing it from the fixture.
For occasional use (non production) with small capacitors (<500 µF) and low bias
voltages (<20 VDC), the SR715/720 can discharge the capacitor internally. Press
the [External] bias key to switch off the bias. The capacitor will then discharge
through the unit in the same manner as the internal bias. Another solution for
somewhat larger bias voltages and/or capacitors is to connect a resistor across
the terminals of the bias supply. Switch off the bias supply and allow the capacitor
to discharge through the resistor before removing it from the fixture. Make certain
the discharge resistor is rated to handle the steady state current that the bias
voltage will generate through it and that the supply can provide this additional
current. For larger capacitors or higher supply voltages it will be necessary to
provide external switches to remove the bias voltage and discharge the capacitor.
MEASUREMENT RATE The SR715/720 has three measurement rates: slow, medium and fast. These
correspond to about 2, 10 and 20 measurements per second on the 1 kHz, 10kHz
and 100 kHz (SR720 only) frequencies and about 1/10 of that for 100 Hz and 120
Hz frequencies. The table below lists the maximum measurement rates for the
different test frequencies. These rates are attainable when the unit is not in
autorange mode, binning is disabled, and neither GPIB nor RS232 interfaces are
active. The measurement rate affects the accuracy, with slow and medium being
the most accurate and fast being the least accurate.
Measurement Range Number of measurements/sec
Frequency
Slow Medium Fast
100 kHz 2.8 14 28
10 kHz 2.8 14 27
1kHz 2.7 13 24
120 Hz 0.7 2.8 7
100 Hz 0.6 2.4 6
The actual measurement time can be calculated from the following equation.
T
meas
= T
s
+ [(N
i
•1/f + T
di
+ T
rs
+ T
d
) • N
m
] + T
calc
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