Protek 9216A, Руководство пользователя, Страница: 25 / 77

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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 9216A has three measurement rates, which are selected by pressing the
Rate button. Slow, medium or fast rate may be selected. Table 3-2 lists the
maximum measurement rates attainable when the meter is not in the auto-range
mode, binning is disabled, and the RS232 interface is active. The measurement
affects the accuracy with slow and medium being the most accurate and fast
being the least accurate.
The actual measurement time can be calculated from the following equation:
T
meas
= T
s
+ ( N
i
/f + T
di
+ T
rs
+ T
d
) N
m
+ T
calc
where T
s
= settling time, N
i
= number of test frequency cycles used in the
measurement, f = test frequency, T
di
= deintegration time, T
rs
= resync time, T
d
=
delay time, N
m
= number of submeasurements per measurement, and T
calc
=
calculation time. T
di
, T
d
and T
calc
are constants, while T
rs
and f are set by the
measurement frequency. N
i
and N
m
are determined by the measurement rate
and T
s
can be set by the user. See Table 3-3 for the appropriate values.
In addition to these factors, the measurement time is increased when using
auto ranging, binning RS232 interface. Binning adds about 2.5 ms to the total
measurement time. Autoranging adds n*(T
meas

1 ms) where n = number of
range changes required and T
meas
is calculated according the equation given
above.
It is difficult to determine the exact amount of time added when using RS-232
interfaces, since it is dependent on baud rate, the speed of the computer, and the
software. It takes about 5 ms for the meter to respond to send back over the
interface.
In general, simple commands and responses like
Table 3-2 Measurement Rates (Number of Measurements/Sec)
Frequency Slow Medium Fast
100KHz 2.8 14 28
10KHz 2.8 14 27
1KHz 0.7 13 24
120Hz 0.7 2.8 7
100Hz 0.6 2.4 6