Voltage Source Reset Circuitry - Keithley 595 Instruction Manual

THEORY OF OPERATION
7.4.4 Reset Circuit
(current or charge) to a 0.2V or 2V full range voltage.
Because of the very low levels of current and charge to
Since sensitive devices are likely to be connected to the out-
which the instrument is sensitive, the preamplifier is re-
put ef the voltage source and may not be removed before
q<ied to have an input bias current of <5fA. Figure 7-9
power is turned off, it is desirable to have the source reset
shows a block diagram of the preamplifier. The input stage
(disconnect) when power is removed. This is advisable
(4302)
and op-amp (U305) form a compound amplifier
because the output of the source is determined by digital
stitisfying the input requirements. Feedback is applied
signals which may be erroneous during power down before
around this compound opanp to set the proper transfer
the analog supplies decay to zero. The reset circuitry is
function of the preamplifier.
shown in Figure 7-8. A half-wave rectifier is formed by
CRW, Rl63, R165 and Cl38. Cl38 is charged through R163
and then partially discharged through ~163 and ~165 as the
17.5.1 Preamplifier Confiw~tions
AC voltage cycles. The result is an average of around l2V
aaoss CE8. This level~is compared to the +6.3V of the D/A S
ifi
reference circuit by U142C. When power is turned on, this
pet c c&&gurations of the preamplifier for current and
will by sensed Within two line cycles, releasing the open
capacitance measurements are shown in Figure 7-10. The
collector output of Ul42C. The gate of
Qls
Will then be
Model 595 measures full range current levels from 2OpA
raised by R139, R162 and Cl42 to its threshold in 1.5s, tur-
to 2001~~ using 10k, 998k, 10oM and 1OGn feedback
ning on KlOl and connecting the output of the source to
resistors. For capacitance, full range meausurements are
fr
the BNC connector (see Figure 7-6). On power down, Cl38
om 2oOpF to 2OnF using feedback capacitors of 1OpE lOOpF
and w,
is discharged in 10111s to 26111s to below 6.3V. Thus, the out-
put of U142C pulls down, turning off Qll5 abruptly. This
~~~
opens KmL disconnecting the output of the source from
the BNC connector.
Before each capacitance measurement, the feedback charge
amplifier is discharged by a low-leakage bridge switch
(simified bv S) which is described in more detail in
7.5 PREAMPLIFIER
The preamplifier of the Model 595 converts the input signal
(+6.3V)
-13
Figure 7-6. Voltage Source Reset Circuitry
7-8