Past Discharge Bridge - Keithley 595 Instruction Manual

THE&Y OF OPEFlFiilON
C304
----iii
K306
-I
K307
C30B
C323
I I
I
0
47rF
R304
1otn
R309
,
IOkn
Figure 7-12. Feedback Element
7.5.4 Fast Discharge Bridge
During capacitance measurements, the preamplifier of the
Model 595 acts as a charge amplifier, integrating all charge
which flows into the instrument input. Because the pre-
amplifier output will eventually saturate at the supply rail,
it is necessary to periodically discharge the preamplifier and
begin integrating again from a known, low value of charge.
Jn order to
m
aximize dynamic range of the measurement
(resolution compared to maximum reading), the preampli-
fier is discharged before every capacitance reading. To
minhize the effects of charge injedion (charge hop), the
baseline chage is measured following discharge and subse-
quent charge measurements are referrenced to this baseline.
The discharge switch is a low leakage diode bridge (Q304
and 4305) as shown in Figure 7-13. The switch is biased
on by passing a current of 2OuA through it. This turns all
four diodes on. Bias current to the top of the bridge is pro-
vided through R351 when the discharge line (U302 pin 11)
goes to 5V. This 5V signal is inverted to -5Vby R353, U314
and R352. The bias current from the bottom of the bridge
thus flows through R350 when discharge causes the out-
put of u314 to go to -5V.
The level of charge injection during switching is high
enough with this switch to require compensation. This is
provided by W, R303, R346 and C320. The re%tors form
and adjustable divider with a &?5OmV output step that
coincides with the switch transition. Through C320, this
corresponds to a f5pC charge injection~~compensation
range.
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