Precisionstep Source; Output Stage - Keithley 595 Instruction Manual

THEORY OF OPERATION
+lO.ZSV +1sv
+6.3V
TO
OUTPUT
STAGE'
Figure 7-5. Precision Step Source
7.4.2 Precision Step Source
In making a capacitance measurement, the Model 595 will
measure charge and divide it by the value of a step in
voltage across the ~capacitor being tested. The step source
circuit is shown in Figure 7-5. The resistor divider formed
by Rll4, Rll9, RlZ4, Rl28, RI31 and Rl34 divides the 6.3V
reference down to 5V, O.lV, O.O5V, 0.O.W and O.OlV. The 5V
level is used by Ul33 and associated components to pro-
duce +5A and +5S supplies from +l5V. The remaining four
levels are multiplexed by lJl30 to the input of U123 actor-
ding to the inputs STEP A, B and C. UE!3 is just a low noise
and drift buffer so
its-output
represents any one ofthe four
precision low level voltages or ground.
by Rl67 and Cl40. Inputs to the inverting amplifier are made
by Rl38 and Rl52 from the step-source and D/A converter
respectively. Gain is -Xl for the step source and -X2 for
the D/A converter.
RFI emissions and susceptibility of the source are reduced
with common mode choke TlO3 and bypass capacitors Cl43
and Cl46.
7.4.3 Output Stage
The outputs of the precision steps source and the D/A con-
verter are combined, buffered, and current limited by the
output stage of the voltage source. A simplified diagram
of the output stage (minus the current limiting circuitry]
is shown in Figure 7-6. The output stage is basically an in-
verting amplifier. The compound op-amp is composed of
Ul., Qll3, 4114, and various resistors and capacitors fox
biasing and frequency compensation. Feedback is provided
The current output of the voltage source is limited to 4mA
maximum. Figure 7-7 shows the additional circuitry of the
output stage that provides current~limitig. Current limiting
occurs at 31~4 *lmA. In the positive half of the com-
plementary out&x&stage, QlO3 turns on when a positive
current~limit occurs. The base current to Qll3 is shunted
away preventing the output capabtity of the stage from ris-
ing any higher. CR114 is kept reverse biased during nor-
mal operation keeping the open-collector output of U142B
off. During current limit, CR114 is forward biased when
QlO3
turns on. This is sensed by U14ZB whose output pulls
down, switching Q107 on and turning on Ul22. The
microprocessor can then detect a current limit condition
in the voltage source via optocoupler UE2. For the negative
half of the complemenary stage, Q104, CRll5, U142A and
Rl4l perform similar functions.
7-5