Ammeter - Keithley 600A Instruction Manual

CIRCUDl! DISCUSSION M3DEL 6OOAELBXF0ME!!PE2
The ZERO ccntrcls function by adjusting the dc voltage of the dummy
electrometer tube grid.
The voltmeter sensitivity is determined by the resistor placed in
series with the meter, R33 through R39. Meter current passing
through Rh2 develops the constant one volt output for any ranse.
C3 and R6 serve to stabilize the feedback loop against oscillation.
B. AMMETER
OUTPUT
CHASSIS GROUND
Y
Grid current of the input
electrometer tube fixes the
minimum current that may be
measured. The Model 600A
grid current will us~&lly
be less than 2 x lo-
amperes. On low current
ranges, the grid current
will be apuarent and may be
balanced out with ZERO con-
trol or subtracted from the
final reading.
PIQURE' 600- 3
(1) Shunt Resistor Method (NORMAL)
In the normal operating connection as shown in Fig. 600-3, current is
measured by placing a resistor across the input terminals and measuring
the voltage drop.
Currents from 3 amperes to lo-13 amperes may be
measured by this method since the range switch selects resistors ranging
from one ohm to 1011 ohms
in
decade steps.
The voltage drop is selected
by the MULTIPLW switch; the setting is the input voltage drop for full-
scale meter deflection.
(2) Feedback Method (FAST)
In the voltmeter discussion above, floating ground has been driven by
the cathode follower and output ground has been connected to the low
impedance side of the input conne&or. In the FAST connection, shown
in Fig. 600-1, the amplifier ground is connected to the low impedance
side of the input; the cathode follower ground floats, and
negative
feedback is applied through the shunt resistor.
In the Model 6OOA, it is possible to use this connection with currents
of 0.1 microamperes or less.
To change the connection, remove the lock
from NORMAL-FAST switch at the bottom of the rear panel and change it to
the FAST position. The advantages of this connection are:
(a) The effect of input capacity is largely neutralized, that
is, the time constant of the input and cable capacity and the
shunt resistor used will be decreased at least 100 times as
4-2
0962R