Ohmmeter - Keithley 600A Instruction Manual

MODEL6GOAEUXXF0~
CIRCUIT DISCUSSION
compared to the NORMAL connection,
correspcnding to a loo-fold
increase in response speed.
(b) The input drop will be reduced~ about 100 times.
If Fig. 600-l is again consulted, it will be seen that this connecticn
converts the 600A into an operational amplifier with a resistor from
the output to the input. Therefore, the following cautions apply:
(a) The input cannot be shorted since this will remove the feed-
back.
(b)
The internal impedance of the current source being measured
should not be less than about one-tenth of the value of the feed-
back resistor used for measurement.
(c) This connection should not be used for measuring the leakage
current of capacitors since the connection of the capacitor to the
input causes the circuit to be transformed into a differentiator
with the resultant extreme sensitivity to very small voltage tran-
sients. For this measurement the NORMAL should be used.
(d) Do not attempt to use the FAST connection for currents exceed-
ing 0.1 microamperes.
C. OHMMETER
(1) Two Terminal Xethod
The Model:600A employs a
linear scale to provide a
megohmmeter of high accur-
acy. The linear ohms scale
is achieved by supplying a
constant current to the
sample and measuring the
voltage drop across it.
The method is shown in
Fig. 600-h.
CIOURE 600-4
(2) Ohms measurement using
GUARD terminal:
There are two disadvantages
with the method just out-
lined:
(a) When measuring resistanoes greater than 1011 ohms, input capa-
city causes the meter reading to be annoyingly slow.
(b) If it is desired to use large voltages across the sample, this
same voltage will appear across the input insulation of the electro-
meter and the input insulation will contribute error to the reading
because of the current also flowing in it.
Og62R
4-3