Keithley 6512, Instruction Manual, Page: 82 / 156

Applications
4-3
*R = displayed resistance.
Figure 4-2 shows the basic circuit configuration for using the
Model 6512 in this manner. A decade current, I, is forced
through the diode under test. The current will develop a for-
ward voltage drop, V
F
, across the diode. The voltage across the
diode can be calculated by multiplying the displayed resis-
tance by the test current (see Table 4-1). For example, assume
that a resistance reading of 50k
Ω
is measured with the instru-
ment on the 200k
Ω
range. The voltage across the diode is:
V
F
= 10µA
×
50k
Ω
= 0.5V
Figure 4-3 shows several examples for typical diodes. The
curves were drawn from data obtained in the manner described.
WARNING
Up to 300V may be present between the
high and low terminals in ohms.
4.3 Diode characterization
When the Model 6512 is placed in the ohms mode, constant
current values between 1nA and 100µA are available at the IN-
PUT jack high and low terminals, as shown in Table 4-1. (Input
high sources the current.) These currents can be used to plot the
I-V (current-voltage) characteristics over a substantial range.
Table 4-1
Diode currents and voltages
Range
Diode
current
Diode voltage (V)*
2k
Ω
, 20k
Ω
200k
Ω
2M
Ω
20M
Ω
200M
Ω
2G
Ω
, 20G
Ω
, 200G
Ω
100µA
10µA
1µA
100nA
10nA
1nA V = (100
×
10
-6
) (R)
V = (10
×
10
-6
) (R)
V = (1
×
10
-6
) (R)
V = (100
×
10
-9
) (R)
V = (10
×
10
-9
) (R)
V = (1
×
10
-9
) (R)
TO A/D
B. EQUIVALENT CIRCUIT
HI
LO
GND
INPUT
TRIAX CABLE
A. CONNECTIONS
V,
Ω
GUARD
HI
LO
SHIELDED
TEST FIXTURE
6512 PREAMP
LO
HI
SHIELDED FIXTURE
VF
DIODE
UNDER
TEST
I
I
6512 SET TO OHMS
VF
V
S
OFF
ON
Figure 4-1
Diode characterization