Keithley 6512, Instruction Manual

Page: 44 / 156

Operation
2-24
Figure 2-17
“Transdiode” logarithmic current configuration
CURRENT
INPUT
OP AMP
TO RANGING
AMPLIFIER
ZERO
CHECK
S
S
NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE
Q1
INPUT
HI
LO
100
Ω
(CHASSIS)
PREAMP
OUT
10M
Ω
MODEL 6512
COM
Further processing of the current response can be achieved
by using the suppress feature. For example, suppress could
be enabled with a reference input current applied. For all
subsequent currents, the natural logarithm of the ratio of the
measured current to the suppressed current would then be
displayed:
V
DISP
= V
SUPPRESS
kT/q (ln (I
READ
/I
O
) - ln (I
SUPPRESS
/
I
O
))
= kT/q (ln (I
READ
/I
SUPPRESS
))
= 0.26/I (ln (I
READ
/I
SUPPRESS
)) @ 25°C
NOTE
The circuit topology of Figure 2-17 works
for positive input currents only. For bipo-
lar input signals, an external offset bias
must be applied, or use a PNP transistor
for Q
1
.
2.9.6 Non-decade current gains
The Model 6512 electrometer input uses internal decade re-
sistance feedback networks for the current ranges. In some
applications, non-decade current gains may be desirable. As
shown in Figure 2-18, an external feedback resistor, R
FB
, can
be used to serve this purpose. Limitations on the magnitude
of the feedback current require that the value of R
FB
be great-
er than 10
2
Ω
.
Note that external feedback can be temporarily calibrated
over a range of ±12% using the calibration program with the
calibration jumper in the disable position. See paragraph
7.4.16 in Section 7.
CURRENT
INPUT
OP AMP
RANGING
AMPLIFIER
ZERO
CHECK
S
NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE
COM
INPUT
HI
LO
100
Ω
(CHASSIS)
PREAMP
OUT
10M
Ω
RFB
Figure 2-18
Non-decade current gains