Non-Decade Current Gains; Figure 2-18 Non-Decade Current Gains - Keithley 6512 Instruction Manual

Operation
CURRENT
Figure 2-17
"Transdiode" logarithmic current configuration
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 = V
DISP SUPPRESS
I ))
O
= kT/q (ln (I
= 0.26/I (ln (I
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
Figure 2-18

Non-decade current gains

2-24
INPUT
COM
Q1
kT/q (ln (I /I ) - ln (I
READ O
/I ))
READ SUPPRESS
/I )) @ 25°C
READ SUPPRESS
NOTE
INPUT
HI
CURRENT
INPUT
LO
COM
R FB
PREAMP
OUT
MODEL 6512
INPUT
10MΩ
HI
LO
100Ω
S
PREAMP
OUT (CHASSIS)
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
be used to serve this purpose. Limitations on the magnitude
of the feedback current require that the value of R
/
SUPPRESS
er than 10
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.
10MΩ
100Ω
(CHASSIS)
ZERO
CHECK
OP AMP
S
NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE
Ω .
2
ZERO
CHECK
OP AMP
S
NOTE: PRESS SHIFT VOLTS TO ENTER
EXTERNAL FEEDBACK MODE
TO RANGING
AMPLIFIER
, can
FB
be great-
FB
RANGING
AMPLIFIER