Section 5: Measurement options
Model 6517B Electrometer Reference Manual
5-16
6517B-901-01 Rev. C / August 2015
Using the above transistors, a minimum RC time constant of 100 µs at maximum input current would
be used. At I
IN
(max) of 100 µA, this value would correspond to 0.4 µF. Note that at 100 nA, this value
would increase the RC response time constant to 100 ms. A minimum capacitance of 100 pF is
recommended.
Although the input signal to this particular circuit is assumed to be a current, conversion to voltage
input could be performed by placing a shunt resistor across the input. However, the nominal voltage
burden of 1 mV must be considered as an error signal that must be taken into account.
Further processing of the current response can be achieved by using the suppress feature. For
example, REL 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
REL
kT/q (ln (I
READ
/I
O
) - ln (I
REL
/I
O
))
= kT/q (ln (I
READ
/I
REL
))
= 0.26/I (ln (I
READ
/I
REL
)) @ 25° C
The circuit topology shown in the figure below works for positive input currents only. For bipolar input
signals, an external offset bias must be applied, or use a PNP transistor for Q1.
Figure 64: Transdiode logarithmic current configuration
