Section 5: Measurement options
Model 6517B Electrometer Reference Manual
5-12
6517B-901-01 Rev. C / August 2015
When using external feedback, the following factors must be taken into account:
The maximum current value that can be supplied by the preamp output is 20 mA in amps and
ohms (1 mA in volts). The maximum voltage span in external feedback is ±20 V.
The input impedance in the external feedback mode is given by the relationship:
Z
IN
= Z
FB
/ A
V
where:
Z
FB
is the impedance of the external feedback network
A
V
is the open-loop gain of the electrometer (typically greater than 55×106).
Note that the input impedance is Z
IN
= 10 M
Ω
| Z
FB
when zero check is enabled.
The voltage at the PREAMP OUT terminal is given by the formula:
V = -IZ
FB
Any feedback elements should be housed in a suitable shielded enclosure (see
Shielded fixture
construction
(on page 5-13)). Insulators connected to Input HI should be made of Teflon or other
high-quality insulating material and should be thoroughly cleaned to maintain the high input
impedance and low input current of the Model 6517B. If these insulators become contaminated,
they can be cleaned with methanol and then dried with clean, pressurized air.
