Section 5: Measurement options
Model 6517B Electrometer Reference Manual
5-14
6517B-901-01 Rev. C / August 2015
External feedback procedure
Use the following procedure to operate the Model 6517B in the external feedback mode.
1. Connect the feedback element between the PREAMP OUT terminal and the Input High terminal.
2. Select the volts (
V
) function.
3. Select external feedback as follows:
a. Press
CONFIG
V
to display the
CONFIGURE DCV
menu.
b. Place the cursor on
EXT-FDBK
and press
ENTER
.
c. Place the cursor on
ON
and press
ENTER
.
d. Use
the
EXIT
key to back out of the menu.
4. The display shows the voltage measured at the output of the input preamplifier (PREAMP OUT).
Non-standard coulombs ranges
In its standard form, the Model 6517B has four coulombs ranges allowing it to measure charge
between 10 fC and 2.1 µC. Different charge measurement ranges can be used by placing an external
feedback capacitor between the PREAMP OUT and Input HI and then placing the instrument in the
external feedback mode.
Charge is related to capacitance and voltage by the formula:
Q = CV
where:
Q is the charge in coulombs
C is the capacitance in farads
V is the voltage in volts
The Model 6517B display reads charge directly in units determined by the value of C. For example, a
10 µF capacitor shows a reading of 10 µC / V.
In practice, the feedback capacitor should be greater than 100 pF for feedback stability and of
suitable dielectric material (such as polystyrene, polypropylene, or Teflon) to ensure low leakage and
low dielectric absorption. The capacitor should be mounted in a shielded fixture like the one shown in
Shielded fixture construction
(on page 5-13).
To discharge the external feedback capacitor, enable zero check. The discharge time constant is
given by: t = (10 M
) (C
FB
). Allow five time constants for discharge to within 1 % of final value.
