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Table 2-7
Full-range PREAMP OUT values
Function
Range
Volts
200mV
2V
20V
200V
Amps
2pA, 2nA, 2µA, 2mA
20pA, 20nA, 20µA, 20mA
200pA, 200nA, 200µA
2k Ω
Ohms*
20k Ω -2G Ω
20G
200G Ω
Coulombs
200pC
2nC
20nC
*WARNING: Open-circuit voltage of 300V present at PREAMP OUT in ohms.
2.9
Using external feedback
The external feedback function provides a means to extend
the capabilities of the Model 6512 Electrometer to such uses
as logarithmic currents, non-decade current ranges, as well
as non-standard coulombs ranges. The following paragraphs
discuss the basic electrometer input circuitry and methods to
implement these functions.
2.9.1 Electrometer input circuitry
A simplified diagram of the electrometer input in the external
feedback mode is shown in Figure 2-15. An input current ap-
plied to the inverting (-) input of the op amp is nulled by a
current fed back through the internal feedback network made
up of R
and C
. Because the output of the op amp ap-
FB
FB
pears at the PREAMP OUT, this internal network can be re-
placed by an external network connected between the
preamp output and Input HI connections. When using exter-
nal feedback, the following factors must be taken into ac-
count:
1. The maximum current value that can be supplied by the
preamp output is 20mA in amps (1mA in volts and
ohms). The maximum voltage span in external feedback
is ±20V.
2. The input impedance in the external feedback mode is
given by the relationship Z
Full-range
the impedance of the external feedback network, and A
value
is the open-loop gain of the electrometer (typically
200mV
greater than 10
= 10M Ω || Z
2V
20V
3. The voltage at the PREAMP OUT terminal is given by
200V
the formula:
200mV
2V
20V
where Z
200mV
2V
4. Any feedback elements should be housed in a suitable
20V
shielded enclosure (see paragraph 2.9.2 below). Insula-
200V
tors connected to Input HI should be made of Teflon or
other high-quality insulating material and should be
200mV
thoroughly cleaned to maintain the high input imped-
2V
ance and low input current of the Model 6512. If these
20V
insulators become contaminated, they can be cleaned
with methanol and then dried with clean, pressurized air.
Figure 2-15
Electrometer input circuitry (external feedback mode)
= Z
IN
6
). Note that the input impedance is Z
when zero check is enabled.
FB
V
IZ
=
–
FB
is the value of the feedback impedance.
FB
ZERO CHECK
OP AMP
100MΩ
HIGH
INPUT
LOW
S
100Ω
COM
PREAMP
OUT
(CHASSIS)
Operation
/A
, where: Z
is
FB
V
FB
V
IN
C FB
R FB
TO RANGING
AMPLIFIER
2-21
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