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MODEL 6008 ELECTROMETER
OPS~TION,
CIRCUIT DESCRIPTION
1.
Connece the voltmeter
co ehe xl and OHMS
GUARD Terminals
as shown in Figure
8.
The OHMS
GUARD Terminal
is at ground with
the FEEDBACK Switch
in NORMAL. laxnwm
output
amplitude
is 10 volts
peak-to-peak?
2.
Adjust
the Model 6008 zero Controls
co obtain
a zero-voltage
reading
on rhe exrernal
voltmeter.
Make cure the latter's
sensitivity
is high
enough
far
a precise
zero adjustment.
b.
When the FEEDBACK Switch
is in FAST position,
the unity-gain
terminals
permit
more convenient
con-
nections
to oscilloscopes
with
a load resistance
of
greeter
than
100 kilohms
without
special
precautions.
In this
mode, the Xl Terminal
is grounded
and ehe
OHMS GUARD Terminal
delivers
en output
equal
co the
input
signel.
SECTION
3.
CIRCUIT
DESCRIPTION
3-l.
CESEUL.
a.
The Keirhley
Yodel bOOR is basically
an eutreme-
ly stable
and iinear
dc ~volcmeter
with
a fuil-scale
sensitivity
oi 10 millivoits
and an input
impedance
of
10L4
ohms shunted
by 20 ?icoiarads.
By using
the frone
p.nel
controls,
shunt
m~istors
and capacitors
are se-
lected
to make meesuremencs
over a total
ot 61 YOLC-
age,
currene,
fes~scance:
.2nA *nl*lnmh
-1"Z-E
r?~~~,mt
and resiseance
dre measured using
precision
resistance
stanaards,
irom 10 ohm wirewound
resistors
to 1011 ohm
glass
sealed,
deposited
carbon
resistors.
coulombs
are measured
using
close
tolerance
polystyrene
film
capacitor
standards.
b.
Batteries
furnish
the neceesarv
amolifier
oower.
3-2.
"OLTHETER OPERATI(IN.
a.
The Model 6008 empioys
matched
insulated-gare,
field-effect
transistors
fallowed
by a eransisror
dif-
ferential
amplifier
with
a high-voltage
comvlementary
oueput
stage.
Figure
9 shows the block
diagram
for
the voltmeter
made of operation.
b.
Voltmeter
operation
of the Model 6008 is as
fallows:
1.
The amplifier
is always
in a unity-gain,
input
voltage
co output
current
converter
contigurarxm.
The internal
circuitry
is arranged
such that
a full
scale
input
voltage
(ei)
results
in exactly
a 1 mil-
liampere
outout
curreoc
through
the divider
string
composed
of
Rm. R177, end the meter.
Voltage
gain
of the circuit
is determined
by the ratio
of R177 to
h.
Output
is taken
across
R177.
2.
The voleage
drop across
the amplifier
is
where & is the amplifier
loop gain,
greater
t!lan
5 I 10' on all
ranges.
c.
'ihe complementary
oufpot
sK.ege, 4109 and 4110,
drives
the amplifier
ground
at the same potential
as
ehe input
signal.
'Thus the input
impedance
is main-
tained
for any value
input
voltage
and the need for
input
dividers
is eliminated.
'The amplifier
*round
is not chassis
ground,
hue it
is connected
directly
to the unity
gain
output.
NOTE
Refer
to Schematic
Diagram
22BOSE for cir-
cuit
designations.
3-3.
"OLTELETER CIRCUIT.
a.
The amplifier
input
staSe
is a pair
of insulated
gate,
field-effect
transiecars,
Qlll
and Qll5,
in a
differential
configuration.
The gate of Qlll
is re-
turned
to amplifier
common, the unity
gain
output.
b.
Depressing
the ZERO CHECK Sutton,
S102, places
the gate
of the active
insulated
gate devices
at zero
p"LSLLLldl.
c.
The input
stage
is fallowed
by a transistor
dif-
ferential
amplifier.
composed of QlOlA-QlOS.
Q106 and
Q107 make up the output
gain
stage,
which is utilized
in e gain multiplier
configureciao.
This
stage
pro-
vides
the remainder
of the high gain
required
by the
r%"L"D
9
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