Keithley 237 Service Manual

SECTION
4
Principles
of
Operation
Note:
If the lOOmA range is
the new
range, it is
a1
ready selected.
The range change
sequence stops at
this point.
If
a
different range is
selected, continue
on
to
the next steps.
5. The
two
voltage
ramps are
disabled
in reverse
order.
6. Relay
K8 opens completing
the
range change.
When the lOOmA
source
range is
selected,
K8 cioses
and
both ramps are
activated to
turn on both FETS. As
long as
the lOOmA range is
selected,
0V
is
maintained
at
the gates
of the FETs
to keep them on.
op
amp
U3
is
a
unity gainamplifier
that
serves
as a
buffer
between the
output
and 1 FEEDBACK.
1 FEEDBACK is
fed back
to
V/I
Controland
Readback
where
it completes
the
feedbackloop and
is also routed
to
the
A/D
converter
for
measurement.
Guard
The xl output
of
U3
is also routed to another
xl
buffer
amplifier
(U4) whose output
is used
as
the driven
guard
for
the instrument. Buffer
U4 isolates
the driven
guard from
1 FEEDBACK.
Volts Ranging
and
Sensing The
voltage range resis
tors are
part of
a voltage divider network.
In general,
OUT + is applied
to
the
top
of
divider.
The output
of
the
divider is
then routed
to a
differential
amplifier.
The
other input
to
the differential
amplifier is effectively
OUT
+‘ The output
of the xl
differentialoutput
is then
fed
back
to
the
V/I
Control
and Readback
circuitry.
op
amp
U2
is
configured
as a
unity
gain
amplifier that
serves
as a
buffer between
the
output and
volts
ranging
circuitry.
In local sense
(K15 closed,
K3 open), the output
is
accessed
internally.
In remote sense
(K15 open, K3
closed),
the output is accessed
at
the bad.
The xl output
ofU2
is applied
to
the
input
of the
differential
amplifier
(U7).
The
output
of
U2
is also
applied to the top
of
the
voltage
divider for
volts ranging. The
bottom of the voltage
di
vider network
is connected to
output common
(OUT
—)
via op amp
U47
which is
configured
as
a xl
buffer. In
lo
cal sense
(K5
closed),
OUT is accessed
internally.
In re
mote
sense
(K5
open),
OUT is accessed at
the bad.
Volts range rebays
K6
and K7 determine
the resistance
of
the bottom
leg
of the voltage
divider.
Table
44 identifies
the
rebays for each voltage
range. The output
of the volt-
age
divider
is
routed
to
the
+
input of the differential
am-
plifier
(U7)
via op
amp
U5
which
serves as a xl
buffer.
op
amp U7
is
configured as a
xl
differential
amplifier.
The
output voltage
(V 0 )
of this
amplifier
willbe
the alge-
braic
difference of
its two
inputs
(Ei
E 2 =
V 0 ).
V 0 ta
(V
FEEDBACK)
is routed
back as a
bootstrap
amplifier feed
back
loop and also to
the A/D
multiplexer
for
measure
ment.
Range
1
Ramp
ov
.
QA
÷
1
OO
K8
QB
Vout
Range
2
Ramp
Figure 4-6.
Damping Relay
Contact
Bounce
1
OOmA
Range
4-7

Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English