Keithley 236 Service Manual

SECTION 4
Principles of
Operation
42
ANALOG
CIRCUITRY
The heart of the
source is
an error amplifier
that along
with its complex
scheme
of feedback elements
is config
uxed
as a bootstrap amplifier.
Input stimulus
for the
am
plifier is provided
by two
user-programmed
digital4o-
analog converters (DACs). The controlling function
(cur
rent or voltage) and
selected
polarity of the
source turns
on one of four precision
clamps
allowing an
input signal
to be
applied
to
the error
amplifier.
The
output voltage
from
the DAC and the
feedback
configuration
applies a
voltage to
the input of the
error amplifier
that will
effect a
constant
level of current
or voltage
at
the
output.
Measurement
is
accomplished by
routing
properly con
ditioned feedback
levels
for voltage and current
through
a
multiplexer
to the
analog4odigital
(A/D)
converter.
Before proceeding to
the
details of circuit
operation,
it
may
be
helpful
to
clarify what
is meant
by
“controffing
function“. If the instrument is programmed
to source
a
specific
current and
it
does
indeed source that current,
then it is
considered
to be current
controlled. That
is, cur
rent is
the controlling
function.
If for some reason
the in
strument
goes
into
compliance, the programmed current
will
not
be
sourced. Instead,
the instrument sources a
constant
voltage
(compliance voltage).
Thus,
voltage
be
comes the controlling function. Conversely,
when
pro-
grammed
to source
voltage,
voltage is the controffing
function unless the
instrument goes into current compli
ance.
In compliance,
current
becomes
the controlling
function.
42i
V/I
Control
and Readback
A
simplified schematic
diagram
of the
V/I
control
and
V/I
readback circuitry is shown
in Figure 4-2. The
de
tailed schematic showing all
circuit
components
is lo
cated on
page
2
of drawing
number
236-126.
Programming current and voltage
sets the output
volt-
age of the
two
digital-to-analog
(DAC)
circuits. Program
ming
current controls the output
of
the 1
DAC
(U23
and
U22),
and
programming
voltage
controls
the output of
the
V
DAC
(U25
and
U24).
Programming
current or volt-
age for zero
output will
result
with
a
0V output from the
respective DAC.
Programming
for
a
full
scale output
wffl
result
with a
-40V output from
the
respective
DAC.
The
output
voltage
from the 1
DAC
is applied
to
current
clamps through
resistor
networks. Op
amp U13 and
di-
ode
CR11 form
the negative
current
clamp
(-4 CLAMP).
The
output from the 1 DAC
is inverted
by
the xl
amplifier
U50.
Op
amp U17
and
diode
CR10
form the positive
cur
rent clamp
(+1
CLAMP).
The output from the
V
DAC is inverted
by
the xl
ampli
fier
U12
(xO,1 for the
1.1V range)
and
similarly applied
to
current clamps through
resistornetworks.
Op
amp U15A
and diode CR12 form
the
positive voltage
clamp
(+V
CLAMP).
The inverted
output
ofUl2 is again
inverted
by
U19.
Op
amp U15B and
diode
CR9 form
the
negative
voltage
clamp
(-V
CLAMP).
During operation,
only one of the
four
precision clamps
wffl be
on
at
one time
to
control the
error
amplifier (U14).
The controllingfunction
and the programmed
polarity
(+
or
—)
wffl determine which clamp
is on. The following
ex-
ample
will explain how this
occurs.
Example
1: This example
is
divided into
two
parts. Part
1
wffl explain circuit
operation with the instrument
sourc
ing
a
constant current.
Part
2
will explain
what happens
when
the source
goes
into voltage compliance.
Part 1: Assume
that the
instrument
is sourcing +5mA
with a 27V compliance
to a
4k bad,
Under
these condi
tions the instrument
is not in compliance
(5mA
x
4k2 =
20V).
Thus, current is
the controlling
function.
The 1 DAC outputs —5V.
Thus, negative
voltage appears
at
the non-inverting
(+)
input ofUl3 and positive
voltage
appears at
the non-inverting
(+)
input ofUl7, The
V
DAC
outputs
approximately —3V.
Positive voltage appears
at
the non-inverting
(+)
input ofUl5A
and negative voltage
appears
at the non-inverting
(+)
input of
U15B.
Since
the
instrument
is
programmed
for a positive out
put, the
±10V
REF
wffl
apply
a
positive
voltage to
the
common side of the clamps.
This positive
voltage will re
verse-bias diodes CR11
and CR9B and
prevent the
-1
CLAMP and
-V
CLAMP
from
turning on.
With the front
end of the
bootstrap
amplifier
set
up
as de
scribed,
-5V
will
be
fed back
(via
1
FEEDBACK)
to
U17
and U13. The
non-inverting(+)
input ofUl7wffl go
to 0V
(-5V +
5V =
0V)
forward-biasing
CR10. As
a result, the +1
CLAMP
turns on.
4-2

Keithley 236 Specifications

General

Brand	Keithley
Model	236
Category	Measuring Instruments
Language	English