Datron 1062 - Page 29

21
3.2.2.B
Input
Current
(lb)
Compensation
(430328
sheets
1 and
5)
During
the
calibration
cycle,
the
microprocessor
notes
and
stores
the
zero
error
due to
the
bias
current
(measured
in a
known
source
resistor).
When DC
is
selected,
this
information
is
recalled by
the
microprocessor,
trans-
ferred
across
the
isolation
barrier
and
latched
into
Ml
3
and M14,
see
Fig.
3.1 1
.
The output
from
the latches
is
applied to
the
binary
resistor
ladder
network, AN2,
providing a 255
step
digital
to
analog
conversion. The
analog
signal is
applied to
the
inverting
input
of
M3
so that
the
output
drives current,
through
the
diode, to
control the current in
the correspond-
ing
transistor
of the
opto-isolator, M23.
The
transistor
of
the
opto-isolator sinks
current to
the —15V
supply until the
voltage
across R198
is equal to
the
voltage applied to
the
inverting
input
of
M3.
The
other
half of
the
opto-isolator
acts as a
current
mirror,
referenced to
the
bootstrap
(BS)
supply.
Thus
the
input
current
correction
is
floated on
the
bootstrap
supply,
tracking
the input
signal
is
divided by
R84
to
R128
and
R129
to
null the bias
current
of
the
preamplifier.
3.
2.2.6
Test
(430328
sheets 1
and
5)
During
the
self-test
routine,
(actuated
from
the
front
panel
or
remotely
programmed)
the DC
isolator
is
checked for
correct
operation.
The
circuitry
is placed
into
the 0.1V
range,
as
described in
3. 2.
1.3,
except that
relay
RL1
is not
energized, (i.e.
the -I- 100
attenuator
is
across
the
input
amplifier).
Filter is
selected
and
F.E.T. Q5
'closed'
via M20-5
causing
a
small
signal to
be
injected
into the
feedback
path
of the
input
amplifier. Thus a
signal
of
—3.125 volts is
output
from
the DC
Isolator
(TL8).
This
signal is then
measured
and'
compared
with a
stored
value.
If
the
measured
signal
is within
±6%
of
the
stored
value,
the
test
continues
with a
IV
range check and a
10V
range
check.
f
Range
^
Output
signal
from DC
Isolator
(TP13)
0.1V
IV
10V
—
3.125
volts
—
0.2193
volts
—
0.06932
volts
J
DC
Isolator
Output
Test
Voltages
3.2.3
Analog to
Digital
Conversion
(Analog
Section)
(430328
sheets
3
and 4)
3. 2.3.1
General
Principles
Section
1 and
Fig.
1.2 of
the User's
Handbook
gives a
very
basic
description
of the
principles
of
the
integration
involved.
The
technique
used
in
the
Autocal
Voltmeter
is
a
quadruple
slope,
the two
extra slopes
being
towards
the end
of the
signal
and
reference
integration
periods
respectively.
Fig.
3.12 is a
simplified
diagram
showing the
essent-
ials
of the
analog
section
of
the A
-
D
conversion
and
should
be used
with
timing
diagram
Fig. 3.13
for
full
appreciation
of
the
circuit
operation.