Test; Dc Isolator - Datron 1062 Service Handbook

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
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.