Filtering; Lnput Current Compensation; Test - Datron 1061 Calibration And Servicing Handbook

T
21
3.2.2.5 lnput
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
knolGr
source resistor)' When DC
is
selected'
this information is
recalled
by the
microprocêssor'
trans-
ferred
across
the
isolation barrier and latched
into
M13
and
M
14, see
Fig'
3.1
1
.
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
RLI
is not
energized,
(i.e.
the +
100
attenuator is
across
the input
amplifier).
Filter is
selected
and F.E'T. 05
'closed'
via
M20-5
causing
a
small
signal
to
be
iniected
into
the feedback path
of
the
input
amplifier. Thus a signal
of
-3.125 volts is output from the DC lsolator
(TLBI.
This signal
is then
measured
and compared
with
a stored
value.
lf
the
measured
signal
is within t.6% of
the stored
value,
the test
continues
with a
1V
range
check and
a
l0V
range
check.
Output
signal
from
DC lsolator
(TP13)
-
3.'125
volts
-
0.2193 volts
-
0.06932
volts
R
ange
0.1v
1V
10v
DC
lsolator OutPut Test Voltages
3.2.3
Analog
to D¡gital Conversion (Analog
Section)
(430328
sheets
3 and
4)
The
output from the latches is
applied
to the
binary
resistor
ladder network, AN2,
providing a 255 step digital
to
analoq
conversíon.
The
analog
signal
is applied
to
the
inu"iting-input
of M3 so that the output
drives current,
if'rough
tt
t
diode,
to control the current in the correspond'
i.ro-trätirto, of
the opto'isolator, M23'
The
transistor
of
iiã
ooto-¡tolutor
sinks
current to the
-15V
supply until
the
;;lü"
across
R198 is equal
to
the voltage applied
to
the
inveri¡ng
¡nPut
of
M3'
3.2.3.1 General PrinciPles
Section
1
and
Fig. 1'2
of
the
User's Handbook
gives
a
very basic description
of
the
p.rinciples
of
the
intugrrtion
involved'
The technique used
in
the
Autocal
Volimeter
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
ihe
analog
section
of
the
A -
D
conversion
and
rr'tori¿
¡t
used
with timing diagram
Fig' 3'13
for
full
appreciation
of the
circuit operation'
M36
FIG.3.10
SIMPLIFIED DC
¡SOLATOR
FILTERING CIRCUIT
R195
R147
R194
c41
98k
or
9M9
c40
RL2
1
-+
Hi
Lo
T
css
V
DC
lsolator
D-A
Converter
I
s
o
L
A
T
I
o
N
B
A
R
R
I
E
R
FIG.3.11
S¡MPLIFIED
DIAGRAM
OF
BIAS CURRENT
COMPENSAT!ON
Calibration
Memory
MPU
+rb