Ohms Assembly; Low Drift Voltage Follower; Constant Current Sou Rce, I - Datron 1061 Calibration And Servicing Handbook

rr
-.:
.. l
I
3'4
OHMS
ASSEMBLY
(circuit
Drawing No' 4303311
RANGE
LOGIC
I+
DC
ISOLATOR
Hi
Lo
I-
FIG.
3.27
SIMPLIF¡ED RESISTANCE
CONVERTER
The instrument functions by
measuring
the
voltage
across
an
unknown
resistance
with a
known
constant
current flowing in
it.
The
converter
can be
split into two
parts: a low drift
voltage
follower and a
constant current
source covering
6
decades
from
100nA
to
1OmA
(see Fig.
3.271.
It
should be noted that when the
Ohms
assembly
is
fitted the
DC
lsolator Lo
is
no longer
directly connnected
to the front/rear panel Lo terminal, but
goes
via
RLI
on
the
Ohms assembly (connector
link
removed
on
side
panel). Lo
becomes
an
act¡ve
terminal
in
resistance meas'
urements.
3.4.1
Low
Drift
Voltage Follower
R9
TP5
¡
Hi
Lo
o8
LO ¡n
o10
D11
lFrom
Dc lsolalorl
ß67
ant Currenl
f
Source
FIG.3.28 SIMPLIFIED DIAGRAM
OF
VOLTAGE
FOLLOWER
CIRCUIT
RL1 such
that the
voltage at front panel Lo
is
at reference
common plus any offset due
to
O10/M3.
This
voltage
offset
drift
is kept
small
for
changes
of
temperature by
compensating
the
input
bias current
of
01O w¡th
the
current
in
R67, which
changes
with
temperature
due
to
the
voltage
drift at
010
emitters.
Ol0
input
bias
current
is
initially
nulled by
R26.
Thus
if
we consider 2-wire
measurement.
I+ is
linked
to
Hi,
I-
is
linked
to
Lo
and
the unknown
resistance
linked
between
Hi and
Lo,
with
a
constant
current flowing from
I+/Hi, through the unknown
resistance
(R*l to Lo/I-.
The
Lo terminal
is maintained at
0V. Therefore
the Hi terminal
(DC lsolator lnput) is at Iconstant
x
R"
volts above Lo.
As
long
as
the error is small
referred
to
reference
0,
the
DVM
will
read the correct
resistance.
lnput protection
is
provided
as
follows:-
Voltage/Current applied
to input
terminals
J+
R9,
D1O,
Dl1
I-
R2. D1,
D2,425,
R23
Lo
R12. R13,
08,
Og
Open
circuit
voltage
l¡m¡t protection:
I+
R15, R16,
06, 07
I-
R6, D7,
D8,o.2,O22
3.4.2
Constant Current
Source
Seven decades
of
ohms
ranges
are provided
by
6
ranges
of
current and
2
ranges
of
DC lsolator voltage
gain
(100mV
range
for
10O,
'l
V
range
otherwise)'
See
Fig.
3.29.
When
kl)'s
is
selected,
O17
(sheet
2)
is turned
on
enabling astable
M6
to
produce a
200H2 signal
to
switch
M5.
Thus
when
gates
B
and
C
of
M5
are
open,
C9
is
charged
up from the
negative reference
(originating from
the analog
section
of the
A
-
D converter).
These gates
then
close
and
A
and
D
open,
sharing
the
charge
with
C8,
the
voltage across CB
equals
the
reference voltage (sheet
1
l.
30
When OHMS
is selected,
the
front
panel
Lo
terminal
is
connected
to the -ve input
of
amplifier
01o/M3,
the
+ìe input
being referred
to
DC isolator
Lo (this
remains
reference
common).
010/M3
.together
with output
foll_
.ïrr
Of
s, will
thus
apply
a
voltage
at the I+
terminal
via
F.E.T.'s/Switches
turned
on
Leakage
path
M2(B)
03,
M2(C)
03.
M2{C}
Cu
rrent
Selector
01 1, M2(A)
01 1,
M2(Al
M1(A}
M1(BI
o4
01,
M1(Dl
01,
M1(Cl
Current
1OmA
1OmA
lmA
1
00FA
10gA
1pA
1
00nA
Range
10Q
1
o0r¿
lko
lokQ
1
ookf)
lMQ
loMo
FIG.3.29
OHMS
CURRENT RANGE SWITCH¡NG