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

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