Philips PM2521 Service Manual page 37

37
1.
4.2.5.
Resistance measurements (^j
The
unknown
resistance
connected
to the
BU2
input
is
fed
via
the 12
1
function
selector
switch contact to
the voltage attenuator
and
is
supplied with
a
constant current IRx,
via
contact 122,
from
the constant current
source
A401
(OQ0063).
CURRENT SOURCE
C405
Fig. 26.
12
measurement
details
ST3376
B11201
The measuring
currents
in
the
OQ0063
are derived
from
a
reference current source
I
REF
adjusted
by
R418
in
series
with potentiometer R41
1
.
The output
current
I
of the reference current
source feeds the current
J.
multipliers n to give
the currents
I
shown
in
the table,
depending on the
selected signals
RNGC,
RNGD
or
RNGE.
As
stated,
the voltage
Vx
developed
across
Rx
is
applied to the
ADC
for
measurement.
However
,
the
ADC
input resistance
is
finite (10IVI12
)
and the
small input current
drawn by
the
ADC
has to be
compensated
to avoid incorrect readings.
This
is
achieved
as
follows;
The
voltage
Vx
across
Rx
is
amplified
by
a factor
of 2
in
the
compensation
amplifier (+Vin),
the gain being
determined by the equal
resistors
R404, R406.
The
output voltage of
2Vx
appears
at
one end
of
R403
and
Vx
is
present
at
the other end.
The
voltage across
R403
is
therefore:
2Vx
-
Vx
=
Vx
As
R403
=
the input resistance
of the
ADC
(10M12
)
the input current
of the
ADC
is
compensated.
In
this
way, the load
imposed by
the
ADC
is
compensated
as