Philips pm2521 - Page 13

10
1.3.2.
5.
Resistance
measuremen
ts
The unknown resistance
is
connected
between
the
V-fi-mA
and
0
input
terminals
and
supplied internally
with
a constant-current
source
dependent
on the range
selected.
This
current results
in a
potential difference
across the
resistor
which
(by
Ohm's
Law) is
proportional
to
the
resistance
value. The
resulting
voltage signal
is applied
as for V
measurements
to the
voltage
attenuator,
the
active
filter
and the
ADC.
The
circuit functions
as shown
in Fig.
7.
A known constant
current
supplied
by the
programmable
current
source
A401
(000063)
flows
through
the unknown
resistor
Rx.
There
are three basic
ranges:
^2
200
k^2
2 20
200
M^2
2
20
Depending
on the range
selected
(manual
or
automatic
selection),
the currents
are
determined
by the
signals
RNGC,
RNGD and
RNGE:
Irx
RNGC
RNGD
RNGE
RANGE
Vx
INDICATION
1mA
-10V
-10V
OV
200 n
-0,2V
200.00 n
1 mA
-10V
-10V
ov
2kfi
-
2V
2.0000kf2
100/tA
-10V
-10V
-10V
20kl2
-
2V
20.000kf2
10m
A
ov
-10V
-10V
200kfi
-
2V
200.00kn
lM
A
-10V
ov
-10V
2Mn
-
2V
2.0000M12
lOOn
A
ov
ov
-10V
20IVir2
-
2V
20.000Mr2
With
respect
to
Fig.
7. Qj
measurements
In
practice, the
internal
resistance
of the
ADC
(10IViJ2)
also
draws
a small amount
of current,
but,
this is com-
pensated
by an equivalent
current
through
the
compensation
amplifier
circuit.