Philips pm2521 - Page 185

13
DESCRIPTION
When
making a
normal
measurement
(..PO...)
the
microcomputer sets all
the relays
and FET's
which are
necessary
to make a
measurement. For instance: Select
the Vtt.
function
and the range 200mV.
The relays
and
FET's
which are now set can be found in
the relay
table. So
ft
is
possible to
determine if the
relays are
really
set.
When making
an AUTOCAL 0,1V
(..PI
...)
the microcomputer
programs
the OQ 0063 so that
a current of
100
(jA
flows
through R606,
which causes a voltage drop of lOOmV
across it.
With this
test it
is
possible to
measure
this
voltage.
In the
position AUTOCAL
IV (..P5...)
the microcomputer programs
the OQ 0063
to give a current
of
1mA
through R606.
The
result
is
IV across R606.
With
the I
ZERO
measurements, normally the offset
is
measured in the I—V
convertor, (current
offset).
In the ..P3...
and the
..P7... tests these IZERO measurements
are
simulated, (see relay table
on page 52
and
page
53
of the PM2521
service manual).
In all
these tests (..PO... to ..P7...)
the
AZ status can be changed. This
means
that
the input of the
ADC can
be
selected. To
do this push the
+/—
button.
Thanks to
this it is possible to measure the input
voltage, or the
voltage across the R606 resistance, on the
input
of the
ADC
(pin 5
and
pin 10 of the
OQ 0064)
Example:
When
there
is no
input signal, the voltage
on pin 5 and
pin 10
of the ADC is 0,460V.
(with respect
tot
the
0 input
socket)
Set 200mV on the input
terminals. After
attenuation the voltage on point 6
of
A601 is lOOmV.
This
voltage of lOOmV causes either a voltage of
0,360V on
pin
5
of
A602
or
a
voltage of
0,360V
on pin 10
of
A602,
depending on the state of the AZ flip-flop. (0,460V—
0,1V
=
0,360V).
So
the input voltage
after attenuation degrees
the voltage on pin 5
or
pin 10
corresponding
to
the
input
signal.
/