Philips pm2521 - Page 44

44
1. 4.4.4.
Analog-to-digjtal
convertor
The
A602
integrated
circuit converts
the unknown input
voltage to a constant
current value
depending on the
magnitude
of
the input voltage.
Two
sensitivities can
be selected by
the
relay
control input
RNGB,
to
give either
IV or 0,1V
(used in the
200mV
measuring
range).
The voltage
converted to a
constant current is the
difference
between the two
inputs of
the
OQ0064.
The auto
zero
circuit is designed to compensate
for
any
internal offset
which could
influence
the measuring
result of the
ADC. This is done by
logic signals AZ and AZ
.
During
the
first
period of
a
measurement the unknown
voltage is supplied via
V606'
and
V608
to
the
+
input
(pins
3 and
5)
of the OQ0064
while the
—
input (pins
12 and
10)
is connected to
zero
via V610
and
V607,
controlled by
the
AZ logic
to
the
gate of
V608'.
At
the same
time, the AZsignal
via
V610
switches the
signal path
via
V606'
to V608
and
the
+
input.
In
the second
period of a
measurement the situation is
reversed.
The
AZ
logic
pulse switches the
input signal to
the
—
input,
and
the
+
input is now
connected to
zero via the gate
of FET V608
and
FET V607'.
The
source of
V607'
is
normally at
OV except
in the temperature measuring
function.
In the
ftp
these two
signals are subtracted, so any
internal
offset is
compensated.
AZ
=
1
for the first measuring period
AZ
=
1
for the second
measuring period.
ST3384
ani30
Fig.
32.
V-l
convertor