Convertor; Active Filter; Analog-To-Digital Convertor - Philips PM2521 Service Manual

15
1.
3.4.3.
Active
filter
Filters
placed
in
feedback
loops
around
an amplifier stage
are
referred to
as
active
filters.
They
have sharper
transition
between
the pass-band
and
stop-band than
their passive
counterparts, inductors
are
not needed,
and
small signals are not further attenuated.
In this application,
good
filtering
is
necessary
especially
when
measuring
small
a.c. signals.
In
this circuit,
the
basic active
filter
elements
are
as
shown:
Fig.
13.
Low-pass
active
filter
ST3367
820503
In
the
PM2521
,
the track
shielding
network on
the
filter
output keeps
the
two
signal lines
that are
connected
to
it
at
the
same
potential;
i.e.
prevents tracking across the
p.c.b.
insulation.
I.3.4.4.
Analog-to-digital Convertor
The
ADC
converts the analog
signal into
a digital signal
by
the 'delta-modulation'
principle.
Basically, the delta-modulation
ADC
counts the
difference
in
the
time taken
to charge
and
to discharge
a
capacitor
about
a
fixed
level,
over
a
fixed period of time.
The number
of charge/discharge cycles within
this
fixed
time depends on
the charge/discharge current
which
is
made
proportional to the
unknown
input
voltage to the
ADC.
Therefore, the
number
of pulses counted within
a
fixed
measuring
period
is
proportional to the
unknown
voltage
V^.
Circuit principles:
The
capacitor
is
charged
by
a
constant reference current added
to the constant current derived
from
V
to
give
(I
ref
-I-
A
I).
^
The
capacitor
is
discharged by the constant reference current minus
the constant current derived from
V
to
give
(I
ref
- A
I).
Each
value of has
a certain
number
of charge/discharge
cycles within the fixed period of
a
specific
number
of clock-pulses counted by
a
timer.
The
fixed
level
between
charging
and
discharging
is
determined by
the voltage
between
the inputs of
a
comparator OV)
a flip-flop
and
clock
signals.
A
simple
example
of the
principle
is
shown
in Fig.
14.
Fig. 14.
Switching
part
ADC AZ
ST3368
811127