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
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