Datasheet

11
LTC1562
1562fa
APPLICATIONS INFORMATION
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external component Z
IN
, usually a resistor or capacitor.
This component must of course be rated to sustain the
magnitude of voltage imposed on it.
Lowpass “T” Input Circuit
The virtual ground INV input in the Operational Filter block
provides a means for adding an “extra” lowpass pole to
any resistor-input application (such as the basic lowpass,
Figure 5, or bandpass, Figure 6a). The resistor that would
otherwise form Z
IN
is split into two parts and a capacitor
to ground added, forming an R-C-R “T” network (Figure
9). This adds an extra, independent real pole at a fre-
quency:
f
RC
P
PT
=
π
1
2
where C
T
is the new external capacitor and R
P
is the
parallel combination of the two input resistors R
INA
and
R
INB
. This pair of resistors must normally have a pre-
scribed series total value R
IN
to set the filter’s gain as
described above. The parallel value R
P
can however be set
arbitrarily (to R
IN
/4 or less) which allows choosing a
convenient standard capacitor value for C
T
and fine tuning
the new pole with R
P
.
INV V1
2nd ORDER
1/4 LTC1562
V2
1562 F09
R2R
Q
R
INB
R
INA
C
T
V
IN
Figure 9. Lowpass “T” Input Circuit
The procedure therefore is to begin with the target extra
pole frequency f
P
. Determine the series value R
IN
from the
gain requirement. Select a capacitor value C
T
such that R
P
= 1/(2πf
P
C
T
) is no greater than R
IN
/4, and then choose
R
INA
and R
INB
that will simultaneously have the parallel
value R
P
and the series value R
IN
. Such R
INA
and R
INB
can
be found directly from the expression:
1
2
1
2
4
2
RRRR
IN IN IN P
±
()
–
A practical limitation of this technique is that the C
T
capaci-
tor values that tend to be required (hundreds or thousands
of pF) can destabilize the op amp in Figure 3 if R
INB
is too
small, leading to AC errors such as Q enhancement. For this
reason, when R
INA
and R
IN
B are unequal, preferably the
larger of the two should be placed in the R
INB
position.
Highpass “T” Input Circuit
A method similar to the preceding technique adds an
“extra” highpass pole to any capacitor-input application
(such as the bandpass of Figure 6b or the highpass of
Figure 7). This method splits the input capacitance C
IN
into
two series parts C
INA
and C
INB
, with a resistor R
T
to ground
between them (Figure 10). This adds an extra 1st order
highpass corner with a zero at DC and a pole at the
frequency:
f
RC
P
TP
=
π
1
2
where C
P
= C
INA
+ C
INB
is the parallel combination of the
two capacitors. At the same time, the total series capaci-
tance C
IN
will control the filter’s gain parameter (H
H
in
Basic Highpass). For a given series value C
IN
, the parallel
value C
P
can still be set arbitrarily (to 4C
IN
or greater).
Figure 10. Highpass “T” Input Circuit
INV V1
2nd ORDER
1/4 LTC1562
V2
1562 F10
R2R
Q
C
INB
R
T
V
IN
C
INA
The procedure then is to begin with the target corner (pole)
frequency f
P
. Determine the series value C
IN
from the gain
requirement (for example, C
IN
= H
H
(159pF) for a highpass).
Select a resistor value R
T
such that C
P
= 1/(2πR
T
f
P
) is at
least 4C
IN
, and select C
INA
and C
INB
that will simultaneously
have the parallel value C
P
and the series value C
IN
. Such
C
INA
and C
INB
can be found directly from the expression:
1
2
1
2
4
2
CCCC
P P IN P
±
()
–