LTC1562-2 Linear Technology, LTC1562-2 Datasheet - Page 11

• Manufacturer Part Number: LTC1562-2
• Description: Very Low Noise/ Low Distortion Active RC Quad Universal Filter
• Manufacturer: Linear Technology

APPLICATIONS
external component Z
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
a capacitor to ground added, forming an R-C-R “T”
network (Figure 9). This adds an extra, independent real
pole at a frequency:
where C
parallel combination of the two input resistors R
R
scribed series total value R
described above. The parallel value R
arbitrarily (to R
convenient standard capacitor value for C
the new pole with R
The procedure therefore is to begin with the target extra
pole frequency f
gain requirement. Select a capacitor value C
= 1/(2 f
R
value R
be found directly from the expression:
INB
INA
2
1
f
P
. This pair of resistors must normally have a pre-
R
and R
IN
P
P
T
2
and the series value R
C
V
is the new external capacitor and R
IN
T
2
1
R C
) is no greater than R
INB
1
Figure 9. Lowpass “T” Input Circuit
P T
R
that will simultaneously have the parallel
R
P
IN
IN
INA
. Determine the series value R
2
/4 or less) which allows choosing a
–
U
P
.
IN
4
C
R
R R
, usually a resistor or capacitor.
T
INB
INFORMATION
IN P
U
IN
INV
IN
1/4 LTC1562-2
IN
2nd ORDER
is split into two parts and
to set the filter’s gain as
. Such R
V1
IN
W
R
/4, and then choose
P
Q
can however be set
V2
R2
T
INA
and fine tuning
1562-2 F09
T
and R
such that R
IN
U
from the
P
INA
INB
is the
and
can
P
A practical limitation of this technique is that the C
tor values that tend to be required (hundreds or thousands
of pF) can destabilize the op amp in Figure 3 if R
small, leading to AC errors such as Q enhancement. For this
reason, when R
larger of the two should be placed in the R
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
two series parts C
between them (Figure 10). This adds an extra 1st order
highpass corner with a zero at DC and a pole at the
frequency:
where C
two capacitors. At the same time, the total series capaci-
tance C
Basic Highpass). For a given series value C
value C
The procedure then is to begin with the target corner (pole)
frequency f
requirement (for example, C
pass). Select a resistor value R
is at least 4C
neously have the parallel value C
Such C
expression:
f
2
1
P
C
P
P
IN
INA
2
P
can still be set arbitrarily (to 4C
= C
will control the filter’s gain parameter (H
2
1
V
R C
P
IN
1
Figure 10. Highpass “T” Input Circuit
and C
. Determine the series value C
IN
T P
INA
C
, and select C
P
C
INA
INA
2
+ C
INA
–
INB
and R
INB
and C
4
R
C C
T
can be found directly from the
C
is the parallel combination of the
IN P
INB
IN
INB
B are unequal, preferably the
INA
INV
, with a resistor R
IN
1/4 LTC1562-2
T
2nd ORDER
and C
such that C
P
= H
and the series value C
V1
R
H
Q
INB
(100pF) for a high-
LTC1562-2
V2
R2
that will simulta-
IN
IN
1562-2 F10
P
INB
IN
from the gain
or greater).
= 1/(2 R
, the parallel
T
position.
to ground
INB
T
11
capaci-
IN
is too
H
into
T
f
IN
P
in
)
.