LTC1562_15 Datasheet, PDF (11/28 Pages) Linear Technology – Very Low Noise, Low Distortion Active RC Quad Universal Filter

LTC1562
APPLICATIONS INFORMATION
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_P
=
1
2πR_PC_T
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.
R_INA
V_IN
R_INB
C_T
R_Q R2
INV V1 V2
2nd ORDER
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1562 F09
Figure 9. Lowpass “T” Input Circuit
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_INB 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_P
=
1
2πR_TC_P
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).
C_INA
V_IN
C_INB
R_T
R_Q R2
INV V1 V2
2nd ORDER
1/4 LTC1562
1562 F10
Figure 10. Highpass “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_PC_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
R_IN
±
1
2
R_IN2 –
4R_INR_P
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_Tf_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
C_P
±
1
2
C_P2 – ₍4C_INC_P)
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