Datasheet
8
LTC1562-2
15622fa
tend to scale with R2.)
At high f
O
these resistors fall below
4k, heavily loading the outputs of the LTC1562-2 and
leading to increased THD and other effects. At the other
extreme, a lower f
O
limit of 20kHz reflects an arbitrary
upper resistor limit of 1MΩ. The LTC1562-2’s MOS input
circuitry can accommodate higher resistor values than
this, but junc
tion leakage current from the input protection
circuitry may cause DC errors.
The 2nd order transfer functions H
LP
(s), H
BP
(s) and
H
HP
(s) (below) are all inverting so that, for example, at DC
the lowpass gain is –H
L
. If two such sections are cas-
caded, these phase inversions cancel. Thus, the filter in the
application schematic on the first page of this data sheet
is a dual DC preserving, noninverting, rail-to-rail lowpass
filter, approximating two “straight wires with frequency
selectivity.”
Figure 4 shows further details of 2nd order lowpass,
bandpass and highpass responses. Configurations to
obtain these responses appear in the next three sections.
Basic Lowpass
When Z
IN
of Figure 3 is a resistor of value R
IN
, a standard
2nd order lowpass transfer function results from V
IN
to V2
(Figure 5):
Vs
Vs
Hs
H
sQs
IN
LP
L
O
O
O
2
2
22
()
()
()
–
/
==
+
()
+
ω
ωω
H
L
= R2/R
IN
is the DC gain magnitude. (Note that the
transfer function includes a sign inversion.) Parameters
APPLICATIONS INFORMATION
WUU
U
f
L
GAIN (V/V)
0.707 H
B
H
B
f
O
f (LOG SCALE)
BANDPASS RESPONSE
f
H
GAIN (V/V)
0.707 H
L
H
P
H
L
H
H
f
P
f (LOG SCALE)
LOWPASS RESPONSE
f
C
f
C
1562-2 F04
GAIN (V/V)
0.707 H
H
H
P
f
P
f (LOG SCALE)
HIGHPASS RESPONSE
Q
f
ff
fff
ff
QQ
ff
QQ
O
HL
OLH
L
O
HO
==
=
+
+
=+
+
–
;
–1
2
1
2
1
1
2
1
2
1
2
2
ff
QQ
ff
Q
HH
Q
Q
CO
PO
PL
=
+
+
=
=
1
1
2
1
1
2
1
1
1
2
1
1
1
1
4
22
2
2
2
––
–
–
ff
QQ
ff
Q
HH
Q
Q
CO
PO
PH
=
+
+
=
=
1
1
2
1
1
2
1
1
1
2
1
1
1
1
4
22
2
1
2
1
2
––
–
–
–
–
Figure 4. Characteristics of Standard 2nd Order Filter Responses
INV V1
2nd ORDER
1/4 LTC1562-2
V2
1562 F05
R2R
Q
R
IN
V
IN
V
OUT
Figure 5. Basic Lowpass Configuration