LTC1562 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1562' PDF : 28 Pages View PDF
LTC1562
TYPICAL APPLICATIONS (Advanced)
Dual 5th Order Lowpass “Elliptic” Filter
RIN2
CIN2
RIN1A RIN1B
VIN1
CIN1
RQ1
R21
5V
RIN1A
VIN2
RIN1B
CIN1
0.1µF
R21
RQ1
1
INVB
20
INVC
2
V1B
19
V1C
3
V2B
18
V2C
5 V+ LTC1562 V – 16
6
SHDN
15
AGND
8
V2A
13
V2D
9
V1A
12
V1D
10
INVA
11
INVD
CIN2
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V –
VOUT1
RQ2
R22
– 5V
0.1µF
R22
RQ2
RIN2
1562 TA15a
VOUT2
Amplitude Response
20
fC = 100kHz
0
– 20
– 40
– 60
– 80
– 100
– 120
10
100
FREQUENCY (kHz)
1000
1562 TA15b
fC (Hz) RIN1A RIN1B
CIN1
RQ1
100k 5.9k 7.5k 680pF 28k
R21
RIN2
CIN2
RQ2
7.5k 6.34k 68pF 9.31k
75k 8.06k 15.4k 560pF 36.5k 13.3k 11.3k 68pF 12.7k
50k 16.9k 35.7k 390pF 56.2k 30.1k 25.5k 68pF 18.7k
R22
11.3k
20k
44.2k
Construction and Instrumentation Cautions
100dB rejections at hundreds of kilohertz require electri-
cally clean, compact construction, with good grounding
and supply decoupling, and minimal parasitic capaci-
tances in critical paths (such as Operational Filter INV
inputs). In a circuit with 5k resistances trying for 100dB
rejection at 100kHz, a stray coupling of 0.003pF around
the signal path can preclude the 100dB. (By comparison,
the stray capacitance between two adjacent pins of an IC
can be 1pF or more.) Also, high quality supply bypass
capacitors of 0.1µF near the chip provide good decoupling
from a clean, low inductance power source. But several
inches of wire (i.e., a few microhenrys of inductance) from
the power supplies, unless decoupled by substantial
capacitance (≥ 10µF) near the chip, can cause a high-Q LC
resonance in the hundreds of kHz in the chip’s supplies or
ground reference, impairing stopband rejection and other
specifications at those frequencies. In demanding filter
circuits we have often found that a compact, carefully laid
out printed circuit board with good ground plane makes a
difference of 20dB in both stopband rejection and distor-
tion performance. Highly selective circuits can even ex-
hibit these issues at frequencies well below 100kHz.
Finally, equipment to measure filter performance can itself
introduce distortion or noise floors; checking for these
limits with a wire replacing the filter is a prudent routine
procedure.
26
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