LTC1562 Linear Technology, LTC1562 Datasheet - Page 26

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

LTC1562
TYPICAL APPLICATIONS
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 0.1 F 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
26
V
V
IN1
IN2
5V
SCHEMATIC INCLUDES PIN NUMBERS FOR 20-PIN PACKAGE.
PINS 4, 7, 14, 17 (NOT SHOWN) ALSO CONNECT TO V
R
R
IN1A
IN1A
R
R
IN1B
IN1B
C
C
IN1
IN1
Dual 5th Order Lowpass “Elliptic” Filter
0.1 F
R
R21
R21
R
Q1
Q1
f
C
100k
75k
50k
(Hz)
10
1
2
3
5
6
8
9
INVB
V1B
V2B
V
SHDN
V2A
V1A
INVA
R
8.06k
16.9k
5.9k
+
IN1A
LTC1562
U
C
C
IN2
IN2
R
15.4k
35.7k
7.5k
–
IN1B
AGND
INVD
INVC
V2D
V1D
V1C
V2C
(Advanced)
V
–
20
19
18
16
15
13
12
11
680pF
560pF
390pF
C
IN1
R
R
R
R22
R22
R
IN2
IN2
Q2
Q2
36.5k
56.2k
R
28k
Q1
1562 TA15a
0.1 F
capacitance ( 10 F) 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.
13.3k
30.1k
7.5k
R21
V
– 5V
V
OUT1
OUT2
6.34k
11.3k
25.5k
R
IN2
68pF
68pF
68pF
C
IN2
–100
–120
–20
– 40
– 60
– 80
20
0
10
9.31k
12.7k
18.7k
f
R
C
Q2
= 100kHz
Amplitude Response
FREQUENCY (kHz)
11.3k
44.2k
R22
20k
100
1562 TA15b
1000