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LTC1560-1: Tiny 1MHz Lowpass Filter Uses No Inductors
Design Note 169
Nello Sevastopoulos
The LTC®1560-1 is a fully integrated continuous-time filter
in an SO-8 package. It provides a 5-pole elliptic response
with a pin selectable cutoff frequency (fC) of 1MHz or
500kHz. Several features distinguish the LTC1560-1 from
other commercially available high frequency, continuoustime monolithic filters:
• 5-pole 0.5MHz/1MHz elliptic in an SO-8 package
• 70dB signal-to-noise ratio (SNR) measured at
0.07% THD
• 75dB signal-to-noise ratio (SNR) measured at
0.5% THD
• 60dB or more stopband attenuation
• No external components required other than supply
and ground decoupling capacitors
The LTC1560-1 delivers accurate fixed cutoff frequencies
of 500kHz and 1MHz without the need for internal or
external clocks. Other cutoff frequencies can be obtained
upon demand; please consult LTC marketing. The
extremely small size of the part makes it suitable for
compact designs that were never before possible using
discrete RC active or RLC passive filter designs.
Frequency and Time-Domain Response
Figure 1 shows a simple circuit for evaluating the performance of the filter. The LTC1560-1 offers a pin-selectable
cutoff frequency of either 500kHz or 1MHz. The filter gain
response is shown in Figure 2. In the 1MHz mode, the
passband gain is flat up to (0.55)(fC) with a typical ripple of
±0.2dB, increasing to ±0.3dB for input frequencies up to
(0.9)(fC). The stopband attenuation is 63dB starting from
(2.43)(fC) and remains at least 60dB for input frequencies
up to 10MHz.
The elliptic transfer function of the LTC1560-1 was chosen
as a compromise between selectivity and transient response. Figure 3a shows the 2-level eye diagram of the
filter. The size of the “eye” opening shows that the filter is
suitable for data communications applications. Additional
phase equalization can be performed with the help of an
external dual op amp and a few passive components. This
is shown in Figure 4, where a 2nd order allpass equalizer is
cascaded with the IC. The allpass function is achieved
through traditional techniques, namely, passing a signal
through a low Q inverting bandpass filter and then performing summation with the appropriate gain factors. Figure 3b
shows the eye diagram of the equalized filter.
DC Accuracy
For applications where very low DC offset and DC accuracy
are required, the DC offset of the filter can be easily
corrected, as shown in Figure 5.
The input amplifier stores the DC offset of the IC across its
feedback capacitor. The total output DC offset is the input
DC offset of the 1/2 LT1364 plus its offset current times the
, LTC and LT are registered trademarks of Linear Technology Corporation.
10
5V
0
LTC1560-1
VIN
2
3
–5V
4
GND VOUT
VIN
GND
SHDN
V
1k
8
7
5
+
5V
0.1µF
2
–
8
VOUT
4
0.1µF
0.01µF
fC = 500kHz
–50
–70
–80
–5V, 1MHz
5V, 500kHz
– 5V
Figure 1. A Typical Circuit for Evaluating
the Full Performance of the LT1560-1
12/97/168
–30
–40
–60
0.01µF
0.1µF
fC = 1MHz
–20
7
LT®1360
(OR – 5V)
+ 6
V – 0.5fC/fC
3
GAIN (dB)
1
–10
0.1µF
DN169 F01
–90
0.1
1
FREQUENCY (MHz)
10
DN169 F02
1560_04.eps
Figure 2. Gain vs Frequency of the 1MHz and 500kHz
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10k resistor (less than 1.85mV). Upon power-up, the initial
settling time of the circuit is dominated by the RC time
constant of the DC correcting feedback path; once the DC
offset of the LTC1560-1 is stored, the transient behavior of
the circuit is dictated by the elliptic filter.
Conclusion
The LTC1560-1 is a 5th order, user friendly, elliptic lowpass
filter suitable for any compact design. It is a monolithic
replacement for larger, more expensive and less accurate
solutions in communications and data acquisition.
Figure 3a. 2-Level Eye Diagram of the LTC1560-1
Before Equalization
Figure 3b. 2-Level Eye Diagram of the Equalized Filter
40.2k
20k
9.75k
5V
22pF
LTC1560-1
1
VIN
2
VIN
3
–5V
GND
4
7
SHDN
V
2
(OR – 5V)
6
+
22pF
6.49k
8
GND VOUT
0.1µF
–
1/2 LT1364
3
5V
6.65k
1
6
+
5
0.1µF
0.1µF
+
49.9Ω
7
1/2 LT1364
5
V – 0.5fC/fC
8
–
VOUT
4
0.01µF
0.1µF
0.01µF
– 5V
DN169 F04
Figure 4. Augmenting the LTC1560-1 for Improved Delay Flatness
10k
5V
0.1µF
10,000pF
6
LTC1560-1
2
VIN
10k 3
1
–
1/2 LT1364
1
2
3
+
–5V
4
GND VOUT
SHDN
VIN
GND
V
–
V+
0.5fC/fC
8
7
1/2 LT1364
1k
5
+
LOAD
5V
– 5V
0.1µF
0.1µF
0.01µF
0.1µF
7
(OR – 5V)
6
5
–
DN169 F05
0.01µF
–5V, 1MHz
5V, 500kHz
Figure 5. A DC Accurate 500kHz/1Mhz Elliptic Filter with an Output Buffer for Driving Cables or Capacitive Loads
For literature on our Monolithic Filters,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2453
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417 ● (408) 432-1900
FAX: (408) 434-0507● TELEX: 499-3977 ● www.linear-tech.com
dn169f LT/TP 1297 340K • PRINTED IN THE USA
 LINEAR TECHNOLOGY CORPORATION 1997
www.BDTIC.com/Linear