Aug 1998 Clock-Tunable, High Accuracy, Quad 2nd Order, Analog Filter Building Blocks

DESIGN FEATURES
Clock-Tunable, High Accuracy,
Quad 2nd Order, Analog Filter
Building Blocks
by Philip Karantzalis
LTC1068-200 Ultralow
The LTC1068 product family consists ware for Windows . The internal sam- Frequency Linear-Phase
of four monolithic, clock-tunable filter pling rate of all the LTC1068 devices Lowpass Filter
Introduction
®
is twice the clock frequency. This
allows the frequency of input signals
to approach twice the clock frequency
before aliasing occurs. Maximum
clock frequency for LTC1068-200,
LTC1068 and LTC1068-25 is 6MHz
with ±5V supplies; that for the
LTC1068-50 is 2MHz with a single 5V
supply. For low power filter applications, the LTC1068-50 power supply
current is 4.5mA with a single 5V
supply and 2.5mA with a single 3V
supply. The LTC1068 products are
available in a 28-pin SSOP surface
mount package. The LTC1068 (the
100:1 part) is also available in a 24pin DIP package. The following four
circuits are typical examples of
application-specific filters that the
LTC1068 products can realize.
RL1 23.2k
RL2 14.3k
LTC1068-200
1
R21 12.4k
VIN
R11 14.3k
R31 10k
2
3
4
R41 15.4k
5
6
7
8
5V
9
10
R43 12.4k
R33 12.4k
11
12
13
R23 10k
14
HPB/NB
HPC/NC
BPB
BPC
LPB
LPC
SB
SC
NC
V–
NC
AGND
V+
CLK
NC
NC
SD
SA
LPD
LPA
BPD
BPA
HPA/NA
HPD/ND
INV D
INV A
28
27
26
25
R22 15.4k
R32 10k
Windows is a registered trademark of Microsoft Corp.
R52 5.11k
R62 9.09k
24
23
0.1µF
21
R64 9.09k
20
18
VOUT
17
16
15
R34 10k
R24 15.4k
RB3 23.2k
0.9
0
GAIN
–10
400kHz
R54 5.11k
19
1.0
10
–5V
22
0.7
–30
0.6
0.5
–40
GROUP
DELAY
–50
Figure 1. Linear-phase lowpass filter: f–3dB = 1Hz = fCLK/400
8
0.4
–60
0.3
–70
0.2
–80
0.1
0.0
–90
0.1
RL3 23.2k
0.8
–20
GROUP DELAY (s)
0.1µF
INV C
INV B
Figure 1 shows an LTC1068-200
linear-phase 1Hz lowpass filter schematic and Figure 2 shows its gain and
group delay responses. The clock frequency of this filter is 400 times the
–3dB frequency (f–3dB or fCUTOFF). The
large clock-to-fCUTOFF frequency ratio
of this filter is useful in ultralow frequency filter applications when
minimizing aliasing errors could be
an important consideration. For
example, the 1Hz lowpass filter shown
in Figure 1 requires a 400Hz clock
frequency. For this filter, the input
frequencies that can generate aliasing errors are in a band from 795Hz to
805Hz (2 × fCLK ±5 × f–3dB). For most
very low frequency signal-processing
applications, the signal spectrum is
less than 100Hz. Therefore, Figure
1’s filter will process very low frequency signals without significant
aliasing errors, since its clock frequency is 400Hz and the aliasing
inputs are in a small band around
800Hz.
GAIN (dB)
building blocks. Each product
contains four matched, low noise,
high accuracy 2nd order switched
capacitor filter sections. An external
clock tunes the center frequency of
each 2nd order filter section. The
LTC1068 products differ only in their
clock-to-center frequency ratio. The
clock-to-center frequency ratio is set
to 200:1 (LTC1068-200), 100:1
(LTC1068), 50:1 (LTC1068-50) or 25:1
(LTC1068-25). External resistors can
modify the clock-to-center frequency
ratio. High precision, high performance, quad 2nd order, dual 4th
order or single 8th order filters can be
designed with an LTC1068 family
product. Designing filters with an
LTC1068 product is fully supported
by the FilterCAD™ 2.0 design soft-
1
FREQUENCY (Hz)
10
Figure 2. Gain and group delay response
of Figure 1’s circuit.
Linear Technology Magazine • August 1998
DESIGN FEATURES
RA1 56.2k
RL2 9.09k
RB1 13.3k
RH2 34k
150
10
140
0
R11 22.6k
VIN
R31 10k
2
3
4
R41 22.6k
5
6
7
8
3.3V
9
0.1µF
10
R43 48.7k
1µF
11
R33 12.7k
12
13
R23 10.7k
14
INV C
INV B
HPC/NC
HPB/NB
BPB
BPC
LPB
LPC
SB
SC
NC
V–
AGND
NC
V+
CLK
NC
NC
SA
SD
LPA
LPD
BPA
BPD
HPD/ND
HPA/NA
INV D
INV A
28
GAIN (dB)
R21 20.5k
130
120
–20
R22 43.2k
27
26
110
–30
GROUP
DELAY
–40
100
R32 43.2k
–50
90
R42 196k
–60
90
23
–70
70
22
–80
25
24
21
60
10k
FREQUENCY (Hz)
1k
500kHz
20
19
17
maximum clock frequency for this
filter is 1MHz with a single 3.3V supply and 2MHz with a single 5V supply.
Typical power supply current is 3mA
with a single 3.3V supply and 4.5mA
with a single 5V supply.
R34 14.3k
16
R24 16.9k
15
100k
Figure 4. Gain and group delay
response of Figure 3’s filter
R44 34.8k
18
GROUP DELAY (µs)
LTC1068-50
1
GAIN
–10
VOUT
RB3 24.9k
RL3 26.7k
Figure 3. Low power, single 3.3V supply, 10kHz, 8th order, linear-phase lowpass filter
The flat group delay response in the
filter’s passband implies a linear
phase. A linear-phase filter has a
transient response with very small
overshoot that settles very rapidly. A
linear-phase lowpass filter is useful
for processing communication signals with minimum intersymbol
interference in digital communications transmitters or receivers. The
Figure 3 is a schematic of an LTC106850-based, single 3.3V, low power,
lowpass filter with linear phase. The
clock-to-fCUTOFF ratio is 50 to 1 (fCUTOFF
is the –3dB frequency). Figure 4 shows
the gain and group delay response.
RL2 23.2k
RH1 28k
RH2 11.3k
LTC1068-25
1
R21 4.99k
VIN
R11 29.4k
2
R31 24.9k
3
4
R41 20.5k
R51 4.99k
R61 11.3k
5
6
7
8
5V
0.1µF
9
10
R43 42.3k
11
R33 59k
12
13
R23 4.99k
14
INV C
INV B
HPB/NB
HPC/NC
BPB
BPC
LPB
LPC
SB
SC
NC
V–
AGND
NC
V+
CLK
NC
NC
SA
SD
LPA
LPD
BPA
BPD
HPA/NA
HPD/ND
INV D
INV A
28
R22 4.99k
27
26
R32 107k
25
24
R62 56.2k
R52 4.99k
23
22
–5V
0.1µF
Figure 5 shows a 70kHz bandpass
filter based on the LTC1068-25 operating with dual 5V power supplies.
The clock-to-center frequency ratio is
25 to 1. Figure 6 shows the gain
response of Figure 5’s bandpass filter. The passband of this filter extends
from 0.95 × fCENTER to 1.05 × fCENTER.
The stopband attenuation is greater
than 40dB at 0.8 × fCENTER and 1.15 ×
fCENTER. The center frequency can be
clock tuned to 80kHz with dual 5V
supplies and to 40kHz with a single
5V supply. With FilterCAD, the
LTC1068-25 can be used to realize
bandpass filters less selective than
that shown in Figure 6, which can be
clock tuned up to 160kHz with dual
5V supplies.
10
1.75MHz
21
19
18
17
16
15
0
R64 10k
20
–10
R54
4.99k
–20
GAIN (dB)
LTC1068-50 Single 3.3V
Low Power Linear-Phase
Lowpass Filter
LTC1068-25 Selective
Bandpass Filter is Clock
Tunable to 80kHz
R44 17.4k
R34 63.4k
R24 7.5k
RH3 15.4k
–30
–40
–50
–60
VOUT
–70
–80
–90
RL3 45.3k
Figure 5. 70kHz, 8th order, bandpass filter
Linear Technology Magazine • August 1998
20
30
40
50 60 70 80
FREQUENCY (kHz)
90
100
Figure 6. Gain response of Figure 5’s filter
9
DESIGN FEATURES
RL2 12.4k
RL1 12.4k
RH2 55.2k
LTC1068
1
R21 10k
R11 22.4k
R31 12.4k
2
3
4
0.47µF
5
6
7
5V
0.1µF
8
9
R33 12.4k
10
R23 10k
11
12
INV C
INV B
HPB/NB
HPC/NC
BPB
BPC
LPB
LPC
SB
SC
AGND
V–
V+
CLK
SA
SD
LPA
LPD
BPA
BPD
HPD
HPA/NA
INV D
INV A
24
R22 10k
23
22
R32 12.4k
21
–5V
20
0.1µF
19
18
17
*COSINE WAVE OUT
16
R34 10k
15
*SINE WAVE OUT
f
fOUT = CLK
128
14
R24 10k
13
RH3 38.4k
RL3 16.5k
fCLK =
128 × fOUT
1
2
CLK
1Q0
1Q1
16
5V
1Q2
10
0.1µF
7
8
9
15
CD4520
GND
1Q3
2Q0
2Q1
2Q2
2Q3
3 ÷2
* PIN 16'S COSINE WAVE OUTPUT IS REFERENCED TO
PIN 15'S SINE WAVE OUTPUT
4 ÷4
5 ÷8
6 ÷16
11 ÷32
12 ÷64
13 ÷128
14 ÷256
Figure 7. Square-wave-to-quadrature oscillator converter
LTC1068 Square-Wave-toQuadrature Oscillator Filter
Figure 7 shows the schematic of a
LTC1068 based filter that is specifically designed to produce a low
harmonic distortion sine and cosine
oscillator from a CMOS-level square
wave input. The reference sine wave
output of Figure 7’s circuit is on pin
15 (BPD on the 24-pin LTC1068 package) and the cosine output is on pin
16 (LPD on the 24-pin LTC1068 package). The output frequency of this
quadrature oscillator is the filter’s
clock frequency divided by 128. The
output of a CMOS CD4520 divide-by128 counter is coupled with a 0.47µ F
capacitor to the input to the LTC1068
filter operating with dual 5V power
supplies. The filter’s clock frequency
is the input to the CD4520 counter.
The LTC1068 filter is designed to
pass the fundamental frequency component of a square wave and attenuate
any harmonic components higher
than the fundamental. An ideal square
wave (50% duty cycle) will have only
odd harmonics (3rd, 5th, 7th and so
on), whereas a typical practical square
wave has a duty cycle less or more
than 50% and will also have even
harmonics (2nd, 4th, 6th and so on).
The filter of Figure 7 has a stopband
notch at the 2nd and 3rd harmonics
for a square wave input with a frequency equal to the filter’s clock
frequency divided by 128. The filter’s
sine wave output (pin 15) is 1VRMS for
a ±2.5V square wave input and has
less than 0.025% THD (total harmonic distortion) for input frequencies
up to 16kHz and less than 0.1% THD
for frequencies up to 20kHz. The cosine output (on pin 16, referenced to
pin 15’s sine wave output) is 1.25VRMS
for a ±2.5V square wave input and
has less than 0.07% THD for frequencies up to 20kHz.
The 20kHz frequency limit is due
to the CD4520; with a 74HC type
divide-by-128 counter, sine and cosine
waves up to 40kHz can be generated
with the LTC1068-based filter of
Figure 7.
The LTC1068
Product Family: Versatile
Filter Building Blocks,
1Hz to 200kHz
The previous four filter examples are
typical of the great variety of filters
that can be designed with the LTC1068
product family. As a general selection
guide, the following is recommended:
for low noise and low frequency filters
less than 20kHz, use the LTC1068200; for low noise and low frequency
filters less than 40kHz, use the
LTC1068; for low power filters up to
20kHz operating with single 3V to 5V
supplies, the LTC1068-50 should be
the choice; and for filters in the frequency range 40kHz to 200kHz, use
the LT1068-25. The FilterCAD design
software will recommend the appropriate LTC1068 device for most filter
applications. For some applicationspecific filter designs, the choice of
the proper LTC1068 device may not
be obvious and the assistance of a
Linear Technology applications engineer will be helpful.
LTC1068-Based
Semicustom Filters
For application-specific filter requirements of single 8th order or dual 4th
order filters, a customized version of
an LTC1068-family product can be
obtained in a 16-pin SO package with
internal thin-film resistors. Clock-tocenter frequency ratios higher or lower
than 200:1, 100:1, 50:1 or 25:1 can
also be obtained. Please contact LTC
Marketing for details.
Authors can be contacted
at (408) 432-1900
10
Linear Technology Magazine • August 1998
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