MAXIM MAX7408CUA

19-1378; Rev 1; 10/98
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
The MAX7408/MAX7411/MAX7412/MAX7415 5th-order,
lowpass, elliptic, switched-capacitor filters (SCFs) operate from a single +5V (MAX7408/MAX7411) or +3V
(MAX7412/MAX7415) supply. The devices draw only
1.2mA of supply current and allow corner frequencies
from 1Hz to 15kHz, making them ideal for low-power
post-DAC filtering and anti-aliasing applications. They
can be put into a low-power mode, reducing supply
current to 0.2µA.
Two clocking options are available: self-clocking (through
the use of an external capacitor) or external clocking for
tighter cutoff-frequency control. An offset-adjust pin
allows for adjustment of the DC output level.
The MAX7408/MAX7412 deliver 53dB of stopband
rejection and a sharp rolloff with a transition ratio of 1.6.
The MAX7411/MAX7415 achieve a sharper rolloff with a
transition ratio of 1.25 while still providing 37dB of stopband rejection. Their fixed response limits the design
task to selecting a clock frequency.
Features
♦ 5th-Order, Elliptic Lowpass Filters
♦ Low Noise and Distortion: -80dB THD + Noise
♦ Clock-Tunable Corner Frequency (1Hz to 15kHz)
♦ Single-Supply Operation
+5V (MAX7408/MAX7411)
+3V (MAX7412/MAX7415)
♦ Low Power
1.2mA (operating mode)
0.2µA (shutdown mode)
♦ Available in 8-Pin µMAX/DIP Packages
♦ Low Output Offset: ±4mV
Ordering Information
PART
TEMP. RANGE
MAX7408CPA
0°C to +70°C
8 Plastic DIP
PIN-PACKAGE
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 µMAX
8 Plastic DIP
8 µMAX
ADC Anti-Aliasing
CT2 Base Stations
MAX7408CUA
MAX7408EPA
MAX7408EUA
Post-DAC Filtering
Speech Processing
MAX7411CPA
0°C to +70°C
8 Plastic DIP
MAX7411CUA
MAX7411EPA
MAX7411EUA
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 µMAX
8 Plastic DIP
8 µMAX
MAX7412CPA
0°C to +70°C
8 Plastic DIP
MAX7412CUA
MAX7412EPA
MAX7412EUA
MAX7415CPA
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 µMAX
8 Plastic DIP
8 µMAX
0°C to +70°C
8 Plastic DIP
MAX7415CUA
MAX7415EPA
MAX7415EUA
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
8 µMAX
8 Plastic DIP
8 µMAX
Applications
Selector Guide
OPERATING
VOLTAGE (V)
TRANSITION RATIO
PART
MAX7408
r = 1.6
+5
MAX7411
r = 1.25
+5
MAX7412
r = 1.6
+3
MAX7415
r = 1.25
+3
Typical Operating Circuit
VSUPPLY
Pin Configuration
0.1µF
VDD
INPUT
CLOCK
IN
CLK
TOP VIEW
SHDN
OUT
MAX7408
MAX7411
MAX7412
MAX7415
GND
OUTPUT
1
IN
2
GND
COM
OS
COM
0.1µF
3
VDD 4
MAX7408
MAX7411
MAX7412
MAX7415
8
CLK
7
SHDN
6
OS
5
OUT
µMAX/DIP
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 1-800-835-8769.
MAX7408/MAX7411/MAX7412/MAX7415
General Description
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
ABSOLUTE MAXIMUM RATINGS
VDD to GND ..............................................................-0.3V to +6V
IN, OUT, COM, OS, CLK, SHDN ................-0.3V to (VDD + 0.3V)
OUT Short-Circuit Duration...................................................1sec
Continuous Power Dissipation (TA = +70°C)
8-Pin DIP (derate 6.90mW/°C above +70°C) ...............552mW
8-Pin µMAX (derate 4.1mW/°C above +70°C) .............330mW
Operating Temperature Ranges
MAX74_ _C_A .....................................................0°C to +70°C
MAX74_ _E_A ..................................................-40°C to +85°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS—MAX7408/MAX7411
(VDD = +5V; filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, SHDN = VDD, OS = COM, 0.1µF from COM to GND,
fCLK = 100kHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
FILTER
Corner-Frequency Range
Clock-to-Corner Ratio
fC
(Note 1)
0.001 to 15
fCLK/fC
100:1
Clock-to-Corner Tempco
10
Output Voltage Range
Output Offset Voltage
0.25
VOFFSET
DC Insertion Gain with Output
Offset Removed
Total Harmonic Distortion plus
Noise
Offset Voltage Gain
COM Voltage Range
Input Voltage Range at OS
Input Resistance at COM
kHz
VIN = VCOM = VDD / 2
VCOM = VDD / 2 (Note 2)
THD+N
AOS
fIN = 200Hz, VIN = 4Vp-p,
measurement bandwidth = 22kHz
OS to OUT
±25
mV
0.2
0.4
dB
-81
dB
1
V/V
VDD
- 0.5
2
VDD
2
VDD
+ 0.5
2
Output, COM internally driven
VDD
- 0.2
2
VDD
2
VDD
+ 0.2
2
V
Measured with respect to COM
RCOM
Clock Feedthrough
V
±4
Input, COM externally driven
VCOM
VOS
0
ppm/°C
VDD - 0.25
110
TA = +25°C
±0.1
V
180
kΩ
5
mVp-p
Resistive Output Load Drive
RL
10
1
kΩ
Maximum Capacitive Load
at OUT
CL
50
500
pF
Input Leakage Current at COM
SHDN = GND, VCOM = 0 to VDD
±0.2
±10
µA
Input Leakage Current at OS
VOS = 0 to VDD
±0.2
±10
µA
27
34
kHz
±12
±20
µA
CLOCK
Internal Oscillator Frequency
fOSC
Clock Output Current
(Internal Oscillator Mode)
ICLK
Clock Input High
VIH
Clock Input Low
VIL
2
COSC = 1000pF (Note 3)
19
4.5
_______________________________________________________________________________________
V
0.5
V
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
(VDD = +5V; filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, SHDN = VDD, OS = COM, 0.1µF from COM to GND,
fCLK = 100kHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
CONDITIONS
SYMBOL
MIN
TYP
MAX
UNITS
5.5
V
mA
POWER REQUIREMENTS
Supply Voltage
VDD
Supply Current
IDD
4.5
Operating mode, no load
1.16
1.5
Shutdown Current
I SHDN
SHDN = GND
0.2
1
Power-Supply Rejection Ratio
PSRR
Measured at DC
70
µA
dB
SHUTDOWN
SHDN Input High
VSDH
SHDN Input Low
VSDL
SHDN Input Leakage Current
4.5
VSHDN = 0 to VDD
V
±0.2
0.5
V
±10
µA
ELECTRICAL CHARACTERISTICS—MAX7412/MAX7415
(VDD = +3V, filter output measured at OUT pin, 10kΩ || 50pF load to GND at OUT, SHDN = VDD, OS = COM, 0.1µF from COM to
GND, fCLK = 100kHz; TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
FILTER CHARACTERISTICS
Corner-Frequency Range
Clock-to-Corner Ratio
fC
(Note 1)
0.001 to 15
fCLK/fC
Clock-to-Corner Tempco
10
Output Voltage Range
Output Offset Voltage
0.25
VOFFSET
DC Insertion Gain with Output
Offset Removed
Total Harmonic Distortion plus
Noise
THD+N
AOS
COM Voltage Range
VCOM
Input Resistance at COM
VIN = VCOM = VDD / 2
VCOM = VDD / 2 (Note 2)
Offset Voltage Gain
Input Voltage Range at OS
kHz
100:1
VOS
fIN = 200Hz, VIN = 2.5Vp-p,
measurement bandwidth = 22kHz
OS to OUT
VDD
- 0.1
2
Measured with respect to COM
RCOM
Clock Feedthrough
0
ppm/°C
VDD - 0.25
±25
mV
0.2
0.4
dB
-79
dB
1
V/V
VDD
2
VDD
+ 0.1
2
±0.1
110
TA = +25°C
V
±4
V
V
180
kΩ
3
mVp-p
Resistance Output Load Drive
RL
10
1
kΩ
Maximum Capacitive Load
at OUT
CL
50
500
pF
Input Leakage Current at COM
Input Leakage Current at OS
SHDN = GND, VCOM = 0 to VDD
VOS = 0 to VDD
±0.2
±10
µA
±0.2
±10
µA
_______________________________________________________________________________________
3
MAX7408/MAX7411/MAX7412/MAX7415
ELECTRICAL CHARACTERISTICS—MAX7408/MAX7411 (continued)
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
ELECTRICAL CHARACTERISTICS—MAX7412/MAX7415 (continued)
(VDD = +3V, filter output measured at OUT pin, 10kΩ || 50pF load to GND at OUT, SHDN = VDD, OS = COM, 0.1µF from COM to
GND, fCLK = 100kHz; TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
19
27
34
kHz
±12
±20
µA
CLOCK
Internal Oscillator Frequency
fOSC
COSC = 1000pF (Note 3)
Clock Output Current
(Internal Oscillator Mode)
ICLK
VCLK = 0 or 3V
Clock Input High
VIH
Clock Input Low
VIL
2.5
V
0.5
V
POWER REQUIREMENTS
Supply Voltage
VDD
Supply Current
IDD
3.6
V
Operating mode, no load
2.7
1.13
1.5
mA
1
Shutdown Current
I SHDN
SHDN = GND
0.2
Power-Supply Rejection Ratio
PSRR
Measured at DC
70
µA
dB
SHUTDOWN
SHDN Input High
VSDH
SHDN Input Low
VSDL
SHDN Input Leakage Current
2.5
V SHDN = 0 to VDD
V
±0.2
0.5
V
±10
µA
ELLIPTIC FILTER (r = 1.6) CHARACTERISTICS—MAX7408/MAX7412
(VDD = +5V for MAX7408, VDD = +3V for MAX7412; filter output measured at OUT; 10kΩ || 50pF load to GND at OUT; SHDN = VDD;
VCOM = VOS = VDD / 2; fCLK = 100kHz; TA = TMIN to TMAX; unless otherwise noted. Typical values are at TA = +25°C.) (Note 3)
PARAMETER
Insertion Gain
with DC Gain Error Removed
(Note 4)
4
MIN
TYP
MAX
fIN = 0.34fC
CONDITIONS
-0.4
-0.2
0.4
fIN = 0.63fC
-0.4
0.2
0.4
fIN = 0.84fC
-0.4
-0.2
0.4
fIN = 0.96fC
-0.4
0.2
0.4
fIN = fC
-0.7
-0.2
0.2
fIN = 1.60fC
-53.4
-50
fIN = 1.90fC
-53.4
-50
fIN = 4.62fC
-53.4
-50
_______________________________________________________________________________________
UNITS
dB
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
(VDD = +5V for MAX7411, VDD = +3V for MAX7415; filter output measured at OUT; 10kΩ || 50pF load to GND at OUT; SHDN = VDD,
VCOM = VOS = VDD / 2; fCLK = 100kHz; TA = TMIN to TMAX; unless otherwise noted. Typical values are at TA = +25°C.) (Note 3)
PARAMETER
CONDITIONS
Insertion Gain
with DC Gain Error Removed
(Note 4)
MIN
TYP
MAX
fIN = 0.38fC
-0.4
-0.2
0.4
fIN = 0.68fC
-0.4
0.2
0.4
fIN = 0.87fC
-0.4
-0.2
0.4
fIN = 0.97fC
-0.4
0.2
0.4
fIN = fC
-0.7
-0.2
0.2
fIN = 1.25fC
-38.5
-34
fIN = 1.43fC
-37.2
-35
fIN = 3.25fC
-37.2
-35
UNITS
dB
Note 1: The maximum fC is defined as the clock frequency fCLK = 100 · fC at which the peak SINAD drops to 68dB with a sinusoidal
input at 0.2fC.
Note 2: DC insertion gain is defined as ∆VOUT / ∆VIN.
Note 3: fOSC (kHz) ≈ 27 · 103 / COSC (COSC in pF).
Note 4: The input frequencies, fIN, are selected at the peaks and troughs of the ideal elliptic frequency responses.
Typical Operating Characteristics
(VDD = +5V for MAX7408/MAX7411, VDD = +3V for MAX7412/MAX7415; fCLK = 100kHz; SHDN = VDD; VCOM = VOS = VDD / 2;
TA = +25°C; unless otherwise noted.)
fC = 1kHz
r = 1.6
0
-0.2
-40
-60
GAIN (dB)
-20
GAIN (dB)
-20
GAIN (dB)
fC = 1kHz
r = 1.25
0
0.2
MAX7408/11-02
20
MAX7408/11-01
20
0
MAX7408/MAX7412
PASSBAND FREQUENCY RESPONSE
MAX7411/MAX7415
FREQUENCY RESPONSE
-40
-60
-0.4
-0.6
-80
-80
-0.8
-100
-100
-1.0
-120
1
2
3
INPUT FREQUENCY (kHz)
4
5
fC = 1kHz
r = 1.6
-1.2
-120
0
MAX7408/11-03
MAX7408/MAX7412
FREQUENCY RESPONSE
0
1
2
3
INPUT FREQUENCY (kHz)
4
5
0
204
408
612
816
1.02k
INPUT FREQUENCY (Hz)
_______________________________________________________________________________________
5
MAX7408/MAX7411/MAX7412/MAX7415
ELLIPTIC FILTER (r = 1.25) CHARACTERISTICS—MAX7411/MAX7415
Typical Operating Characteristics (continued)
(VDD = +5V for MAX7408/MAX7411, VDD = +3V for MAX7412/MAX7415; fCLK = 100kHz; SHDN = VDD; VCOM = VOS = VDD / 2;
TA = +25°C; unless otherwise noted.)
-0.6
-0.8
-1.0
fC = 1kHz
r = 1.25
-150
-200
-250
-300
408
612
816
1.02k
0
0.2
1.15
1.14
1.13
1.12
1.11
2.5
3.0
3.5
4.0
4.5
5.0
5.5
1.0
1.2
1.4
0
1.6
0.2
0.4
0.6
MAX7408/11-10
-20
-30
MAX7408/11-08
1.18
-60
B
-70
1.16
1.15
1.14
VDD = +3V
1.13
SEE TABLE A
-30
-40
-50
-60
1.12
-70
1.11
-80
A
-60
-40
-20
0
20
40
60
80
100
0
1
2
0
1
2
Table A. THD + Noise Test Conditions
LABEL
fIN
(Hz)
fC
(kHz)
fCLK
(kHz)
MEASUREMENT
BANDWIDTH (kHz)
A
200
1
100
22
B
1k
5
500
80
A
3
4
5
AMPLITUDE (Vp-p)
6
3
AMPLITUDE (Vp-p)
B
A
B
-90
1.10
-80
-90
1.6
-20
VDD = +5V
1.17
-40
-50
1.4
0
-10
TEMPERATURE (°C)
SEE TABLE A
1.2
SUPPLY CURRENT vs. TEMPERATURE
MAX7411
TOTAL HARMONIC DISTORTION PLUS NOISE
vs. INPUT SIGNAL AMPLITUDE
-10
1.0
MAX7408
TOTAL HARMONIC DISTORTION PLUS NOISE
vs. INPUT SIGNAL AMPLITUDE
SUPPLY VOLTAGE (V)
0
0.8
INPUT FREQUENCY (kHz)
1.19
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
1.16
0.8
1.20
MAX7408/11-07
1.17
0.6
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (Hz)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
0.4
THD + NOISE (dB)
204
-400
-600
-400
0
-300
-500
-350
-1.4
-200
MAX7408/11-09
-0.4
-100
fC = 1kHz
r = 1.25
-100
PHASE SHIFT (DEGREES)
PHASE SHIFT (DEGREES)
-0.2
fC = 1kHz
r = 1.6
-50
0
MAX7408/11-05
0
GAIN (dB)
0
MAX7408/11-04
0.2
-1.2
MAX7411/MAX7415
PHASE RESPONSE
MAX7408/MAX7412
PHASE RESPONSE
MAX7408/11-06
MAX7411/MAX7415
PASSBAND FREQUENCY RESPONSE
THD + NOISE (dB)
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
_______________________________________________________________________________________
4
5
5th-Order, Lowpass,
Elliptic, Switched-Capacitor
MAX7415
TOTAL HARMONIC DISTORTION PLUS NOISE
vs. INPUT SIGNAL AMPLITUDE
MAX7412
TOTAL HARMONIC DISTORTION PLUS NOISE
vs. INPUT SIGNAL AMPLITUDE
-20
-30
-40
-50
-60
-70
SEE TABLE A
-10
THD + NOISE (dB)
-30
-40
-50
-60
B
B
-70
B
A
-80
-80
A
-90
0
0.5
1.0
1.5
A
-90
2.0
2.5
0
3.0
0.5
INTERNAL OSCILLATOR PERIOD
vs. SMALL CAPACITANCE (in pF)
VDD = +5V
60
40
10
20
0
VDD = +3V
6
4
1000 1500 2000 2500 3000 3500
27.1
27.0
26.9
26.8
COSC = 1000pF
26.7
26.6
0
50
100
150
200
250
300
2.0
350
2.5
3.0
3.5
4.0
4.5
CAPACITANCE (nF)
SUPPLY VOLTAGE
INTERNAL OSCILLATOR FREQUENCY
vs. TEMPERATURE
DC OFFSET VOLTAGE
vs. TEMPERATURE
DC OFFSET VOLTAGE
vs. SUPPLY VOLTAGE
27.0
VDD = +5V
-1.0
VDD = +3V
-1.5
-2.0
-2.5
VDD = +5V
26.0
-3.0
COSC = 1000pF
-30
-10
10
30
50
TEMPERATURE (°C)
70
90
110
MAX7408/11-18
5.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-3.5
25.5
-0.5
DC OFFSET VOLTAGE (mV)
DC OFFSET VOLTAGE (mV)
VDD = +3V
26.5
-0.5
5.0
0
MAX7408/11-17
0
MAX7408/11-16
27.5
-50
27.2
CAPACITANCE (pF)
28.0
OSCILLATOR FREQUENCY (kHz)
8
0
500
3.0
27.3
2
0
2.5
27.4
OSCILLATOR FREQUENCY (kHz)
VDD = +3V
MAX7408/11-14
12
OSCILLATOR PERIOD (ms)
OSCILLATOR PERIOD (µs)
80
2.0
INTERNAL OSCILLATOR FREQUENCY
vs. SUPPLY VOLTAGE
INTERNAL OSCILLATOR PERIOD
vs. LARGE CAPACITANCE (in nF)
MAX7408/11-13
VDD = +5V
100
1.5
AMPLITUDE (Vp-p)
AMPLITUDE (Vp-p)
120
1.0
MAX7408/11-15
THD + NOISE (dB)
-20
0
MAX7408/11-12
SEE TABLE A
MAX7408/11-11
0
-10
-4.0
-40
-20
0
20
40
60
TEMPERATURE (°C)
80
100
2.5
3.0
3.5
4.0
4.5
5.0
5.5
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________
7
MAX7408/MAX7411/MAX7412/MAX7415
Typical Operating Characteristics (continued)
(VDD = +5V for MAX7408/MAX7411, VDD = +3V for MAX7412/MAX7415; fCLK = 100kHz; SHDN = VDD; VCOM = VOS = VDD / 2;
TA = +25°C; unless otherwise noted.)
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
Pin Description
PIN
NAME
1
COM
FUNCTION
Common Input Pin. Biased internally at mid-supply. Bypass externally to GND with 0.1µF capacitor. To
override internal biasing, drive with an external supply.
2
IN
3
GND
Filter Input
Ground
4
VDD
Positive Supply Input, +5V for MAX7408/MAX7411 or +3V for MAX7412/MAX7415
5
OUT
Filter Output
6
OS
Offset Adjust Input. To adjust output offset, bias OS with a resistive voltage-divider between an external
supply and ground. Connect OS to COM if no offset adjustment is needed.
7
SHDN
Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDD for normal operation.
8
CLK
Clock Input. Connect an external capacitor (COSC) from CLK to GND to set the internal oscillator
frequency. To override the internal oscillator, connect to an external clock.
Detailed Description
The MAX7408/MAX7411/MAX7412/MAX7415 family of
5th-order, elliptic, lowpass filters provides sharp rolloff
with good stopband rejection. All parts operate with a
100:1 clock-to-corner frequency ratio and a 15kHz
maximum corner frequency.
Most switched-capacitor filters (SCFs) are designed
with biquadratic sections. Each section implements two
pole-zero pairs, and the sections can be cascaded to
produce higher order filters. The advantage to this
approach is ease of design. However, this type of
design is highly sensitive to component variations
if any section’s Q is high. The MAX7408/MAX7411/
MAX7412/MAX7415 use an alternative approach, which
is to emulate a passive network using switched-capacitor integrators with summing and scaling. The passive
network may be synthesized using CAD programs, or
may be found in many filter books. Figure 1 shows a
basic 5th-order ladder elliptic filter structure.
A switched-capacitor filter that emulates a passive ladder filter retains many of the same advantages. The
component sensitivity of a passive ladder filter is low
when compared to a cascaded biquadratic design,
RS
+
-
VIN C1
C2
C4
L2
L4
C3
C5
Figure 1. 5th-Order Ladder Elliptic Filter Network
8
RL
because each component affects the entire filter shape
rather than a single pole-zero pair. In other words, a
mismatched component in a biquadratic design has a
concentrated error on its respective poles, while the
same mismatch in a ladder filter design spreads its
error over all poles.
Elliptic Characteristics
Lowpass elliptic filters such as the MAX7408/MAX7411/
MAX7412/MAX7415 provide the steepest possible
rolloff with frequency of the four most common filter
types (Butterworth, Bessel, Chebyshev, and elliptic).
The high Q value of the poles near the passband edge
combined with the stopband zeros allows for the sharp
attenuation characteristic of elliptic filters, making these
devices ideal for anti-aliasing and post-DAC filtering in
single-supply systems (see the Anti-Aliasing and PostDAC Filtering section).
In the frequency domain, the first transmission zero
causes the filter’s amplitude to drop to a minimum level.
Beyond this zero, the response rises as the frequency
increases until the next transmission zero. The stopband begins at the stopband frequency, fS. At frequencies above fS, the filter’s gain does not exceed the gain
at fS. The corner frequency, fC, is defined as the point
where the filter output attenuation falls just below the
passband ripple. The transition ratio (r) is defined as
the ratio of the stopband frequency to the corner frequency:
r = fS / fC
The MAX7408/MAX7412 have a translation ratio of 1.6
and typically 53dB of stopband rejection. The
MAX7411/MAX7415 have a transition ratio of 1.25 (providing a steeper rolloff) and typically 37dB of stopband
rejection.
_______________________________________________________________________________________
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
RIPPLE
0.1µF
VDD
fC
GAIN (dB)
TRANSITION RATIO = fS
fC
INPUT
SHDN
OUT
IN
OUTPUT
COM
0.1µF
fS
CLOCK
MAX7408
MAX7411
MAX7412
MAX7415
CLK
50k
OS
0.1µF
GND
PASSBAND
50k
STOPBAND
fC
FREQUENCY
fS
Figure 2. Elliptic Filter Response
Figure 3. Offset Adjustment Circuit
Clock Signal
External Clock
These SCFs are designed for use with external clocks
that have a 40% to 60% duty cycle. When using an
external clock, drive the CLK pin with a CMOS gate
powered from 0 to VDD. Varying the rate of the external
clock adjusts the corner frequency of the filter:
f
fC = CLK
100
Internal Clock
When using the internal oscillator, the capacitance
(COSC) on CLK determines the oscillator frequency:
fOSC (kHz) =
50k
27
⋅
103
COSC (pF)
Since COSC is in the low picofarads, minimize the stray
capacitance at CLK so that it does not affect the internal oscillator frequency. Varying the rate of the internal
oscillator adjusts the filter’s corner frequency by a
100:1 clock-to-corner frequency ratio. For example, an
internal oscillator frequency of 100kHz produces a
nominal corner frequency of 1kHz.
Input Impedance vs. Clock Frequencies
The MAX7408/MAX7411/MAX7412/MAX7415’s input
impedance is effectively that of a switched-capacitor
resistor (see the following equation), and is inversely
proportional to frequency. The input impedance values
determined by the equation represent the average input
impedance, since the input current is not continuous. As
a rule, use a driver with an output resistance less than
10% of the filter’s input impedance.
Estimate the input impedance of the filter by using the
following formula:
ZIN =
1
(fCLK
⋅
CIN )
where fCLK = clock frequency and CIN = 1pF.
Low-Power Shutdown Mode
The MAX7408/MAX7411/MAX7412/MAX7415 have a
shutdown mode that is activated by driving SHDN low.
In shutdown mode, the filter supply current reduces to
0.2µA, and the output of the filter becomes high impedance. For normal operation, drive SHDN high or connect to VDD.
Applications Information
Offset (OS) and Common-Mode (COM)
Input Adjustment
COM sets the common-mode input voltage and is
biased at mid-supply with an internal resistor-divider. If
the application does not require offset adjustment, connect OS to COM. For applications where offset adjustment is required, apply an external bias voltage
through a resistor-divider network to OS, as shown in
Figure 3. For applications that require DC level shifting,
adjust OS with respect to COM. (Note: Do not leave OS
unconnected.) The output voltage is represented by
these equations:
VOUT = (VIN − VCOM ) + VOS
VCOM =
VDD
2
(typical)
where (VIN - VCOM) is lowpass filtered by the SCF and
OS is added at the output stage. See the Electrical
_______________________________________________________________________________________
9
MAX7408/MAX7411/MAX7412/MAX7415
VSUPPLY
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
to the system ground and GND to the negative supply.
Figure 4 shows an example of dual-supply operation.
Single-supply and dual-supply performance are equivalent. For either single-supply or dual-supply operation,
drive CLK and SHDN from GND (V- in dual supply
operation) to VDD . Use the MAX7408/MAX7411 for
±2.5, and use the MAX7412/MAX7415 for ±1.5V. For
±5V dual-supply applications, see the MAX291/
MAX292/MAX295/MAX296 and MAX293/MAX294/
MAX297 data sheets.
V+
VDD
INPUT
V+
CLOCK
V-
IN
CLK
MAX7408
MAX7411
MAX7412
MAX7415
SHDN
OUT
COM
*
OUTPUT
Input Signal Amplitude Range
OS
0.1µF
0.1µF
GND
V-
The optimal input signal range is determined by observing the voltage level at which the signal-to-noise plus
distortion (SINAD) ratio is maximized for a given corner
frequency. The Typical Operating Characteristics show
the THD+Noise response as the input signal’s peak-topeak amplitude is varied.
Anti-Aliasing and Post-DAC Filtering
*CONNECT SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.
Figure 4. Dual-Supply Operation
Characteristics table for the input voltage range of COM
and OS. Changing the voltage on COM or OS significantly from mid-supply reduces the dynamic range.
Power Supplies
The MAX7408/MAX7411 operate from a single +5V
supply and the MAX7412/MAX7415 operate from a single +3V supply. Bypass V DD to GND with a 0.1µF
capacitor. If dual supplies are required, connect COM
When using the MAX7408/MAX7411/MAX7412/
MAX7415 for anti-aliasing or post-DAC filtering, synchronize the DAC (or ADC) and the filter clocks. If the
clocks are not synchronized, beat frequencies may
alias into the desired passband.
Harmonic Distortion
Harmonic distortion arises from nonlinearities within the
filter. These nonlinearities generate harmonics when a
pure sine wave is applied to the filter input. Table 1 lists
typical harmonic distortion values with a 10kΩ load at
TA = +25°C.
Table 1. Typical Harmonic Distortion
FILTER
fCLK
(kHz)
fIN
(Hz)
500
1k
MAX7408
TYPICAL HARMONIC DISTORTION (dB)
2nd
3rd
4th
5th
-85.5
-78.4
-92.8
-86.9
-88.2
-83.1
-93
-89.5
-90
-80
-92
-88
4
100
200
500
1k
MAX7411
4
100
200
-88
-86
-92
-88
500
1k
-86.6
-93.1
-90
-85.6
100
200
-88.2
-85.1
-88.9
-85.7
500
1k
-87
-86
-90
-90
-90
-87
-90
-90
MAX7412
2
MAX7415
2
100
10
VIN
(Vp-p)
200
______________________________________________________________________________________
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
TRANSISTOR COUNT: 1457
8LUMAXD.EPS
________________________________________________________Package Information
______________________________________________________________________________________
11
MAX7408/MAX7411/MAX7412/MAX7415
Chip Information
MAX7408/MAX7411/MAX7412/MAX7415
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
PDIPN.EPS
Package Information (continued)
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1998 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.