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General Description
The MAX7480 8th-order, lowpass, Butterworth,
switched-capacitor filter (SCF) operates from a single
+5V supply. The device draws only 2.9mA of supply
current and allows corner frequencies from 1Hz to
2kHz, making it ideal for low-power post-DAC filtering
and anti-aliasing applications. The MAX7480 features a
shutdown mode, which reduces the supply current to
0.2µA.
Two clocking options are available: self-clocking
(through the use of an external capacitor) or external
clocking for tighter corner-frequency control. An offset
adjust pin allows for adjustment of the DC output level.
The MAX7480 Butterworth filter provides a maximally
flat passband response. The fixed response simplifies
the design task to selecting a clock frequency.
Applications
ADC Anti-Aliasing
Post-DAC Filtering
Features
♦ 8th-Order, Lowpass Butterworth Filter
♦ Low Noise and Distortion: -73dB THD + Noise
♦ Clock-Tunable Corner Frequency (1Hz to 2kHz)
♦ 100:1 Clock-to-Corner Ratio
♦ +5V Single-Supply Operation
♦ Low Power
2.9mA (Operating Mode)
0.2µA (Shutdown Mode)
♦ Available in 8-Pin SO/DIP Package
♦ Low Output Offset: ±5mV
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
________________________________________________________________
Maxim Integrated Products
1
OS
OUTV
DD
1
2
87CLK
SHDNIN
GND
COM
SO/DIP
TOP VIEW
3
4
6
5
MAX7480
V
DD
IN
CLK
OUT
GND
INPUT
0.1µF
0.1µF
CLOCK
SHDN
OUTPUT
V
SUPPLY
COM
OS
MAX7480
Typical Operating Circuit
19-1416; Rev 0; 1/99
Pin Configuration
Ordering Information
PART
MAX7480ESA
MAX7480EPA -40°C to +85°C
-40°C to +85°C
TEMP. RANGE PIN-PACKAGE
8 SO
8 Plastic DIP
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VDD= +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF from COM to GND, SHDN =
V
DD
, f
CLK
= 100kHz, TA= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at TA= +25°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.
VDDto GND..............................................................-0.3V to +6V
IN, OUT, COM, OS, CLK............................-0.3V to (V
DD
+ 0.3V)
SHDN........................................................................-0.3V to +6V
OUT Short-Circuit Duration...................................................1sec
Continuous Power Dissipation (T
A
= +70°C)
8-Pin SO (derate 5.88mW/°C above +70°C)................471mW
8-Pin DIP (derate 9.09mW/°C above +70°C)...............727mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10sec).............................+300°C
C
OSC
= 1000pF (Note 4)
VOS= 0 to (VDD- 1V) (Note 3)
SHDN = GND, V
COM
= 0 to V
DD
(Note 1)
Input, COM externally driven
fIN= 200Hz, VIN= 4Vp-p,
measurement bandwidth = 22kHz
VIN= V
COM
= V
DD
/ 2
V
COM
= V
DD
/ 2 (Note 2)
CONDITIONS
40 53 67f
OSC
Internal Oscillator Frequency
±0.1 ±10Input Leakage Current at OS
±0.1 ±10Input Leakage Current at COM
50 500C
L
10 1R
L
Resistive Output Load Drive
10Clock Feedthrough
75 125R
COM
Input Resistance at COM
100:1f
CLK
/f
C
Clock-to-Corner Ratio
0.001 to 2f
C
Corner Frequency
V
COM
1A
OS
OS Voltage Gain to OUT
-73THD+N
Total Harmonic Distortion
plus Noise
10Clock-to-Corner Tempco
0.25 V
DD
-
0.25Output Voltage Range
±5 ±25V
OFFSET
Output Offset Voltage
-0.1 0.15 0.3
DC Insertion Gain with
Output Offset Removed
MIN TYP MAX
SYMBOLPARAMETER
V
CLK
= 0 or 5V
0.5V
IL
Clock Input Low
V
DD
- 0.5V
IH
Clock Input High
±24 ±40I
CLK
Clock Input Current
V
V
µA
kHz
µA
µA
pF
kΩ
mVp-p
kΩ
V
V/V
dB
dB
mV
V
ppm/°C
kHz
UNITS
Maximum Capacitive Load at
OUT
COM Voltage Range
V
DD
/ 2 V
DD
/ 2 V
DD
/ 2
- 0.5 + 0.5
V
COM
±0.1V
OS
Input Voltage Range at OS V
V
DD
/ 2 V
DD
/ 2 V
DD
/ 2
- 0.2 + 0.2
FILTER CHARACTERISTICS
CLOCK
Output, COM internally biased
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VDD= +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, OS = COM, 0.1µF from COM to GND, SHDN =
V
DD
, f
CLK
= 100kHz, TA= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at TA= +25°C.)
CONDITIONS
MIN TYP MAX
SYMBOLPARAMETER
Measured at DC
SHDN = GND, CLK driven from 0 to V
DD
Operating mode, no load, IN = OS = COM
0.5V
SDL
SHDN Input Low
V
DD
- 0.5V
SDH
SHDN Input High
60PSRRPower-Supply Rejection Ratio
0.2 1I
SHDN
Shutdown Current
2.9 3.5Supply Current I
DD
4.5 5.5V
DD
Supply Voltage
V
V
dB
µA
mA
V
UNITS
SHDN Input Leakage Current V
SHDN
= 0 to V
DD
±0.1 ±10 µA
POWER REQUIREMENTS
SHUTDOWN
FILTER CHARACTERISTICS
(VDD= +5V, filter output measured at OUT, 10kΩ || 50pF load to GND at OUT, SHDN = VDD, V
COM = VOS = VDD/2, fCLK
= 100kHz, T
A
= T
MIN
to T
MAX
, unless otherwise noted. Typical values are at TA= +25°C.)
Note 1: The maximum f
C
is defined as the clock frequency f
CLK
= 100 · fCat which the peak SINAD drops to 68dB with a sinusoidal
input at 0.2f
C
.
Note 2: DC insertion gain is defined as ∆V
OUT
/ ∆VIN.
Note 3: OS voltages above V
DD
- 1V saturate the input and result in a 75µA typical input leakage current.
Note 4: f
OSC
(kHz) ≅ 53 · 103/ C
OSC
(pF).
fIN= 0.5f
C
-0.1 0.0
fIN= 2f
C
fIN= f
C
-3.5 -3.0 -2.5
Insertion Gain Relative to
DC Gain
-48 -43
fIN= 3f
C
dB
-76 -70
CONDITIONS UNITS
MIN TYP MAX
PARAMETER
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
4 _______________________________________________________________________________________
-120
-80
-100
-40
-60
0
-20
20
0 2.01.0 3.0 4.00.5 2.51.5 3.5 4.5 5.0
FREQUENCY RESPONSE
MAX7480 toc01
INPUT FREQUENCY (kHz)
GAIN (dB)
fC = 1kHz
-3.5
-2.5
-3.0
-2.0
-0.5
0
-1.0
-1.5
0.5
0 202 404 606 808 1010
PASSBAND FREQUENCY RESPONSE
MAX7480 toc02
INPUT FREQUENCY (Hz)
GAIN (dB)
fC = 1kHz
2.70
2.85
2.80
2.75
2.90
2.95
3.00
-40 200-20 40 60 80 100
SUPPLY CURRENT vs. TEMPERATURE
MAX7480 toc05
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
NO LOAD
INTERNAL OSCILLATOR FREQUENCY vs.
C
OSC
CAPACITANCE
MAX7480 toc08
CAPACITANCE (nF)
FREQUENCY (kHz)
1000
0.01
0.1
1
10
100
0.1 100 1000110
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
4.5 4.84.6 4.7 5.04.9 5.25.1 5.45.3 5.5
DC OFFSET VOLTAGE
vs. SUPPLY VOLTAGE
MAX7480-06
SUPPLY VOLTAGE (V)
DC OFFSET VOLTAGE (mV)
VIN = V
COM
0.95
0.96
0.97
0.98
0.99
1.01
1.00
1.02
1.03
1.04
1.05
4.5 4.6 4.8 4.94.7 5.0 5.1 5.2 5.3 5.4 5.5
NORMALIZED INTERNAL OSCILLATOR
FREQUENCY vs. SUPPLY VOLTAGE
MAX7480-09
SUPPLY VOLTAGE (V)
NORMALIZED OSCILLATOR FREQUENCY
C
OSC
= 530pF
640
480
560
400
160
80
240
320
0
0 400 800 1200 1600 2000
PHASE RESPONSE
MAX7480 toc03
INPUT FREQUENCY (Hz)
PHASE SHIFT (DEGREES)
fC = 1kHz
2.70
2.85
2.80
2.75
2.90
2.95
3.00
4.5 4.94.84.6 4.7 5.0 5.1 5.2 5.3 5.4 5.5
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX7480 toc04
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
NO LOAD
-1.5
-1.0
0
-0.5
0.5
1.0
-40 0-20 20 40 60 80 100
OFFSET VOLTAGE vs. TEMPERATURE
MAX7401 toc07
TEMPERATURE (°C)
OFFSET VOLTAGE (mV)
VIN = V
COM
= V
DD
/ 2
Typical Operating Characteristics
(VDD= +5V, f
CLK
= 100kHz, SHDN = VDD, V
COM
= VOS= VDD/ 2, TA= +25°C, unless otherwise noted.)
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
_______________________________________________________________________________________ 5
Typical Operating Characteristics (continued)
(VDD= +5V, f
CLK
= 100kHz, SHDN = VDD, V
COM
= VOS= VDD/ 2, TA= +25°C, unless otherwise noted.)
0.97
1
0.99
0.98
1.01
1.02
1.03
-40 200-20 40 60 80 100
NORMALIZED OSCILLATOR FREQUENCY
vs. TEMPERATURE
MAX7480 toc10
TEMPERATURE (°C)
NORMALIZED OSCILLATOR FREQUENCY
C
OSC
= 530pF
-90
-70
-80
-50
-60
-40
-30
-10
-20
0
0 1.0 1.5 2.00.5 2.5 3.0 3.5 4.0 4.5 5.0
TOTAL HARMONIC DISTORTION PLUS NOISE
vs. INPUT SIGNAL AMPLITUDE
MAX7480 toc11
AMPLITUDE (Vp-p)
THD+N (dB)
NO LOAD
(SEE TABLE A)
A
B
Table A. THD+N vs. Input Signal
Amplitude Test Conditions
22
22
MEASUREMENT
BANDWIDTH (kHz)
100
200
f
CLK
(kHz)
1200B
2400A
f
C
(kHz)
f
IN
(Hz)
TRACE
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
6 _______________________________________________________________________________________
NAME FUNCTION
1 COM
Common Input Pin. Biased internally at mid-supply. Bypass externally to GND with a 0.1µF capacitor. To
override internal biasing, drive with an external supply.
2 IN Filter Input
PIN
3 GND Ground
4 V
DD
+5V Supply Input
8 CLK
Clock Input. To override the internal oscillator, connect to an external clock; otherwise, connect an external
capacitor (C
OSC
) from CLK to GND to set the internal oscillator frequency.
7
SHDN
Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDDfor normal operation.
6 OS
Offset Adjust Input. To adjust output offset, bias OS externally. Connect OS to COM if no offset adjustment is
needed. Refer to
Offset and Common-Mode Input Adjustment
section.
5 OUT Filter Output
Pin Description
_______________Detailed Description
The MAX7480 Butterworth filter operates with a 100:1
clock-to-corner frequency ratio and a 2kHz maximum
corner frequency.
Lowpass Butterworth filters provide a maximally flat
passband response, making them ideal for instrumentation applications that require minimum deviation from
the DC gain throughout the passband.
Figure 1 shows the difference between Bessel and
Butterworth filter frequency responses. With the filter
cutoff frequencies set at 1kHz, trace A shows the
Bessel filter response and trace B shows the
Butterworth filter response.
Background Information
Most switched-capacitor filters (SCFs) are designed
with biquadratic sections. Each section implements two
filtering poles, and the sections are 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. An alternative approach is to emulate a passive network using switched-capacitor integrators with summing and scaling. Figure 2 shows a
basic 8th-order ladder filter structure.
A switched-capacitor filter such as the MAX7480 emulates a passive ladder filter. The filter’s component sensitivity is low when compared to a cascaded biquad
design, because each component affects the entire filter shape, not just one pole-zero pair. In other words, a
mismatched component in a biquad design will have a
concentrated error on its respective poles, while the
same mismatch in a ladder filter design results in an
error distributed over all poles.
-100
-60
-80
-20
-40
0
20
0.1 0.5 10.2
A
B
2510
FREQUENCY (kHz)
GAIN (dB)
A: BESSEL FILTER RESPONSE; fC = 1kHz
B: BUTTERWORTH FILTER RESPONSE; f
C
= 1kHz
Figure 1. Bessel vs. Butterworth Filter Frequency Response
L3
L5 L7
C8
R2
C4C2
V
IN
+
-
V
0
L1
R1
C6
Figure 2. 8th-Order Ladder Filter Network
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
_______________________________________________________________________________________ 7
Clock Signal
External Clock
The MAX7480 SCF is designed for use with external
clocks that have a 40% to 60% duty cycle. When using
an external clock with these devices, drive CLK with a
CMOS gate powered from 0 to VDD. Varying the rate of
the external clock adjusts the corner frequency of the
filter as follows:
fC= f
CLK
/ 100
Internal Clock
When using the internal oscillator, connect a capacitor
(C
OSC
) between CLK and ground. The value of the
capacitor determines the oscillator frequency as follows:
Minimize the stray capacitance at CLK so that it does
not affect the internal oscillator frequency. Vary the rate
of the internal oscillator to adjust 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 MAX7480’s input impedance is effectively that of a
switched-capacitor resistor, and is inversely proportional to frequency. The input impedance values determined below represent the average input impedance,
since the input current is not continuous. As a rule, use
a driver with an output impedance less than 10% of the
filter’s input impedance. Estimate the input impedance
of the filter using the following formula:
where f
CLK
= clock frequency and CIN= 2.31pF.
Low-Power Shutdown Mode
This device features a shutdown mode that is activated
by driving SHDN low. In shutdown mode, the filter’s supply current reduces to 0.2µA (typ) and its output
becomes high impedance. For normal operation, drive
SHDN high or connect to VDD.
___________Applications Information
Offset and Common-Mode
Input Adjustment
The voltage at COM sets the common-mode input voltage and is biased at mid-supply with an internal resistor-divider. Bypass COM with a 0.1µF capacitor and
connect OS to COM. For applications requiring offset
adjustment or DC level shifting, apply an external bias
voltage through a resistor-divider network to OS, as
shown in Figure 3. (Note: Do not leave OS unconnected.) The output voltage is represented by this equation:
V
OUT
= (VIN- V
COM
) + V
OS
with V
COM
= VDD/ 2 (typical), where (VIN- V
COM
) is
lowpass-filtered by the SCF and VOS is added at the
output stage. See the
Electrical Characteristics
for the
voltage range of COM and OS. Changing the voltage
on COM or OS significantly from mid-supply reduces
the filter’s dynamic range.
Power Supplies
The MAX7480 operates from a single +5V supply.
Bypass VDDto GND with a 0.1µF capacitor. If dual
supplies (±2.5V) are required, connect COM to system
ground and connect GND to the negative supply.
Figure 4 shows an example of dual-supply operation.
Single- and dual-supply performances are equivalent.
For either single- or dual-supply operation, drive CLK
and SHDN from GND (V- in dual-supply operation) to
VDD. For ±5V dual-supply applications, use the
MAX291–MAX297.
Input Signal Amplitude Range
The optimal input signal range is determined by observing the voltage level at which the total harmonic distortion plus noise (THD+N) is minimized for a given corner
frequency. The
Typical Operating Characteristics
shows a graph of the device’s THD+N response as the
input signal’s peak-to-peak amplitude is varied. This
measurement is made with OS and COM biased at midsupply.
Z
1
f C
IN
CLK IN
=
()
⋅
f (kHz)
C
; C in pF
OSC
3
OSC
OSC
=
⋅53 10
V
DD
V
SUPPLY
IN
CLK
GND
INPUT
OUTPUT
50k
50k
50k
OUT
0.1µF
0.1µF
0.1µF
CLOCK
SHDN
COM
OS
MAX7480
Figure 3. Offset Adjustment Circuit
MAX7480
8th-Order, Lowpass, Butterworth,
Switched-Capacitor Filter
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.
8
_____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Anti-Aliasing and Post-DAC Filtering
When using the MAX7480 for anti-aliasing or post-DAC
filtering, synchronize the DAC and the filter clocks. If
the clocks are not synchronized, beat frequencies may
alias into the passband.
The high clock-to-corner frequency ratio (100:1) also
eases the requirements of pre- and post-SCF filtering.
At the input, a lowpass filter prevents the aliasing of frequencies around the clock frequency into the passband. At the output, a lowpass filter attenuates the
clock feedthrough.
A high clock to corner-frequency ratio allows a simple
RC lowpass filter, with the cutoff frequency set above
the SCF corner frequency to provide input anti-aliasing
and reasonable output clock attenuation.
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
the MAX7480’s typical harmonic-distortion values with
a 10kΩ load at TA= +25°C.
V
DD
V+ = +2.5V
V- = -2.5V
IN
CLK
GND
INPUT
OUTPUTOUT
0.1µF
CLOCK
*DRIVE SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.
SHDN
COM
OS
0.1µF
MAX7480
*
V+
V-
Figure 4. Dual-Supply Operation
5th
3rd
-89-68
-93-73
200
100
f
CLK
(kHz)
4th
2nd
-85
-91
TYPICAL HARMONIC DISTORTION (dB)
-82
-89
4
2
1
MAX7480
V
IN
(Vp-p)
f
C
(kHz)
FILTER
400
200
f
IN
(Hz)
Table 1. Typical Harmonic Distortion
TRANSISTOR COUNT: 1116
Chip Information
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