MAX291/MAX292/
MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
LE
AVAILAB
General Description
The MAX291/MAX292/MAX295/MAX296 are easy-to-use,
8th-order, lowpass, switched-capacitor filters that can be
set up with corner frequencies from 0.1Hz to 25kHz
(MAX291/MAX292) or 0.1Hz to 50kHz (MAX295/MAX296).
The MAX291/MAX295 Butterworth filters provide maximally flat passband response, and the MAX292/MAX296
Bessel filters provide low overshoot and fast settling. All
four filters have fixed responses, so the design task is
limited to selecting the clock frequency that controls the
filter’s corner frequency.
An external capacitor is used to generate a clock using
the internal oscillator, or an external clock signal can be
used. An uncommitted operational amplifier (noninverting
input grounded) is provided for building a continuoustime lowpass filter for post-filtering or anti-aliasing.
Produced in an 8-pin DIP/SO and a 16-pin wide SO
package, and requiring a minimum of external components, the MAX291 series delivers very aggressive performance from a tiny area.
Features
o 8th-Order Lowpass Filters:
Butterworth (MAX291/MAX295)
Bessel
(MAX292/MAX296)
o Clock-Tunable Corner-Frequency Range:
0.1Hz to 25kHz (MAX291/MAX292)
0.1Hz to 50kHz (MAX295/MAX296)
o No External Resistors or Capacitors Required
o Internal or External Clock
o Clock to Corner Frequency Ratio:
100:1 (MAX291/MAX292)
50:1 (MAX295/MAX296)
o Low Noise: -70dB THD + Noise (Typ)
o Operate with a Single +5V Supply or
Dual ±5V Supplies
o Uncommitted Op Amp for Anti-Aliasing or ClockNoise Filtering
o 8-Pin DIP and SO Packages
Ordering Information
Applications
ADC Anti-Aliasing Filter
Noise Analysis
DAC Post-Filtering
50Hz/60Hz Line-Noise Filtering
Functional Diagrams
Typical Operating Circuit
+5V
PART
MAX291CPA
MAX291CSA
MAX291CWE
MAX291C/D
MAX291EPA
MAX291ESA
MAX291EWE
MAX291MJA
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
-55°C to +125°C
Ordering Information continued at end of data sheet.
* Contact factory for dice specifications.
** Contact factory for availability and processing to MIL-STD-883.
Pin Configurations
7
INPUT
8
V+
IN
OUT
OP OUT
5
1
OUTPUT
TOP VIEW
3
CLK 1
MAX29_
CLOCK
8
IN
7
V+
OP OUT 3
6
GND
OP IN- 4
5
OUT
V- 2
CLK
6
PIN-PACKAGE
8 Plastic DIP
8 SO
16 Wide SO
Dice*
8 Plastic DIP
8 SO
16 Wide SO
8 CERDIP**
V-
OP IN-
4
2
Pin Configurations appear at end of data sheet.
-5V at end of data sheet.
Functional Diagrams continued
UCSP
a trademark of
Maxim
Integrated Products, Inc.
PinisConfiguration
is 8-pin
DIP/SO.
MAX29_
DIP/SO
16-pin Wide SO at end of data sheet.
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
19-4526; Rev 5; 5/10
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V+ to V-).......................................................12V
Input Voltage at Any Pin.............V- + (-0.3V) ≤ VIN ≤ V+ + (0.3V)
Continuous Power Dissipation
8-Pin Plastic DIP (derate 9.09mW/°C above +70°C) ...727mW
8-Pin SO (derate 5.88mW/°C above +70°C)................471mW
16-Pin Wide SO (derate 9.52mW/°C above +70°C) ....762mW
8-Pin CERDIP (derate 8.00mW/°C above +70°C)........640mW
Operating Temperature Ranges
MAX29_C_ _ ........................................................0°C to +70°C
MAX29_E_ _ .....................................................-40°C to +85°C
MAX29_MJA ..................................................-55°C to +125°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow) .......................................+240°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
(V+ = 5V, V- = -5V, filter output measured at OUT pin, 20kΩ load resistor to ground at OUT and OP OUT, fCLK = 100kHz
(MAX291/MAX292) or fCLK = 50kHz (MAX295/MAX296), TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
FILTER CHARACTERISTICS
Corner-Frequency Range
Clock to Corner
Frequency Ratio
Clock to Corner
Frequency Tempco
MAX291/MAX292
MAX295/MAX296
MAX291/MAX292
MAX295/MAX296
MAX291
MAX292
MAX295
MAX296
0.1-25k
0.1-50k
100:1
50:1
10
40
5
60
fIN = 0.50 Fo
MAX291
MAX292
Insertion Gain Relative to
DC Gain
MAX296
2
ppm/°C
-0.02
-0.1
-3.2
fIN = 1.00 Fo
-2.2
-2.7
fIN = 2.00 Fo
-43.0
-48.0
fIN = 3.00 Fo
-70.0
-76.0
fIN = 0.25 Fo
-0.1
-0.2
-0.3
fIN = 0.50 Fo
-0.6
-0.8
-1.0
fIN = 1.00 Fo
-2.7
-3.0
-3.3
fIN = 2.00 Fo
-11.0
-13.0
-15.0
fIN = 3.00 Fo
-30.0
-34.0
fIN = 4.00 Fo
-47.0
-51.0
fIN = 6.00 Fo
-74.0
-78.0
fIN = 0.50 Fo
MAX295
Hz
-0.02
-0.1
-3.2
fIN = 1.00 Fo
-2.2
-2.7
fIN = 2.00 Fo
-43.0
-48.0
fIN = 3.00 Fo
-70.0
-76.0
fIN = 0.25 Fo
-0.1
-0.2
-0.3
fIN = 0.50 Fo
-0.6
-0.8
-1.0
fIN = 1.00 Fo
-2.7
-3.0
-3.3
fIN = 2.00 Fo
-11.0
-13.0
-15.0
fIN = 3.00 Fo
-30.0
-34.0
fIN = 4.00 Fo
-47.0
-51.0
fIN = 6.00 Fo
-74.0
-78.0
dB
Maxim Integrated
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 5V, V- = -5V, filter output measured at OUT pin, 20kΩ load resistor to ground at OUT and OP OUT, fCLK = 100kHz
(MAX291/MAX292) or fCLK = 50kHz (MAX295/MAX296), TA = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
Output DC Swing
Output Offset Voltage
MIN
TYP
MAX
UNITS
±150
±400
V
mV
0
-0.15
dB
±4
IN = GND
DC Insertion Gain Error with
Output Offset Removed
0.15
Total Harmonic Distortion
plus Noise
TA = +25°C, fCLK = 100kHz
Clock Feedthrough
fCLK = 100kHz
-70
dB
6
mVp-p
CLOCK
Internal Oscillator
Frequency
COSC = 1000pF
Internal Oscillator
Current Source/Sink
VCLK = 0V or 5V
29
Clock Input High
(Note 1)
35
43
kHz
±70
±120
µA
4.0
Low
UNCOMMITTED OP AMP
Input Offset Voltage
Output DC Swing
Input Bias Current
V
±10
1.0
V
±50
mV
V
µA
±5.500
V
11.000
22
12
V
±4
0.05
POWER REQUIREMENTS
Supply Voltage
Dual Supply
±2.375
Single Supply
V- = 0V, GND = V±2
V+ = 5V, V- = -5V, VCLK = 0V to 5V
V+ = 2.375V, V- = -2.375V, VCLK = -2V to 2V
Supply Current
4.750
15
7
mA
Note 1. Guaranteed by design.
Typical Operating Characteristics
(V+ = 5V, V- = -5V, TA = +25°C, fCLK = 100kHz (MAX291/MAX292) or fCLK = 50kHz (MAX295/MAX296), unless otherwise noted.)
350
300
250
200
150
100
50
1.030
1.020
1.010
1.000
0.990
NORMALIZED OSCILLATOR FREQUENCY
400
1nF EXTERNAL
CAPACITOR CLK
MAX291/2/5/6-02
OSCILLATOR PERIOD (µs)
450
NORMALIZED OSCILLATOR FREQUENCY
MAX291/2/5/6-01
500
NORMALIZED INTERNAL OSCILLATOR
FREQUENCY vs. TEMPERATURE
NORMALIZED INTERNAL OSCILLATOR
FREQUENCY vs. SUPPLY VOLTAGE
1nF EXTERNAL
CAPACITOR CLK
1.06
MAX291/2/5/6-03
INTERNAL OSCILLATOR PERIOD vs.
CAPACITANCE VALUE
1.03
1.00
0.97
0.94
0
0
2
4
6
8
10
12
CAPACITANCE (nF)
Maxim Integrated
14
16
18
2.0
2.5
3.0
3.5
4.0
4.5
SUPPLY VOLTAGE (V)
5.0
5.5
-60 -40 -20
0
20 40 60
80 100 120 140
TEMPERATURE (°C)
3
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
Typical Operating Characteristics (continued)
(V+ = 5V, V- = -5V, TA = +25°C, fCLK = 100kHz (MAX291/MAX292) or fCLK = 50kHz (MAX295/MAX296), unless otherwise noted.)
MAX295
GAIN (dB)
GAIN (dB)
-0.3
-0.4
-20
-20
-40
-40
-60
Fo = 1kHz
0
GAIN (dB)
MAX291
Fo = 1kHz
0
20
MAX291/2/5/6-05
Fo = 1kHz
-0.2
20
MAX291/2/5/6-04
0
-0.1
MAX292/MAX296
FREQUENCY RESPONSE
MAX291/MAX295
FREQUENCY RESPONSE
MAX291/2/5/6-06
MAX291/MAX295
FREQUENCY RESPONSE
-60
MAX296
-80
-0.5
-80
-0.6
-100
-100
-0.7
-120
-120
MAX291
MAX292
600
800
1k
1
2
3
4
0
5
2
4
6
8
INPUT FREQUENCY (Hz)
INPUT FREQUENCY (Hz)
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX291/MAX295
FREQUENCY RESPONSE
MAX292/MAX296
FREQUENCY RESPONSE
0
14
-10
13
GAIN (dB)
12
11
10
Fo = 1kHz
9
8
7
10
0
Fo = 1kHz
-2
-20
-4
-30
-6
MAX296
MAX292
GAIN (dB)
100kHz EXTERNAL CLOCK
15
SUPPLY CURRENT I+ OR |I-|(mA)
0
MAX291/2/5/6-09
400
INPUT FREQUENCY (Hz)
MAX291/2/5/6-08
16
200
MAX291/2/5/6-07
0
MAX295
MAX291/MAX295
-40
-8
-50
-10
-60
-12
-70
-14
6
3.5
4.0
4.5
5.0
400
800
1.2k
1.6k
INPUT FREQUENCY (Hz)
SUPPLY CURRENT vs. TEMPERATURE
MAX291/MAX295
PHASE RESPONSE
100kHz EXTERNAL CLOCK
I+ OR | I- |
14
13
12
Fo = 1kHz
-160
-240
-320
MAX291
-400
0
400
800
1.2k
1.6k
2k
MAX292/296 PHASE RESPONSE
-80
PHASE SHIFT (Degrees)
15
2k
INPUT FREQUENCY (Hz)
0
MAX291/2/5/6-10
16
SUPPLY CURRENT (mA)
0
5.5
0
fo = 1kHz
-50
-100
MAX291/2/5/6-12
3.0
SUPPLY VOLTAGE, V+ OR |V-|
PHASE SHIFT (Degrees)
2.5
MAX291/2/5/6-11
2.0
-150
-200
-250
-300
-480
11
-560
-350
MAX295
10
-60 -40 -20 0
20 40 60 80 100 120 140
TEMPERATURE (°C)
4
0
400
800
1.2k
INPUT FREQUENCY (Hz)
1.6k
2k
0
400
800
1.2k
1.6k
2k
INPUT FREQUENCY (Hz)
Maxim Integrated
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
Typical Operating Characteristics (continued)
(V+ = 5V, V- = -5V, RLOAD = 5kΩ, TA = +25°C, unless otherwise noted.)
-4
FC = 2kHz
-12
-16
-16
-20
-20
-24
-24
FC = 20kHz
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
B: fCLK = 1MHz Fo = 1kHz
INPUT FREQ. = 1kHz
MEAS. BANDWIDTH = 80kHz
-55
-60
A
-65
-70
FC = 1kHz
-75
-28
-28
A: fCLK = 200kHz Fo = 2kHz
INPUT FREQ. = 200Hz
MEAS. BANDWIDTH = 30kHz
-45
THD + NOISE (dB)
-12
-40
-50
-8
FC = 20kHz
-8
GAIN (dB)
B
-80
1.0
1.1
1.2
1.4
1.3
1
1.5
2
4
3
5
6
7
8
INPUT FREQUENCY (F/FC)
AMPLITUDE (Vp-p)
MAX296 LOW-VOLTAGE PHASE RESPONSE
MAX291 LOW-FREQUENCY
PHASE RESPONSE
MAX292 THD + NOISE vs.
INPUT SIGNAL AMPLITUDE
PHASE SHIFT (Degrees)
-90
-180
-270
FC = 20kHz
-360
FC = 2kHz
FC = 20kHz
-320
-400
-540
-480
-630
-560
FC = 1kHz
1.1
1.2
1.3
1.5
1.4
1
MAX291/2/5/6-19
D
-65
-40
4
5
6
C: fCLK = 200kHz Fo = 4kHz
INPUT FREQ. = 400Hz
MEAS. BANDWIDTH = 30kHz
-45
THD + NOISE (dB)
THD + NOISE (dB)
3
8
7
AMPLITUDE (Vp-p)
-50
-60
-70
D: fCLK = 1MHz Fo = 20kHz
INPUT FREQ. = 2kHz
MEAS. BANDWIDTH = 80kHz
-55
-60
D
-65
-70
-75
-75
C
-80
1
2
3
4
5
6
7
AMPLITUDE (Vp-p)
Maxim Integrated
2
MAX296 THD + NOISE vs.
INPUT SIGNAL AMPLITUDE
D: fCLK = 1MHz Fo = 20kHz
INPUT FREQ. = 2kHz
MEAS. BANDWIDTH = 80kHz
-55
A
INPUT FREQUENCY (F/FC)
C: fCLK = 200kHz Fo = 4kHz
INPUT FREQ. = 400Hz
MEAS. BANDWIDTH = 30kHz
-50
B
-65
-70
MAX295 THD + NOISE vs.
INPUT SIGNAL AMPLITUDE
-45
10
-60
-80
INPUT FREQUENCY (F/FC)
-40
9
B: fCLK = 1MHz Fo = 1kHz
INPUT FREQ. = 1kHz
MEAS. BANDWIDTH = 80kHz
-55
-75
1.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
A: fCLK = 200kHz Fo = 2kHz
INPUT FREQ. = 200Hz
MEAS. BANDWIDTH = 30kHz
-45
-50
-240
-450
0
-40
-80
-160
10
MAX291/2/5/6-18
0
V+ = +2.5V
V- = -2.5V
0
9
MAX291/2/5/6-20
V+ = +2.5V
V- = -2.5V
MAX291/2/5/6-17
MAX291/2/5/6-16
INPUT FREQUENCY (F/FC)
THD + NOISE (dB)
GAIN (dB)
-4
PHASE SHIFT (Degrees)
V+ = +2.5V
V- = -2.5V
MAX291/2/5/6-14
0
MAX291/2/5/6-13
V+ = +2.5V
V- = -2.5V
0
MAX291 THD + NOISE vs.
INPUT SIGNAL AMPLITUDE
MAX291 LOW-VOLTAGE
FREQUENCY RESPONSE
MAX291/2/5/6-15
MAX296 LOW-VOLTAGE
FREQUENCY RESPONSE
C
-80
8
9
10
1
2
3
4
5
6
7
8
9
10
AMPLITUDE (Vp-p)
5
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
_____________________Pin Description
16-PIN
NAME
FUNCTION
1, 2, 7,
8, 9, 10,
15, 16
N.C.
No Connect
1
3
CLK
Clock Input. Use internal or
external clock.
2
4
V-
Negative Supply pin. Dual
supplies: -2.375V to -5.500V.
Single supplies: V- = 0V.
3
5
OP OUT
Uncommitted Op-Amp Output
4
6
OP IN-
Inverting Input to the uncommitted op amp. The noninverting op
amp is internally tied to ground.
5
11
OUT
Filter Output
6
12
GND
Ground. In single-supply operation, GND must be biased to
the mid-supply voltage level.
7
13
V+
Positive Supply pin. Dual supplies: +2.375V to +5.500V. Single
supplies: +4.75V to +11.0V.
8
14
IN
Filter Input
A
AMPLITUDE (5V/div)
8-PIN
B
C
TIME (200µs/div)
A: 3kHz INPUT SIGNAL
B: MAX292 BESSEL FILTER RESPONSE WITH Fo = 10kHz
C: MAX291 BUTTERWORTH FILTER RESPONSE WITH Fo = 10kHz
Figure 1. Bessel vs. Butterworth Filter Responses
The MAX291/MAX295 give more attenuation outside the
passband. The phase and frequency response curves in
the Typical Operating Characteristics reveal the differences
between the two types of filters.
MAX291/MAX292/MAX295/MAX296 phase shift and gain
do not vary significantly from part to part. Typical phase
shift and gain differences are less than 0.5% at the corner
frequency (FC).
_______________Detailed Description
Corner Frequency and Filter Attenuation
Lowpass Butterworth filters such as the MAX291/
MAX295 provide maximally flat passband response, making
them ideal for instrumentation applications that require minimum deviation from the DC gain throughout the passband.
The MAX291/MAX292 operate with a 100:1 clock to corner
frequency ratio and a 25kHz maximum corner frequency,
where corner frequency is defined as the point where the
filter output is 3dB below the filter’s DC gain. The
MAX295/MAX296 operate with a 50:1 clock to corner frequency ratio with a 50kHz maximum corner frequency.
The 8 poles provide 48dB of attenuation per octave.
Lowpass Bessel filters such as the MAX292/MAX296
delay all frequency components equally, preserving the
shape of step inputs, subject to the attenuation of the higher frequencies. They also settle faster than Butterworth filters. Faster settling can be important in applications that
use a multiplexer (mux) to select one signal to be sent to
an analog-to-digital converter (ADC)—an anti-aliasing filter
placed between the mux and the ADC must settle quickly
after a new channel is selected by the mux.
The difference in the filters’ responses can be observed
when a 3kHz square wave is applied to the filter input
(Figure 1, trace A). With the filter cutoff frequencies set at
10kHz, trace C shows the MAX291/MAX295 Butterworth
filter response and trace B shows the MAX292/MAX296
Bessel filter response. Since the MAX292/MAX296 have a
linear phase response in the passband, all frequency
components are delayed equally, which preserves the
square wave. The filters attenuate higher frequencies of
the input square wave, giving rise to the rounded edges at
the output. The MAX291/MAX295 delay different frequency components by varying times, causing the overshoot
and ringing shown in trace C.
6
Background Information
Most switched-capacitor filters are designed with biquadratic sections. Each section implements two filtering
poles, 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 can display poor sensitivity if any section’s Q is high.
An alternative approach is to emulate a passive network
using switched-capacitor integrators with summing and
scaling. The passive network can be synthesized using
CAD programs, or can be found in many filter books.
Figure 2 shows the basic ladder filter structure.
A switched-capacitor filter that emulates a passive ladder
filter retains many of its advantages. 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 pair. That is, a mismatched
component in a biquad design will have a concentrated
Maxim Integrated
MAX291/MAX292/MAX295/MAX296
8th-Order, Lowpass,
Switched-Capacitor Filters
+5V
7
R1
L1
L3
L5
L7
+5V
1
3
0V
C2
C4
C6
C8
4
R2
VIN
VO
+1V TO +4V
INPUT SIGNAL
RANGE
8
V+
CLK
OUT
OP OUT
GND
IN
OUTPUT
0.1µF
MAX29_
OP IN-
5
10k
6
V-
10k
2
0.1µF
0V
Pin Configuration is 8-pin DIP.
Figure 2. 8th-Order Ladder Filter Network
Figure 3. +5V Single-Supply Operation
error on its respective poles, while the same mismatch in a
ladder filter design will spread its error over all poles.
The MAX291/MAX292/MAX295/MAX296 input impedance
is effectively that of a switched-capacitor resistor (see
equation below, and Table 1), and it is inversely proportional to frequency. The input impedance values determined
below represent average input impedance, since the input
current is not continuous. The input current flows in a series
of pulses that charge the input capacitor every time the
appropriate switch is closed. A good rule of thumb is that
the driver’s input source resistance should be less than
10% of the filter’s input impedance. The input impedance
of the filter can be estimated using the following formula:
Z = 1 / (fCLK * C)
where: fCLK = Clock Frequency
The input impedance for various clock frequencies is
given below:
clock frequency over the clock range 100kHz to 1MHz.
Varying the rate of an external clock will dynamically adjust the corner frequency of the filter.
Ideally, the MAX291/MAX292/MAX295/MAX296 should
be clocked symmetrically (50% duty cycle). MAX291/
MAX292/MAX295/MAX296 can be operated with clock
asymmetry of up to 60/40% (or 40/60%) if the clock
remains HIGH and LOW for at least 200ns. For example,
if the part has a maximum clock rate of 2.5MHz, then the
clock should be high for at least 200ns, and low for at
least 200ns.
Table 1. Input Impedance for Various Clock
Frequencies
PART
C (pF)
10kHz
(MΩ)
100kHz
(MΩ)
1000kHz
(kΩ)
MAX291
MAX292
MAX295
MAX296
2.24
3.28
4.47
4.22
44.6
30.5
22.4
23.7
4.46
3.05
2.24
2.37
446
305
224
237
Clock-Signal Requirements
The MAX291/MAX292/MAX295/MAX296 maximum recommended clock frequency is 2.5MHz, producing a cutoff
frequency of 25kHz for the MAX291/MAX292 and 50kHz
for the MAX295/MAX296. The CLK pin can be driven by
an external clock or by the internal oscillator with an external capacitor. For external clock applications, the clock
circuitry has been designed to interface with +5V CMOS
logic. Drive the CLK pin with a CMOS gate powered from
0V and +5V when using either a single +5V supply or dual
+5V supplies. The MAX291/MAX292/MAX295/MAX296
supply current increases slightly (
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