MAX7418 Datasheet, PDF (9/14 Pages) Maxim Integrated Products – 5th-Order, Lowpass, Switched-Capacitor Filters

5th-Order, Lowpass,
Switched-Capacitor Filters
Typical Operating Characteristics (continued)
(V_DD = +5V for MAX7418–MAX7421, V_DD = +3V for MAX7422–MAX7425, f_CLK = 2.2MHz, SHDN = V_DD, V_COM = V_OS = V_DD / 2,
T_A = +25°C, unless otherwise noted.
DC OFFSET VOLTAGE
vs. TEMPERATURE
3.0
DC OFFSET VOLTAGE
vs. SUPPLY VOLTAGE
2.5
2.5
2.0
2.0
V_DD = 5V
1.5
1.5
1.0
1.0
V_DD = 3V
0.5
0.5
0
-40 -15
10
35
60
85
TEMPERATURE (°C)
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5
SUPPLY VOLTAGE (V)
Pin Description
PIN
NAME
FUNCTION
1
COM
Common Input Pin. Biased internally at midsupply. Bypass COM externally to GND with a 0.1µF capacitor.
To override internal biasing, drive COM with an external supply.
2
IN
Filter Input
3
GND Ground
4
V_DD
Positive Supply Input: +5V for MAX7418–MAX7421, +3V for MAX7422–MAX7425. Bypass V_DD to GND with
a 0.1µF capacitor.
5
OUT Filter Output
Offset Adjust Input. To adjust output offset, connect OS to an external supply through a resistive voltage-
6
OS
divider (Figure 4). Connect OS to COM if no offset adjustment is needed. See the Offset and Common-Mode
Input Adjustment section.
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 (C_OSC) from CLK to ground. To override the internal oscillator,
connect CLK to an external clock: f_C = f_CLK /100.
_______________Detailed Description
The MAX7418/MAX7421/MAX7422/MAX7425 elliptic
lowpass filters provide sharp rolloff with good stopband
rejection. The MAX7419/MAX7423 Bessel filters provide
low overshoot and fast settling responses, and the
MAX7420/MAX7424 Butterworth filters provide a maxi-
mally flat passband response. All parts operate with a
100:1 clock-to-corner frequency ratio.
Most switch 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 MAX7418–MAX7425 use an
alternative approach, which is to emulate a passive net-
work using switched-capacitor integrators with sum-
ming 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 lad-
der filter structure.
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