MAX7401 Ver la hoja de datos (PDF) - Maxim Integrated
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Lista de partido
MAX7401 Datasheet PDF : 12 Pages
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8th-Order, Lowpass, Bessel,
Switched-Capacitor Filters
Pin Description
PIN NAME
FUNCTION
1
COM
Common Input. Biased internally at mid-supply. Bypass externally to GND with a 0.1µF capacitor. To over-
ride internal biasing, drive with an external supply.
2
IN
Filter Input
3
GND Ground
4
VDD
Positive Supply Input: +5V for MAX7401, +3V for MAX7405
5
OUT Filter Output
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.
7
SHDN Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDD for normal operation.
8
CLK
Clock Input. To override the internal oscillator, connect to an external clock; otherwise, connect an external
capacitor (COSC) from CLK to GND to set the internal oscillator frequency.
_______________Detailed Description
The MAX7401/MAX7405 Bessel filters provide low over-
shoot and fast settling responses. Both parts operate
with a 100:1 clock-to-corner frequency ratio and a 5kHz
maximum corner frequency.
Lowpass Bessel filters such as the MAX7401/MAX7405
delay all frequency components equally, preserving the
shape of step inputs (subject to the attenuation of the
higher frequencies). Bessel filters settle quickly—an
important characteristic in applications that use a multi-
plexer (mux) to select an input signal for 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.
Figure 1 shows the difference between Bessel and
Butterworth filters when a 1kHz square wave is applied
to the filter input. With the filter cutoff frequencies set at
5kHz, trace B shows the Bessel filter response and
trace C shows the Butterworth filter response.
Background Information
Most switched-capacitor filters (SCFs) are designed with
biquadratic sections. Each section implements two filter-
ing 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 net-
work using switched-capacitor integrators with summing
and scaling. Figure 2 shows a basic 8th-order ladder filter
structure.
A
2V/div
B
2V/div
C
2V/div
200µs/div
A: 1kHz INPUT SIGNAL
B: BESSEL FILTER RESPONSE; fC = 5kHz
C: BUTTERWORTH FILTER RESPONSE; fC = 5kHz
Figure 1. Bessel vs. Butterworth Filter Response
R1
+
- VIN
L1
C2
L3
C4
L5
C6
L7
C8
R2 V0
Figure 2. 8th-Order Ladder Filter Network
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