Rainbow Electronics MAX7427 - Manual (Page 8)

MAX7426/MAX7427
5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
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Detailed Description
The MAX7426/MAX7427 family of 5th-order, elliptic,
lowpass filters provides sharp rolloff with good stop-
band rejection. All parts operate with a 100:1 clock-to-
corner frequency ratio.
Most SCFs are designed with biquadratic sections.
Each section implements two pole-zero pairs, and the
sections can be cascaded to produce higher-order fil-
ters. The advantage to this approach is ease of design.
However, this type of design is highly sensitive to com-
ponent variations if any section’s Q is high. The
MAX7426/MAX7427 use an alternative approach, which
is to emulate a passive network using switched-capaci-
tor 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 lad-
der filter retains many of the same advantages. The
component sensitivity of a passive ladder filter is low
when compared to a cascaded biquadratic design,
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 MAX7426/MAX7427
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 allow for the sharp attenuation characteristic of
elliptic filters, making these devices ideal for anti-alias-
ing and post-DAC filtering in single-supply systems
(see the Anti-Aliasing and Post-DAC Filtering section).
In the frequency domain (Figure 2), the first transmis-
sion zero causes the filter’s amplitude to drop to a mini-
mum level. Beyond this zero, the response rises as the
frequency increases until the next transmission zero.
The stopband begins at the stopband frequency, f
S . At
frequencies above f S , the filter’s gain does not exceed
the gain at f S . The corner frequency, f C , 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 = f S / f C
The MAX7426/MAX7427 have a transition ratio of 1.25
and typically 37dB of stopband rejection.
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
C4 C2
L4
C5 C3 C1 V IN
+
-
R L
L2 R S
Figure 1. 5th-Order Ladder Elliptic Filter Network
NAME FUNCTION
1 COM
Common Input Pin. Biased internally at midsupply. 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 Positive Supply Input, +5V for MAX7426 or +3V for MAX7427
8 CLK
Clock Input. Connect an external capacitor (C OSC ) from CLK to GND to set the internal oscillator
frequency. To override the internal oscillator, connect to an external clock.
7 SHDN Shutdown Input. Drive low to enable shutdown mode; drive high or connect to V
DD
for normal operation.
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.
5 OUT Filter Output
Pin Description