MAX7427EUA Maxim Integrated Products, MAX7427EUA Datasheet - Page 8

• Manufacturer Part Number: MAX7427EUA
• Description: Active Filter
• Manufacturer: Maxim Integrated Products

Specifications of MAX7427EUA

• Number Of Channels: 1
• Cutoff Frequency: 12 KHz
• Supply Voltage (max): 3.6 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Operating Supply Voltage: 3 V
• Supply Voltage (min): 2.7 V
• Package / Case: uMAX-8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

5th-Order, Lowpass, Elliptic,
Switched-Capacitor Filters
8
Figure 1. 5th-Order Ladder Elliptic Filter Network
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
PIN
_______________________________________________________________________________________
1
2
3
4
5
6
7
8
+
-
V
NAME
SHDN
IN
R
COM
GND
OUT
CLK
V
S
OS
IN
DD
C1
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.
Filter Input
Ground
Positive Supply Input, +5V for MAX7426 or +3V for MAX7427
Filter Output
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.
Shutdown Input. Drive low to enable shutdown mode; drive high or connect to V
Clock Input. Connect an external capacitor (C
frequency. To override the internal oscillator, connect to an external clock.
C2
L2
Detailed Description
C3
C4
L4
C5
R
L
OSC
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.
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
frequencies above f
the gain at f
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:
The MAX7426/MAX7427 have a transition ratio of 1.25
and typically 37dB of stopband rejection.
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
FUNCTION
) from CLK to GND to set the internal oscillator
S
. The corner frequency, f
S
, the filter’s gain does not exceed
r = f
Elliptic Characteristics
S
/ f
DD
Pin Description
C
for normal operation.
C
Clock Signal
External Clock
, is defined as
S
. At