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MAX7412 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX7412 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page 5th-Order, Lowpass, Elliptic, Switched-Capacitor Filters 8 _______________________________________________________________________________________ Detailed Description The MAX7408/MAX7411/MAX7412/MAX7415 family of 5th-order, elliptic, lowpass filters provides sharp rolloff with good stopband rejection. All parts operate with a 100:1 clock-to-corner frequency ratio and a 15kHz maximum corner frequency. Most switched-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 MAX7408/MAX7411/ MAX7412/MAX7415 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 MAX7408/MAX7411/ MAX7412/MAX7415 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 allows for the sharp attenuation characteristic of elliptic filters, making these devices ideal for anti-aliasing and post-DAC filtering in single-supply systems (see the Anti-Aliasing and Post- DAC Filtering section). In the frequency domain, the first transmission zero causes the filter’s amplitude to drop to a minimum level. Beyond this zero, the response rises as the frequency increases until the next transmission zero. The stop- band begins at the stopband frequency, fS. At frequen- cies above fS, the filter’s gain does not exceed the gain at fS. The corner frequency, fC, 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 fre- quency: r = fS / fC The MAX7408/MAX7412 have a translation ratio of 1.6 and typically 53dB of stopband rejection. The MAX7411/MAX7415 have a transition ratio of 1.25 (pro- viding a steeper rolloff) and typically 37dB of stopband rejection. C4 C2 L4 C5 C3 C1 VIN + - RL L2 RS Figure 1. 5th-Order Ladder Elliptic Filter Network NAME FUNCTION 1 COM Common Input Pin. Biased internally at mid-supply. Bypass externally to GND with 0.1µF capacitor. To override internal biasing, drive with an external supply. 2 IN Filter Input PIN 3 GND Ground 4 VDD Positive Supply Input, +5V for MAX7408/MAX7411 or +3V for MAX7412/MAX7415 8 CLK Clock Input. Connect an external capacitor (COSC) 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 VDD 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 |
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