Datasheet MAX7409, MAX7410, MAX7413, MAX7414 (Maxim) - 8

5th-Order, Lowpass, Switched-Capacitor Filters
RS L2 L4 + 2V/div V C1 IN C3 C5 R - L A 2V/div B Figure 2. 5th-Order Ladder Filter Network 2V/div C
Clock Signal External Clock
200µs/div The MAX7409/MAX7410/MAX7413/MAX7414 family of A: 1kHz INPUT SIGNAL SCFs is designed for use with external clocks that have B: MAX7409 BESSEL FILTER RESPONSE; fC = 5kHz a 50% ±10% duty cycle. When using an external clock C: MAX7410 BUTTERWORTH FILTER RESPONSE; fC = 5kHz with these devices, drive CLK with a CMOS gate pow- ered from 0 to VDD. Varying the rate of the external Figure 1. Bessel vs. Butterworth Filter Response clock adjusts the corner frequency of the filter as fol- lows: The difference between Bessel and Butterworth filters fC = fCLK / 100 can be observed when a 1kHz square wave is applied
Internal Clock
to the filter input (Figure 1, trace A). With the filter cutoff When using the internal oscillator, connect a capacitor frequencies set at 5kHz, trace B shows the Bessel filter (C response and trace C shows the Butterworth filter OSC) between CLK and ground. The value of the capacitor determines the oscillator frequency as follows: response.
Background Information
fOSC (kHz) = 30 x 103/ COSC (pF) Most switched-capacitor filters (SCFs) are designed with Minimize the stray capacitance at CLK so that it does biquadratic sections. Each section implements two filter- not affect the internal oscillator frequency. Vary the rate ing poles, and the sections are cascaded to produce of the internal oscillator to adjust the filter’s corner fre- higher-order filters. The advantage to this approach is quency by a 100:1 clock-to-corner frequency ratio. For ease of design. However, this type of design is highly example, an internal oscillator frequency of 100kHz sensitive to component variations if any section’s Q is produces a nominal corner frequency of 1kHz. high. An alternative approach is to emulate a passive net-
MAX7409/MAX7410/MAX7413/MAX7414
work using switched-capacitor integrators with summing
Input Impedance vs. Clock Frequencies
and scaling. Figure 2 shows a basic 5th-order ladder filter The MAX7409/MAX7410/MAX7413/MAX7414’s input structure. impedance is effectively that of a switched-capacitor A switched-capacitor filter such as the MAX7409/ resistor (see the following equation), and is inversely MAX7410/MAX7413/MAX7414 emulates a passive ladder proportional to frequency. The input impedance values filter. The filter’s component sensitivity is low when com- determined below represent the average input imped- pared to a cascaded biquad design, because each ance, since the input current is not continuous. As a component affects the entire filter shape, not just one rule, use a driver with an output impedance less than pole-zero pair. In other words, a mismatched component 10% of the filter’s input impedance. Estimate the input in a biquad design will have a concentrated error on its impedance of the filter using the following formula: respective poles, while the same mismatch in a ladder Z filter design results in an error distributed over all poles. IN = 1 / ( fCLK x 2.1pF) For example, an fCLK of 100kHz results in an input impedance of 4.8MΩ.
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