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MAX292CPA Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX292CPA Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 10 page _______________Detailed Description Lowpass Butterworth filters such as the MAX291/ MAX295 provide maximally flat passband response, making them ideal for instrumentation applications that require mini- mum deviation from the DC gain throughout the passband. Lowpass Bessel filters such as the MAX292/MAX296 delay all frequency components equally, preserving the shape of step inputs, subject to the attenuation of the high- er frequencies. They also settle faster than Butterworth fil- ters. Faster settling can be important in applications that use a multiplexer (mux) to select one signal to be sent to 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 by the mux. The difference in the filters’ responses can be observed when a 3kHz square wave is applied to the filter input (Figure 1, trace A). With the filter cutoff frequencies set at 10kHz, trace C shows the MAX291/MAX295 Butterworth filter response and trace B shows the MAX292/MAX296 Bessel filter response. Since the MAX292/MAX296 have a linear phase response in the passband, all frequency components are delayed equally, which preserves the square wave. The filters attenuate higher frequencies of the input square wave, giving rise to the rounded edges at the output. The MAX291/MAX295 delay different frequen- cy components by varying times, causing the overshoot and ringing shown in trace C. The MAX291/MAX295 give more attenuation outside the passband. The phase and frequency response curves in the Typical Operating Characteristics reveal the differences between the two types of filters. MAX291/MAX292/MAX295/MAX296 phase shift and gain do not vary significantly from part to part. Typical phase shift and gain differences are less than 0.5% at the corner frequency (FC). Corner Frequency and Filter Attenuation The MAX291/MAX292 operate with a 100:1 clock to corner frequency ratio and a 25kHz maximum corner frequency, where corner frequency is defined as the point where the filter output is 3dB below the filter’s DC gain. The MAX295/MAX296 operate with a 50:1 clock to corner fre- quency ratio with a 50kHz maximum corner frequency. The 8 poles provide 48dB of attenuation per octave. Background Information Most switched-capacitor filters are designed with biqua- dratic sections. Each section implements two filtering poles, and the sections can be cascaded to produce high- er-order filters. The advantage to this approach is ease of design. However, this type of design can display poor sen- sitivity if any section’s Q is high. An alternative approach is to emulate a passive network using switched-capacitor integrators with summing and scaling. The passive network can be synthesized using CAD programs, or can be found in many filter books. Figure 2 shows the basic ladder filter structure. A switched-capacitor filter that emulates a passive ladder filter retains many of its advantages. The filter’s com- ponent sensitivity is low when compared to a cascaded biquad design because each component affects the entire filter shape, not just one pole pair. That is, a mismatched component in a biquad design will have a concentrated 8th-Order, Lowpass, Switched-Capacitor Filters 6 _______________________________________________________________________________________ _____________________Pin Description Filter Input 14 8 Inverting Input to the uncommit- ted op amp. The noninverting op amp is internally tied to ground. 6 4 Filter Output 11 5 Ground. In single-supply oper- ation, GND must be biased to the mid-supply voltage level. 12 6 Positive Supply pin. Dual sup- plies: +2.375V to +5.500V. Single supplies: +4.75V to +11.0V. 13 7 Uncommitted Op-Amp Output 5 3 Negative Supply pin. Dual supplies: -2.375V to -5.500V. Single supplies: V- = 0V. 4 2 8-PIN Clock Input. Use internal or external clock. 3 1 No Connect 1, 2, 7, 8, 9, 10, 15, 16 FUNCTION 16-PIN IN OP IN- OUT GND V+ OP OUT V- CLK N.C. NAME A B C A: 3kHz INPUT SIGNAL B: MAX292 BESSEL FILTER RESPONSE WITH Fo = 10kHz C: MAX291 BUTTERWORTH FILTER RESPONSE WITH Fo = 10kHz TIME (200 µs/div) Figure 1. Bessel vs. Butterworth Filter Responses |
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