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MAX7491CEE Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX7491CEE Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 18 page Dual Universal Switched-Capacitor Filters _______________________________________________________________________________________ 9 2nd-Order Filter Stage The MAX7490/MAX7491 are dual biquad filters. The biquad topology allows the use of standard filter tables and equations to implement simultaneous lowpass, bandpass, and notch or highpass filters. Topologies such as Butterworth, Chebyshev, Bessel, elliptic, as well as custom algorithms are possible. Internal Common Voltage The COM pin sets the common-mode input voltage and is internally biased to VDD/2 with a resistor-divider. The resistors used are typically 250k Ω for the MAX7490, and typically 80k Ω for the MAX7491. The common- mode voltage is easily overdriven by an external volt- age supply if desired. The COM pin should be bypassed to the analog ground with at least a 0.1µF capacitor. Inverting Inputs Locate resistors that are connected to INV_ as close as possible to INV_ to reduce stray capacitance and noise pickup. INV_ are inverting inputs to continuous-time op amps, and behave like a virtual ground. There is no sampling energy present on these inputs. Outputs Each switched-capacitor section, together with two to four external resistors, can generate all standard 2nd- order functions: bandpass, lowpass, highpass, and notch (band-reject) functions. Three of these functions are simultaneously available. The maximum signal swing is limited by the power-supply voltages used. The amplifiers’ outputs in the MAX7490/MAX7491 are able to swing to within approximately 0.2V of either supply. Driving coaxial cable, large capacitive loads, or total resistive loads less than 10k Ω will degrade the total harmonic distortion (THD) performance. Note that the effective resistive load at the output must include both the feedback resistors and any external load resistors. Low-Power Shutdown Mode The MAX7490/MAX7491 have a shutdown mode that is activated by driving SHDN low. In shutdown mode, the filter supply current reduces to <1µA (max), and the fil- ter outputs become high impedance. The COM input also becomes high impedance during shutdown. For normal operation, drive SHDN high or connect to VDD. __________Applications Information Designing with the MAX7490/MAX7491 begins by selecting the mode that best fits the desired circuit requirements. Table 1 lists the available modes and their relative advantages and disadvantages. Table 2 lists the different nomenclature used in the explanations that follow. Mode 1 Figure 2 shows the MAX7490/MAX7491s’ configuration of Mode 1. This mode provides 2nd-order notch, low- pass, and bandpass filter functions. The gain at all three outputs is inversely proportional to the value of R1. The center frequency, fO, is fixed at fCLK/100. High- Q bandpass filters can be built without exceeding the bandpass amplifier’s output swing (i.e., HOBP does not Σ ∫ ∫ NA/HPA (3) SHDN VDD (8) GND (7) CLK (9) EXTCLK (10) INVB (13) COM (11) R R INVA (4) (6) SA (5) BPA (2) LPA (1) + - Σ ∫ ∫ NB/HPB (14) SB (12) BPB (15) LPB (16) + - Figure 1. Block Diagram |
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