Dual Universal Switched-Capacitor Filters

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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

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

__________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

Q bandpass filters can be built without exceeding the

Σ

∫

∫

NA/HPA (3)

SHDN

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