8th-Order, Lowpass, Bessel,

Switched-Capacitor Filters

_______________________________________________________________________________________

9

GND to the negative supply. Figure 4 shows an example

of dual-supply operation. Single- and dual-supply perfor-

mance are equivalent. For either single- or dual-supply

operation, drive CLK and SHDN from GND (V- in dual-

tions, use the MAX291–MAX297.

Input Signal Amplitude Range

The optimal input signal range is determined by observ-

ing the voltage level at which the total harmonic distor-

tion plus noise (THD+N) is minimized for a given corner

frequency. The

Typical Operating Characteristics show

graphs of the devices’ THD+N response as the input

signal’s peak-to-peak amplitude is varied. These mea-

surements are made with OS and COM biased at mid-

supply.

Anti-Aliasing and Post-DAC Filtering

When using the MAX7401/MAX7405 for anti-aliasing or

post-DAC filtering, synchronize the DAC and the filter

clocks. If the clocks are not synchronized, beat frequen-

cies may alias into the passband.

The high clock-to-corner frequency ratio (100:1) also

eases the requirements of pre- and post-SCF filtering. At

the input, a lowpass filter prevents the aliasing of fre-

quencies around the clock frequency into the passband.

At the output, a lowpass filter attenuates the clock

feedthrough.

A high clock-to-corner frequency ratio allows a simple

RC lowpass filter, with the cutoff frequency set above

the SCF corner frequency, to provide input anti-aliasing

and reasonable output clock attenuation.

Harmonic Distortion

Harmonic distortion arises from nonlinearities within the

filter. These nonlinearities generate harmonics when a

pure sine wave is applied to the filter input. Table 1 lists

the MAX7401/MAX7405’s typical harmonic-distortion

values with a 10k

V+

V-

IN

CLK

GND

INPUT

OUTPUT

OUT

0.1

µF

CLOCK

*DRIVE SHDN TO V- FOR LOW-POWER SHUTDOWN MODE.

SHDN

COM

OS

0.1

µF

MAX7401

MAX7405

*

V+

V-

Figure 4. Dual-Supply Operation

5th

3rd

-92

-79

-93

-83

500

100

(kHz)

4th

2nd

-92

-90

TYPICAL HARMONIC DISTORTION (dB)

-89

-91

4

5

1

MAX7401

(Vp-p)

(kHz)

FILTER

1000

200

(Hz)

Table 1. Typical Harmonic Distortion

TRANSISTOR COUNT: 1116

Chip Information

-88

-82

-88

-83

500

100

-88

-87

-83

-87

2

5

1

MAX7405

1000

200