8th-Order, Lowpass, Elliptic,

Switched-Capacitor Filters

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11

with VCOM = VDD / 2 (typical), and where (VIN - VCOM)

is lowpass filtered by the SCF, and VOS is added at the

output stage. See the

Electrical Characteristics for

COM and OS input voltage ranges. Changing the volt-

age on COM or OS significantly from midsupply

reduces the filter’s dynamic range.

Power Supplies

The MAX7400/MAX7403 operate from a single +5V

supply. The MAX7404/MAX7407 operate from a single

+3V supply. Bypass VDD to GND with a 0.1µF capaci-

tor. If dual supplies are required, connect COM to the

system ground and GND to the negative supply. Figure

5 shows an example of dual-supply operation. Single-

supply and dual-supply performance are equivalent.

For single-supply or dual-supply operation, drive CLK

and SHDN from GND (V- in dual-supply operation) to

VDD. For a ±2.5V supply, use the MAX7400 or MAX7403;

for a ±1.5V supply, use MAX7404 or MAX7407. For ±5V

dual-supply applications, use the MAX291–MAX297.

Input Signal Amplitude Range

The ideal input signal range is determined by observ-

ing the voltage level at which the total harmonic

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

corner frequency. The

Typical Operating Character-

istics show THD+N response as the input signal’s

peak-to-peak amplitude is varied. These measurements

are made with OS and COM biased at midsupply.

Anti-Aliasing and Post-DAC Filtering

When using the MAX7400/MAX7403/MAX7404/

MAX7407 for anti-aliasing or post-DAC filtering, syn-

chronize the DAC and the filter clocks. If the clocks are

not synchronized, beat frequencies 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 pass-

band. 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. Such nonlinearities generate harmonics when a

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

typical harmonic distortion values with a 10k

Ω load and

an input signal of 4Vp-p (MAX7400/MAX7403) or 2Vp-p

(MAX7404/MAX7407), at TA = +25°C.

VDD

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

MAX7400

MAX7403

MAX7404

MAX7407

*

V+

V-

Figure 5. Dual-Supply Operation

2nd

-88

100

TYPICAL

HARMONIC

DISTORTION (dB)

1

fC

(kHz)

-89

100

200

1

-84

fIN

(Hz)

200

4th

3rd

5th

-89

500

MAX7400

5

1000

-86

-89

-82

-88

-93

-77

500

MAX7403

5

1000

-87

-91

FILTER

-81

-91

-90

-80

4

VIN

(Vp-p)

4

fCLK

(kHz)

-85

100

1

200

-85

500

MAX7404

5

1000

-86

-85

-82

-84

-86

-81

2

-85

100

1

200

-86

500

MAX7407

5

1000

-86

-85

-82

-86

-85

-84

2

Table 1. Typical Harmonic Distortion