ADMP441

Data Sheet

Rev. B | Page 10 of 16

DIGITAL FILTER CHARACTERISTICS

The ADMP441 has an internal digital band-pass filter. A

high-pass filter eliminates unwanted low frequency signals.

A low-pass filter allows the user to scale the pass band with

the sampling frequency as well as perform required noise

reduction.

High-Pass Filter

The ADMP441 incorporates a high-pass filter to remove unwanted

dc and very low frequency components. Table 7 shows the high-

pass characteristics for a nominal sampling rate of 48 kHz. The

cutoff frequency scales with changes in sampling rate.

Table 7. High-Pass Filter Characteristics

Frequency

Attenuation

3.7 Hz

–3.0 dB

10.4 Hz

–0.5 dB

21.6 Hz

–0.1 dB

This digital filter response is in addition to the natural high-

pass response of the ADMP441 MEMS acoustic transducer that

has a −3 dB cutoff of 60 Hz.

Low-Pass Filter

The analog-to-digital converter in the ADMP441 is a single-bit,

high order, sigma-delta (Σ-Δ) running at a high oversampling

ratio. The noise shaping of the converter pushes the majority of

the noise well above the audio band and gives the microphone a

wide dynamic range. However, it does require a good quality

low-pass filter to eliminate the high frequency noise.

Figure 13 shows the response of this digital low-pass filter

included in the microphone. The pass band of the filter extends

48 kHz sampling rate results in a pass band of 20.3 kHz and a

half amplitude corner at 24 kHz; the stop-band attenuation of

the filter is greater than 60 dB. Note that these filter specifica-

tions scale with sampling frequency.

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

NORMALIZED FREQUENCY (

Figure 13. Digital Low-Pass Filter Magnitude Response