There is no commonly accepted unit of measurement to express the instantaneous level of a digital signal output from the
microphone, as opposed to the RMS level of the signal. Some measurement systems express the instantaneous level of an individual
sample in units of D, where 1.0 D is digital full scale (see Figure 11). In this case, a −26 dBFS sine wave has peaks at 0.05 D.
For more information about digital microphone sensitivity, see the AN-1112 Application Note, Microphone Specifications Explained.
SYNCHRONIZING MICROPHONES
Stereo INMP441 microphones are synchronized by the WS signal, so audio captured from two microphones sharing the same clock
will be in sync. If the mics are enabled separately, this synchronization may take up to 0.35 ms after the enable signal is asserted
while internal data paths are flushed.
DIGITAL FILTER CHARACTERISTICS
The INMP441 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 INMP441 incorporates a high-pass filter to remove unwanted DC and very low frequency components. This 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 ATTENTUATION
3.7 Hz −3 dB
10.4 Hz −0.5 dB
This digital filter response is in addition to the natural high-pass response of the INMP441 MEMS acoustic transducer that has a -3 dB
cutoff of 60 Hz.
LOW-PASS FILTER
The analog-to-digital converter in the INMP441 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 12 shows the response of this digital low-pass filter included in the microphone. The pass band of the filter extends to 0.423 ×
f
S
and, in that band, has an unnoticeable 0.04 dB of ripple. The high-frequency cutoff of −6 dB occurs at 0.5 × f
S
. A 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 specifications scale with sampling frequency.
Page 13 of 21
Document Number: DS-INMP441-00
Revision: 1.1
