MP34DT05-A: The input clock frequency range is 1.2MHz - 3.25MHz. What is the likely impact if the clock is outside of this range? Will the SNR or sensitivity be affected? Thank you

[PBenn.1]

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[Eleon BORLINI]

Hi @PBenn.1​ , the input clock frequency directly impact the SNR value. This because the SNR is typically defined as the ratio between the 1kHz 94dBSPL (sinusoidal) acoustic signal and the A-weighted noise integrated in a predefined band. The typical integration band is the audio bandwidth, from 20Hz to 20kHz. MEMS digital microphone (not only from ST) are most times PDM microphone, that is the mic output is a 1-bit-pulse density modulated stream. This PDM is composed by audio signal plus oversampling noise above the audio band, and the "zero" of the spectrum (i.e. the frequency from which the noise starts to increase) is directly related to the input clock frequency: the higher the clock frequency, the higher the pole frequency (so 3.25MHz, For this reason, when the PDM signal is reconstructed to obtain a PCM (decimation procedure, you can simplify it assuming to use a low pass filter at 20kHz), the oversampling noise can fall into the noise integration band and the noise can increase, worsening the SNR parameter. Sensitivity is almost the same across the 1.2MHz - 3.25MHz clock range. Typical SNR vs clock values (from digital microphone characterization data) are the following ones:

Regards

[PBenn.1]

Hi Eleon

Thanks for the quick and thoughtful reply. If the input clock frequency were lower than the specified frequency range, would the pole frequency be lowered? And would the upper end of the audio frequency range be lowered as well?

Thanks again Eleon

[Eleon BORLINI]

Hi @PBenn.1​ , the mic behavior outside that frequency range is not specified in the datasheet (so technically is not suggested...) but ok, the microphone is alive also outside that range. Other digital microphones has a "low power mode" centered around 768kHz. Although not declared, also MP34DT05-A can work well around that frequency (but not below 500kHz): in this range the SNR must be calculated in a narrower band (typically 20Hz-8kHz) and is near 62.5 dB(A). Unfortunately there is an ill-working range around 1MHz (a typical audio frequency is 1.024MHz), were the threshold between low power and normal mode is. Which is your desired operating frequency? Regards

[PBenn.1]

Hi Eleon

Again, thanks for the quick and thoughtful reply. I noticed the "low power mode" that you mentioned on the MP23DB02MM from ST Micro.

Can you describe the ill-working range around 1 MHz with more details? The frequency that we are considering is 1.024 MHz so I am very interested to know more about that. Thanks Eleon!

[Eleon BORLINI]

Hi @PBenn.1​ , well, it depends on the threshold frequency value of the specific device (this parameter cannot be tuned during process or production test). It is typically around 900kHz +-50kHz, but there is a non-zero (even if low) eventuality to have some issues at 1MHz... I cannot guarantee, but there is a good probability you should not encounter risks for clock frequency 1.024MHz. Regards

[PBenn.1]

Thanks Eleon - I appreciate your help :thumbs_up: :thumbs_up: