understanding PDM gain

[mjaszczykowski]

Dear Nordic,

In our application, we use nRF52840 (nRF5 SDK) and a PDM microphone to listen to the very quiet signals.

We would like to understand how exactly gain works. I could not find more information than this:
https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpdm.html

Is the gain just purely digital multiplication/division and can be applied after processing producing the same result?
Or can increased gain improve the SNR for a quiet signal? 

Best regards,
Marcin

[Kazi Afroza Sultana]

Hello,

''The gain set in the register will alter the numerical value for each sampling.'' -It's a straightforward gain stage that amplifies or attenuates the signal, which is a multiplication of the gain value with each individual sample which then results in larger or smaller represented swing.

The filter stage of each channel is followed by a digital volume control, to attenuate or amplify the output samples in a range of -20 dB to +20 dB around the  default (reset) setting, defined by GPDM,default. The gain is controlled by the GAINL and GAINR registers.

As an example, if the goal is to achieve 2500 RMS output samples (16 bit) with a 1 kHz 90 dBA signal into a -26 dBFS sensitivity PDM microphone, the user will have to sum the PDM module's default gain ( GPDM,default ) and the gain introduced by the microphone and acoustic path of his implementation (an attenuation would translate into a negative gain), and adjust GAINL and GAINR by this amount. Assuming that only the PDM module influences the gain,  GAINL and GAINR must be set to -GPDM,default dB to achieve the requirement.

With GPDM,default=3.2 dB, and as GAINL and GAINR are expressed in 0.5 dB steps, the closest value to program would be 3.0 dB, which can be calculated as:

GAINL = GAINR = (DefaultGain - (2 * 3))