Where,

P

mum allowed input power (Full Scale). P

power on the input pin and “A” is the gain of the pre-amplifier

in decibels.

Written into voltages, the equation is:

(2)

Or in decibels:

Digital Output (dBFS) = Input (dBV) - Reference (dB) + A

Where,

Input = 20 Log V

Ref = 20 Log V

A is the Gain (dB)

For the LMV1024/LMV1026 the reference voltage V

1.5V

eters are fixed inside the device. Knowing this, Equation (2)

can be simplified:

Digital Output (dBFS) = V

Digital Output (dBFS) = V

The sensitivity of the digital microphone is the sensitivity of a

conventional microphone plus the input to output transfer of

the LMV1024. The sensitivity of a typical digital microphone

is therefore: −44 + 15.2 = −28.8 dB(FS/Pa).

Digital Output = SP +C+S

Where,

SP is the Sound Pressure in dB SPL

C is the dB SPL to dBPa conversion (−94 dB)

S is the Sensitivity in dB(V/Pa)

Taking the example of busy traffic (70 dB SPL) again results

in the following digital output (dBFS):

Digital Output (dBFS) = SP -C+S

Digital Output (dBFS) = 70 - 94 - 28.8 = −52.8 dBFS

ANALOG-TO-DIGITAL CONVERTER

The ADC used in the LMV1024/LMV1026 is an one bit sigma

delta converter with a Pulse Density Modulated output signal

(PDM). The output of this ADC can be either High (one) or

Low (zero). Assume that the LMV1024/LMV1026 input is at

the minimum level. In that case the DATA output will produce

almost only “zeros”. When the input increases, the amount of

“ones” increases too. At mid-point, where the input is 0V, the

number of “zeros” will equal the number of “ones”. At the

time that the input approaches the maximum level, the DATA

output produces a majority of “ones”. Figure 6 shows the

resulting DATA output as function of the input.

An important characteristic of the sigma delta converter is

that the noise is shifted out of the band to frequencies above

the band of interest. The band that can be used (Audio

Bandwidth) relates directly the applied clock frequency.

Table 1 shows the relation between the Clock Frequency

and a couple of common Audio Bandwidths.

TABLE 1. Audio Bandwidth vs. Clock Frequency

Clock Frequency

Audio Bandwidth

408 kHz

3.4 kHz

960 kHz

8 kHz

1.2 MHz

10 kHz

1.92 MHz

16 kHz

2.4 MHz

20 kHz

The high corner of the band of interest (knee) is determined

by the clock frequency divided by 2 times the OSR. The

factor of two comes from the Nyquist theorem. The over

sampling ratio (OSR) of this particular ADC is chosen at 60.

This sets the high corner of the band at the clock frequency

divided by 120. For instance when a bandwidth of 10 kHz is

desired, the clock frequency needs to be 1.2 MHz or higher.

Figure 7 depicts the noise shaping effect in a frequency

spectrum plot, where a 1 kHz signal is applied.

20133472

FIGURE 6. DATA Output versus Input Amplitude

www.national.com

14