lmv1024 National Semiconductor Corporation, lmv1024 Datasheet - Page 14

• Manufacturer Part Number: lmv1024
• Description: Ic Pre Amp Mic Mono Ab Adc 6usmd
• Manufacturer: National Semiconductor Corporation

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Or in decibels:Digital Output (dBFS) = Input (dBV) - Refer-
ence (dB) + A
Where,Input = 20 Log V
(V
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
put (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
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 Band-
width) relates directly the applied clock frequency. Table 1
shows the relation between the Clock Frequency and a cou-
ple of common Audio Bandwidths.
RMS
TABLE 1. Audio Bandwidth vs. Clock Frequency
P
)A is the Gain (dB)
(1.06 V
RMS
INPUT
) and the Gain A is 15.7 dB. These param-
(dBV) + 15.2
INPUT
INPUT
Audio Bandwidth
3.4 kHz
8 kHz
10 kHz
16 kHz
20 kHz
(dBV) - 0.5 + 15.7Digital Out-
(V
RMS
201334 Version 3 Revision 4
FIGURE 6. DATA Output versus Input Amplitude
)Ref = 20 Log V
REF
REF
(2)
is
Print Date/Time: 2009/07/20 17:12:49
14
Where,SP is the Sound Pressure in dB SPLC 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 + SDigital 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 out-
put produces a majority of “ones”. Figure 6 shows the result-
ing DATA output as function of the input.
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 de-
picts the noise shaping effect in a frequency spectrum plot,
where a 1 kHz signal is applied.
FIGURE 7. Frequency Spectrum
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