LMV1024_09 Datasheet, PDF (15/18 Pages) National Semiconductor (TI) – PDM Output with Pre-Amplifier for Electret Microphones

To eliminate the noise above the band of interest a low pass
decimation filter is implemented in the baseband chip or DSP.
The resulting frequency spectrum contains only the frequency
components left within the band of interest. Figure 8 depicts
the frequency spectrum after filtering.
same I/O lines (Data and Clock). To avoid overlap between
the drivers of the microphones, one microphone always goes
into a high impedance state before the second microphone
starts driving the data-line. The LMV1024 is positive edge
triggered while the LMV1026 is negative edge triggered. The
timing between the two microphones is shown in Figure 10.
For exact timing values, please see the Electrical Character-
istics table.
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FIGURE 8. Frequency Spectrum after Filtering
STEREO OPERATION
The LMV1024 and the LMV1026 are designed to operate to-
gether in a stereo solution with two microphones. One micro-
phone will have a LMV1024 built-in and the other will have a
LMV1026 built-in. These two microphones share the same
interface lines to minimize wiring (Figure 9).
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FIGURE 9. Stereo Application
Both microphones produce valid data in only one half of a
clock cycle to allow the two microphones to operate on the
FIGURE 10. Timing
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LAYOUT CONSIDERATIONS
To obtain the best possible performance from the micro-
phone, special care needs to be taken for the design of the
PCB. Especially the V_IN trace is very sensitive as it is con-
nected to the high impedance electret element. It is essential
to isolate and shield the V_IN trace as much as possible from
the digital signal traces (DATA and CLOCK). This needs to
be done to avoid any switching noise coupling directly into the
input of the IC. An example of a PCB layout is given in Figure
11. The microphone PCB has two capacitors. One capacitor
(100 nF) is connected to the reference pin of the LMV1024/
LMV1026. The other capacitor (10 nF) is used as decoupling
for high frequencies on the supply. No capacitors should be
placed on the data output of the LMV1024/LMV1026 since it
will only load the output driver and would degrade the perfor-
mance. This is opposite to the regular analog phantom biased
microphones, where capacitors are needed to improve RFI.
FIGURE 11. PCB Layout
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201334 Version 3 Revision 4 Print Date/Time: 2009/07/20 17:12:49
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