Audio Precision PDM-DRVR User manual

Overview
PDM microphones typically have low power output
drivers, designed for the very short wiring connection
to a downstream circuit within a device.
Many testing situations require relatively long cable
runs between the PDM DUT and the analyzer. A
typical example is the testing of a PDM microphone
within an acoustic chamber, while the analyzer is
located outside of the chamber at some distance. The
PDM Line Driver boosts the PDM data signal from
the microphone to enable this longer cable run while
maintaining data integrity.
Using the PDM Line Driver
Place the PDM Line Driver near the PDM DUT. Con-
nect the PDM DUT to the PDM Line Driver using
three short 50 Ω coaxial cables. We recommend using
the 36 cm (14 in.) BNC to BNC cables provided with
the PDM Line Driver.
PDM Line Driver User’s Guide
The PDM Line Driver (model PDM-DRVR) enables long
cable runs between a PDM Device Under Test (DUT) and
the PDM input on an Audio Precision analyzer.
APx analyzer PDM module
PDM
line driver
PDM
microphone
Vdd line Bit clock line Data line

Connections to the PDM device under test.
Connections to the APx PDM module.
Connect the output of the PDM Line Driver to the
PDM input in the APx analyzer, using three 50 Ω
coaxial cables no longer than indicated in the chart
shown above, taking the data rate into consideration.
The cables must each be of the same length, to main-
tain synchronization between the PDM data and the
clock signal.
Cable length versus data rate
The maximum cable length is dependent upon the data
rate of the PDM signal, which is the Bit Clock rate.
At the lowest bit clock rates (below 1 MS/s), the cable
run can be up to 46 m (150 ft.) At a typical data rate
of 3.072 MS/s (baseband audio rate of 48 kS/s over-
sampled at x64), the cable run can be as long as 14 m
(45 ft.). At rates above about 11 MS/s, the recom-
mended cable length is 30 cm (12 in.) or less, and the
advantage of using the PDM Line Driver is lost. See
the chart at the top of the page.
Cable length performance predictions
• 1.57 ns/ft, propagation delay of the cable
• 5 ns, propagation delay of the cable driver at the
PDM microphone.
• 5 ns, setup time requirement for the PDM mod-
ule.
• 11.5 ns offset, PDM clock to data
• 3.072 MHz, PDM microphone bit clock rate
• 150 feet, practical maximum cable length
Block Diagram
Vdd
Bit Clock
Data
0.1uF
7SZ125
1
2
3
4
5
APX Data
APX Bit Clk
APX Vdd
Cables
To
APx PDM
(10m)
DUT Data
DUT Bit Clk
DUT Vdd
Cables
To
DUT
(<=1m)
PDM Line Driver Block Diagram.
The PDM Line Driver is powered from the Vdd sup-
plied by the APx PDM module.
5750 SW Arctic Drive
Beaverton, Oregon USA
ap.com
Copyright © 2013 Audio Precision
8211.0309 rev 000
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