Page 13
Biasing an Electret
The 1901 board has a 3 to 3.3 Volt supply on pin 2 of PL1. If you have a 2-pin device you will
probably need a 2.2 kOhm resistor to feed this voltage to the microphone. The junction of where
the resistor joins to the microphone is where the audio is taken off. There is no need to add a
decoupling capacitor in the signal line as there is already one on the 1901 input. The microphone
screen/ground line can connect to Pin 7 of PL2 (i.e. Ground) and the signal line to Pin 9.
If the microphone is a 3-wire version then PL1 pin 2 can be connected to the microphone power
terminal and the other two connections are given above.
There are load of tutorials regarding how to feed and care for your Electret microphone on the
Internet. Using the 1901 power to bias the electret microphone will allow you to get running and try
out the Gain settings etc., but ideally the bias voltage should come from a separate regulated and
filtered source.
MEM’s Micropho
nes
These are more modern devices that are very similar to the Electret device, but because they are
produced with the diaphragm as part of the silicon amplifier, they can be made very small. They
generally have two types of output. One being an analogue signal, which may be higher (-33
dBV/Pa) than that of the Electret device and the other being a digital Pulse width signal which will
need recovering before it can be used.
Some devices even have serial data converters built in, such that they can be used directly with
other digital circuits directly via I
2
S, these cannot be directly interfaced to the 1901 module.
Like the Condenser Electret microphone, the MEM’s microphone needs a DC supply of a few micro
-
amps to power the internal circuitry.
Powering a MEM’s microphone
Ideally, the MEM’s microphone should be powered separately to reduce the risk of any noise being
picked up by the internal biasing and amplification circuits. Most MEM’s microphones require fixed
voltages of 1.8 Volts or 3.3 Volts.
Microphone Sensitivities
In the previous sections of this document, you will see that various nominal sensitivity values have
been quoted for each type of microphone. These values are all done at 1 Pa which is created with a
sound level of around 94dB SPL. To assist the user making use of a microphone a table is provided at
the end of this document which converts the dBV/Pa value into a peak to peak voltage and
recommended 1901 Gain setting that should be tried.
1901 gain setting
The NEDSP1901 module replaces the obsolete NEDSP1068 module.
The gain setting of the NEDSP1901-PCB module is set to maximum (RV1 set fully clockwise). The
Gain links on the pcb should be set to maximum (30 dB) for Dynamic, maybe a little less for an
Electret device, but may need to be set much lower to accommodate the higher output level
expected from the MEM’s module.
The output of the 1901 module has a buffer amplifier allowing it to drive up to 1 Volt (or more) into
a 1 k Ohm load. This is adjustable using the OUT trimmer (RV2).
