Rainbow Electronics MAX9814 User Manual
Page 9
Setting the AGC Threshold
To set the output-voltage threshold at which the micro-
phone output is clamped, an external resistor-divider
must be connected from MICBIAS to ground with the out-
put of the resistor-divider applied to TH. The voltage V
TH
determines the peak output-voltage threshold at which
the output becomes clamped. The maximum signal
swing at the output is then limited to two times V
TH
and
remains at that level until the amplitude of the input signal
is reduced. To disable AGC, connect TH to MICBIAS.
Microphone Bias Resistor
MICBIAS is capable of sourcing 20mA. Select a value for
R
MICBIAS
that provides the desired bias current for the
electret microphone. A value of 2.2kΩ is usually sufficient
for a microphone of typical sensitivity. Consult the micro-
phone data sheet for the recommended bias resistor.
Bias Capacitor
The BIAS output of the MAX9814 is internally buffered
and provides a low-noise bias. Bypass BIAS with a
470nF capacitor to ground.
Input Capacitor
The input AC-coupling capacitor (C
IN
) and the input
resistance (R
IN
) to the microphone amplifier form a
highpass filter that removes any DC bias from an input
signal (see the
Typical Application Circuit/Functional
Diagram
). C
IN
prevents any DC components from the
input-signal source from appearing at the amplifier out-
puts. The -3dB point of the highpass filter, assuming
zero source impedance due to the input signal source,
is given by:
Choose C
IN
such that f
-3dB_IN
is well below the lowest
frequency of interest. Setting f
-3dB_IN
too high affects the
amplifier’s low-frequency response. Use capacitors with
low-voltage coefficient dielectrics. Aluminum electrolytic,
tantalum, or film dielectric capacitors are good choices
for AC-coupling capacitors. Capacitors with high-voltage
coefficients, such as ceramics (non-C0G dielectrics), can
result in increased distortion at low frequencies.
Output Capacitor
The output of the MAX9814 is biased at 1.23V. To elimi-
nate the DC offset, an AC-coupling capacitor (C
OUT
)
must be used. Depending on the input resistance (R
L
)
of the following stage, C
OUT
and R
L
effectively form a
highpass filter. The -3dB point of the highpass filter,
assuming zero output impedance, is given by:
Shutdown
The MAX9814 features a low-power shutdown mode.
When SHDN goes low, the supply current drops to
0.01µA, the output enters a high-impedance state, and
the bias current to the microphone is switched off.
Driving SHDN high enables the amplifier. Do not leave
SHDN unconnected.
Power-Supply Bypassing
and PCB Layout
Bypass the power supply with a 0.1µF capacitor to
ground. Reduce stray capacitance by minimizing trace
lengths and place external components as close to the
device as possible. Surface-mount components are
recommended. In systems where analog and digital
grounds are available, connect the MAX9814 to analog
ground.
f
1
2
R
C
3dB_OUT
L
OUT
−
=
Ч
Ч
π
f
1
2
R
C
3dB_IN
IN
IN
−
=
Ч
Ч
π
MAX9814
Microphone Amplifier with AGC and
Low-Noise Microphone Bias
_______________________________________________________________________________________
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