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CMAMP110MR

Microphone Amplifier

California Micro Devices Corp 

CMAMP110

Application Information

CMAMP110 Design Considerations

1. General Overview

The CMAMP110 is very straightforward to apply, but a

few precautions are worth noting. It is recommended

that the power supply pin, V+ (pin 4), be bypassed with

a parallel combination of a 4.7µF and a 0.1µF capacitor

to minimize noise and hum pickup. In addition, a 100Ω

resistor may be inserted in series to form a RC filter

between the the power supply feed and the V+ pin to

the CMAMP110. Also, in that regard, lead dress for the

microphone inputs on the PCB should either be

shielded from or spaced from noise sources. The V−

input on the device (pin 7) should be connected to the

“clean analog ground”.

2. Passive Component Selection

Selection of the microphone input coupling capacitors

can be made using the following equation:

Where:

C

=

-----------------1-----------------

2π ⋅ RIN ⋅ fMIN

RIN = Microphone Input Impedance (40KΩ typical)

ƒMIN = Minimum Input Frequency

For example, for ƒMIN = 20Hz, C should be selected

for 0.22µF (or greater). The above equation may also

be used to select the output coupling capacitor.

3. Microphone Bias

The CMAMP110 provides a nominal 4.6 volt stable ref-

erence and 2.5KΩ resistor to bias the microphone. For

a bias current of 800µA, a 2.6 volt bias voltage results.

However, if other voltages are desired, a resistor can

be inserted in series with the microphone bias output

(pin 3) whose value, REXT , can be chosen using the

following equation:

Where:

VBMIC = 4.6 volts

RBIAS = 2.5 KΩ

VB = Desired Microphone Bias Voltage

IB = Microphone Bias Current

For example, an electret condenser microphone that

operates on 1.5V and 0.5mA, REXT should be selected

to be 3.9KΩ (nearest standard value).

4. Microphone Select

The CMAMP110 has provisions for the selection of

either the internal microphone and external micro-

phone inputs. The external microphone is selected

when the MIC_SELECT signal (pin 1) is at Logic "1" (>

3.5 volts), and the internal microphone is selected by

applying Logic "0" to the pin (< 1.5V). The switching

time is approximately 2 ms. For static applications

where only one input is used, pin 1 can simply be

strapped to either V− or V+.

5. Internal Microphone

The CMAMP110 provides for a differential (floating)

input for the Internal microphone where INT_MIC+ (pin

6) is the “non-inverting” input and INT_MIC− (pin 5) is

the “inverting” input. The CMAMP110 may be operated

single-ended by connecting a 0.22µF (or greater)

capacitor between either pin 5 or pin 6 and V−.

6. Start Up

Application of the CMAMP110 requires AC coupling

the inputs and outputs (see Passive Component Selec-

tion). Both the Internal Microphone inputs (INT_MIC+

and INT_MIC−) and the External Microphone input

(EXT_MIC_IN) are DC biased at half the supply volt-

age with an equivalent impedance of about 500KΩ. As

a result, a start up transient will occur as the capacitors

charge to half of the rail. The time for the amplifier

inputs and outputs to settle is given by:

REXT

=

V----B---M----I--C-----–----(---I--B----⋅---R----B---I--A---S--)----–----V----B--

IB

© 2004 California Micro Devices Corp. All rights reserved.

01/09/04 430 McCarthy Blvd., Milpitas, California 95035L Tel: (408) 263-3214 L Fax: (408) 263-7846 L www.calmicro.com

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