A21SP16 datasheet(27/37 Pages) STMICROELECTRONICS

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A21SP16

Application information

5.1

Differential configuration principle

The A21SP16 is a monolithic fully-differential input/output class D power amplifier. The

A21SP16 also includes a common-mode feedback loop that controls the output bias value

to average it at VCC/2 for any DC common mode input voltage. This allows the device to

always have a maximum output voltage swing, and by consequence, maximizes the output

power. Moreover, as the load is connected differentially compared to a single-ended

topology, the output is four times higher for the same power supply voltage.

The advantages of a full-differential amplifier are:



High PSRR (power supply rejection ratio).



High common mode noise rejection.



Virtually zero pop without additional circuitry, giving a faster start-up time compared to

conventional single-ended input amplifiers.



Easier interfacing with differential output audio DAC.



No input coupling capacitors required due to common mode feedback loop.

The main disadvantage is:



As the differential function is directly linked to external resistor mismatching, paying

particular attention to this mismatching is mandatory in order to obtain the best

performance from the amplifier.

5.2

Gain in typical application schematic

Typical differential applications are shown in Figure 1 on page 4.

In the flat region of the frequency-response curve (no input coupling capacitor effect), the

differential gain is expressed by the relation:

with Rin expressed in k

Due to the tolerance of the internal 150 k

 feedback resistor, the differential gain will be in

the range (no tolerance on Rin):

diff

Out

–

-------------------------------

300

Rin

----------

273

Rin

----------

diff

327

Rin

----------



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