STMicroelectronics Pop-free 120 mW stereo headphone amplifier DEMOTS488S DEMOTS488S Data Sheet

Doc ID 11971 Rev 5

25/32

In the flat frequency response region (with no effect from C

in

, C

out

, C

feed

), the output voltage 

is:

The gain A

V

 is:

Two capacitors are needed to properly bypass the TS488 (TS489), a power supply capacitor 
C

s

 and a bias voltage bypass capacitor C

b

.

 has a strong influence on the THD+N in the high frequency range (above 7kHz) and 

indirectly on the power supply disturbances. With 1 µF, you can expect THD+N performance 
to be similar to the one shown in the datasheet. If C

s

 is lower than 1 µF, the THD+N 

increases in the higher frequencies and disturbances on the power supply rail are less 
filtered. On the contrary, if C

s

 is higher than 1 µF, the disturbances on the power supply rail 

are more filtered.

 has an influence on the THD+N in the low frequency range. Its value is critical on the 

PSRR with grounded inputs in the lower frequencies:

■

If C

b

 is lower than 1 µF, the THD+N improves and the PSRR worsens.

■

If C

b

 is higher than 1 µF, the benefit on the THD+N and PSRR is small.

When the standby mode is activated an internal circuit of the TS488 (TS489) is charged 
(see 

). A time required to change the internal circuit is a few microseconds.

V

OUT

V

IN

R

feed

R

in

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

–

⎝

⎠

⎛

⎞

⋅

V

IN

A

V

⋅

=

=

A

V

R

feed

R

in

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

–

=

25K

25K

600K

600K

Vin1

BYPASS

Vin2

Vout1

Vout2

GND

TS488/9