SSM6322

Data Sheet

Rev. 0 | Page 16 of 20

APPLICATIONS INFORMATION

HEADPHONE DRIVERS IN MOBILE PHONES

In a headphone driver application, some high performance

audio DACs can be configured as a voltage output or a current

output. Typically, the current output configuration results in the

best THD + N performance.

For a current output configuration, implement an current to

voltage (I to V) circuit to convert the differential current signal

from the R channel and the L channel to the differential voltage

signal, followed by a difference amplifier circuit (see Figure 41).

For a voltage output configuration, the conditioning circuit is a

difference amplifier circuit, which converts the differential

signal from the R channel or L channel to a single-ended signal

(see Figure 39).

Current output audio DACs are typically used to achieve the

best THD + N performance (see Figure 41). Six amplifiers and

many passive components are required to perform current

mode signal conditioning, which consumes more PCB area and

more power. Area consumption and power consumption are

important considerations in mobile phone applications.

R

DAC

V+

V–

DIFFERENTIAL TO

SINGLE-END

VOLTAGE OUTPUT

L

V+

V–

Figure 39. Voltage Output DAC Configuration

The SSM6322 is an integrated solution for mobile phone

applications requiring low distortion and noise performance

while directly driving a low impedance load. The device also

saves more PCB area and power than the current discrete

solution.

The SSM6322 contains an additional buffer to support high

current drive capabilities. The buffer is also capable of being

configured in true high-Z mode in the audio band, which is

desirable in some portable applications for the multiplexing of

other signals on the same output port.

COMMON-MODE CONTROL CIRCUIT

The differential output stage of the DAC can be modeled as two

voltage sources, which both have the same amplitude and a 180°

voltage source (see Figure 40).

In a typical current output DAC signal chain (see Figure 41),

four amplifiers are configured as an I to V circuit. The non-

inverting inputs are connected to a dc voltage that is the output

common-mode level of the DAC, making the voltage at the I+/I−

terminals a dc signal. This signal makes the voltage drop at two

which makes the DAC achieve the best distortion performance.

In the SSM6322, the input difference amplifier performs the I to V

conversion.

SSM6322

INPUT

REF2

AUDIO

DAC

GND2

VEE2 VCC2

GAIN2

VP2

VN2

9

4

5

6

11

12

7

Figure 40. Common-Mode Circuit Without Common-Mode Control

R

DAC

I+

I–

DIFFERENTIAL TO

SINGLE-END

ITOV

CURRENT OUTPUT

L

I+

I–

Figure 41. Current Output DAC Configuration