Data Sheet

SSM6322

Rev. 0 | Page 19 of 20

DESIGN GUIDELINES

The performance of the SSM6322 is such that any minor

external interference can destroy the circuit. When using this

device, consider the following:



The sensing ground of the input stage is sensitive to

external interference. In the PCB layout, it is recommended

to refer the sensing ground to the output interface ground

(in high fidelity headphone driver applications, the output

interface is the jack). As shown in Figure 48, the dashed

outline enclosed ground is the input stage sensing ground,

which must be routed directly to the ground of the jack.

Note that Figure 48 only shows one channel; for the other

channel, route the sensing ground to the jack ground

separately.



The SSM6322 circuit is different with a typical current output

DAC signal chain (see Figure 41); there is only one op amp

that performs the differential I to V conversion. The power

across the noninverting grounded resistor is fixed, but the

power across the feedback resistor varies with the output

signal. This variability creates a mismatch between the two

resistors, as well as distortion if the heat cannot be well

dissipated. Low drift (25 ppm/°C) metal film or thin film

resistors are suggested to avoid this situation (see Figure 46).



If there is a resistor between the final output and the

headphone, the resistor must be low drift (25 ppm/°C) and

metal film or thin film to avoid distortion when driving

heavy loads.



Use a low dropout regulator (LDO) as the power supply.

Place the decoupling capacitors (0.1 μF and 4.7 μF) near

the amplifier power pins. If there is switching power on the

board, keep the switching power circuit and return path far

away from the SSM6322 circuit.



For better heat dissipation, solder the exposed pad of the

LFCSP package to the board pad and, using vias, connect

the exposed pad to a large, solid copper plane at the

opposite side of the board. The copper plane can be

connected to the negative supply plane or ground plane.



Shielding is important in mobile phone applications. To

reach <−100 dB THD + N specifications, even small

interferences can degrade THD + N performance,

particularly when listening to music and browsing the

internet simultaneously. Metal shielding helps prevent

performance degradation.



The maximum input filter capacitor values are 2.2 nF.

1µF

AUDIO

DAC

1nF

1nF

1nF

1kΩ

51kΩ

1kΩ

VOUT2

SD2

OUTPUT

INPUT

GND3

FILT2

GAIN2

VEE2

VP2

GND2

REF2

VN2

VCC2

499Ω

15

13

14

8

7

12

11

6

4

9

5

10

SSM6322

VNFB2

Figure 48. Sensing Ground of Input Stage