SSM6322

Data Sheet

Rev. 0 | Page 18 of 20

SSM6322 IN A HEADPHONE DRIVER APPLICATION

SSM6322 Circuit with Current Output DAC

For an audio DAC with a differential current output, two gain

resistors convert the current to voltage (see Figure 46). The

resistor value is determined by the DAC output full-scale

current and the input stage output range (the output range is

±3 V at a ±5 V supply). Assuming that the DAC single-ended

output current is ±1.5 mA, and that the differential current is

±3 mA, the output of the input stage is ±3 V when using two

1 kΩ gain resistors. The feedback capacitors, in parallel with the

gain resistors, form a single-pole, low-pass filter. The SSM6322

can handle up to a 1 kΩ and 2.2 nF resistor capacitor combination.

Typically, audio DACs generate a dc offset current, which is

converted to an input common-mode voltage at the input of the

SSM6322. The REF1 and REF2 pins of the SSM6322 set the

input common-mode voltage of each channel. The voltage at

the REF1 and REF2 pins is achieved by an internal 15 μA

current source and an external resistor; a 51 kΩ resistor is

suggested to achieve a 0.765 V voltage. A 1 μF capacitor can be

used in parallel with the resistor to remove noise.

A 499 Ω resistor and 1 nF capacitor can be added between the

input stage and output stage for a second single-pole, low-pass

filter, as shown in Figure 46.

For better gain matching and better distortion performance,

all 1 kΩ and 499 Ω resistors must to be of 0.1% tolerance and a

25 ppm/°C temperature coefficient. The 1 nF capacitors must be

NP0 capacitors. There are no specific requirements for the

51 kΩ resistor and the 1 μF capacitor at REF1 and REF2.

SSM6322 Circuit with Voltage Output DAC

For audio DACs that output a differential voltage, four gain

resistors convert the differential voltage to single-ended voltage

(see Figure 47). The feedback capacitors must be in parallel with

the gain resistors to form the single-pole, low-pass filter. As shown

in Figure 47, four 1 kΩ resistors and two 1 nF capacitors are

used to achieve a gain of 1 and a first-order, 159 kHz cutoff

frequency low-pass filter.

For REF1 and REF2, refer to the DAC data sheet for the

common-mode voltage; then, calculate the resistor value at

REF1 and REF2. As shown in Figure 47, a 51 kΩ resistor is

suggested to obtain a 0.765 V voltage.

A 499 Ω resistor and 1 nF capacitor can be added between the

input stage and output stage for a second single-pole, low-pass

filter, as shown in Figure 47.

For better gain matching and better distortion performance, all

1 kΩ and 499 kΩ resistors must be of a 0.1% tolerance and

25ppm/°C temperature coefficent; the 1 nF capacitor must be

NP0 capacitor.

There are no specific requirement for the 51 kΩ resistor and

1 μF capacitor at REF1 and REF2.

1µF

AUDIO

DAC

1nF

1nF

1nF

1kΩ

51kΩ

1kΩ

VOUT2

VNFB2

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

Figure 46. SSM6322 Circuit with Current Output DAC

1µF

AUDIO

DAC

1nF

1nF

1nF

1kΩ

1kΩ

1kΩ

51kΩ

1kΩ

VOUT2

VNFB2

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

Figure 47. SSM6322 Circuit with Voltage Output DAC