SSM2163

REV. 0

–12–

AGND

ACOM

DGND

SSM2163

TTL/CMOS

LOGIC

CIRCUITS

+5V

POWER

SUPPLY

+5V

GND

10

µF

0.1

µF

0.1

µF

10

µF

+

+

Figure 24. Use Separate Traces to Reduce Power Supply

Noise

Even if the system includes separate analog and digital power

supplies, both the analog and digital power pins of the

SSM2163 should be connected to the analog supply. While this

connection will inject a small amount of digital noise into the

analog ground, the effect is small due to the SSM2163’s low

digital logic input currents and capacitances. If, on the other

hand, the SSM2163’s digital supply pins are connected to a

digital power supply, then noise from the digital supply will be

coupled into the SSM2163 and degrade performance.

APPLICATIONS

An 8-Input, 2-Output Mixer

A single-chip, 8-input, 2-output mixer using the SSM2163 is

shown in Figure 25. With this circuit, any of the eight channels

can be attenuated by 0 dB to –63 dB and mixed into the right,

left, or both outputs under software control.

10µF

10µF

10µF

10µF

10µF

10µF

10µF

10µF

+

+

+

+

+

+

+

+

5

22

7

20

9

18

11

16

27

26

25

24

28

14

15

3

NC

4

10

+5V

+

10µF

0.1µF

LEFT CHANNEL

OUTPUT

RIGHT CHANNEL

OUTPUT

DATA OUT

SHIELD

SHIELD

SHIELD

SHIELD

SHIELD

AGND

ACOM

DGND

212

1

6

8

19

21

23

17

13

10µF

0.1µF

+

DATA IN

CLK

WRITE

LD

SYSMUTE

SSM2163

INPUT 1 LEFT

INPUT 2 LEFT

INPUT 3 LEFT

INPUT 4 LEFT

INPUT 1 RIGHT

INPUT 2 RIGHT

INPUT 3 RIGHT

INPUT 4 RIGHT

DATA IN

CLOCK

CHIP SELECT

SYSTEM MUTE

Figure 25. An 8-Input, 2-Output Mixer

This circuit demonstrates ac coupling of the inputs. As

previously mentioned, this eliminates level shifting concerns

from previous stages.

The circuit of Figure 25 also demonstrates single +5 V supply

operation. The output of the ACOM supply-splitter, Pin 13,

provides the pseudo-ground reference which is required when

operating from a single supply.

ATTENUATOR

MIXER

SWITCH

CHANNEL

SELECT

MIXER

BUFFER AMP

AGND

MIXER SUMMING

AMPLIFIER

R1

R2

R1=R2

ACOM

10µF

0.1µF

Σ

Figure 23. Single-Supply Pseudo-Ground Reference

(ACOM) Generator

For single supply operation, the inputs can either be ac-coupled,

as shown in Figure 25, or referenced to the pseudo-ground out-

put. AC coupling eliminates dc offset and offset drift

differentials between the input source and the SSM2163. In

addition, ac coupling reduces the risk of “clicks” when switching

between multiple dc-coupled inputs which have different

reference levels.

Since the input coupling capacitors are in series with the 10 k

Ω

input of the attenuator, the impedance of these capacitors will

influence low frequency gain. The inexpensive 10

µF aluminum

electrolytic capacitors shown will limit the gain error to 0.66 dB

at 20 Hz.

If the entire system is operating from a single supply, then

typically the input voltage is already referenced to the midpoint

of the supply. In this case, the SSM2163 can be referenced to

the same midpoint reference source, as shown in Figure 22c.

This connection will eliminate dc offset errors caused by having

the input signal common and the SSM2163 analog common

referenced to different voltages. DC offset errors can also be

eliminated, of course, by using the ac coupling technique

discussed above.

Dual-Supply Operation

The SSM2163 will also operate from dual supplies, ranging

from

±4 V to ±7 V. (See Figure 22a.) In this case, input signals

can be referenced to power supply ground. The ACOM output

(Pin 13) is left open (no connection) for dual supply operation.

Supply Decoupling

Optimizing the performance of the SSM2163, or any low noise

device, requires careful attention to power supply decoupling.

Since the SSM2163 can operate from a single +5 V supply, it

seems convenient to simply tap into the digital logic power

supply. Unfortunately, the logic supply is often a switch-mode

design, which generates noise in the 20 kHz to 1 MHz range. In

addition, fast logic gates can generate glitches hundred of millivolts

in amplitude due to wiring resistances and inductances.

If a separate analog power supply is not available, the SSM2163

can be powered directly from the system power supply. Separate

power and ground traces should be provided for the analog

section, if possible. This arrangement, shown in Figure 24, will

isolate the analog section from the logic switching transients.

Even if a separate power supply trace is not available, however,

generous supply bypassing will reduce supply line induced

noise. Local supply bypassing, consisting of a 10

µF tantalum

electrolytic in parallel with a 0.1

µF ceramic capacitor, is

recommended in all applications. (See the SSM2163 Power

Supply Connections figures.)