SSM2402/SSM2412

REV. A

–11–

Figure 8. Switcher Functional Block Diagram

Overall performance of the 16

× 1 stereo switcher is noteworthy.

Input-to-output frequency response is flat to within 1 dB over a

10 Hz to 50 kHz band. Total harmonic distortion plus noise is

less than 0.03%, from 20 Hz to 20 kHz. SMPTE intermodula-

tion distortion is less than 0.02%. The use of

±18 V dc power

supplies produces a +30 dBm clip level, even when driving

600

Ω loads.

Table I. Circuit Performance Specifications

Max Input Level

+30 dBu

Input Impedance, Unbalanced

100 k

Ω

Input Impedance, Balanced

200 k

Ω

Common-Mode Rejection (20 Hz to 20 kHz)

>70 dB

Common-Mode Voltage Limit

±98 V Peak

Max Output Level

+30 dBu/dBm

Output Impedance

67

Ω

Gain Control Range

±2 dB

Output Voltage Slew Rate

6 V/

µs

Frequency Response (

±0.05 dB)

20 Hz to 20 kHz

Frequency Response (

±0.5 dB)

10 Hz to 50 kHz

THD + Noise (20 Hz to 20 kHz, +8 dBu)

0.005%

THD + Noise (20 Hz to 20 kHz, +24 dBu)

0.03%

IMD (SMPTE 60 Hz & 4 kHz, 4:1, +24 dBu)

0.02%

Crosstalk (20 Hz to 20 kHz)

>80 dB

S/N Ratio @ 0 dB Gain

135 dB

The routing switcher bus carries high level unbalanced audio,

but is driven with low impedance sources. With the output im-

pedance of the SSM2015 at virtually 0

Ω and the SSM2402

switch ON, resistance is typically 60

Ω. Bus-to-bus crosstalk is

exceptionally low. For example, assuming 14 pF coupling be-

tween buses and 20 kHz signal, the crosstalk (isolation) exceeds

80 dB. The 14 pF would be representative for the 16

× 1 stereo

design shown. Shielding of the buses with a printed circuit

board ground plane and physically isolating the input and out-

put circuits will reduce the crosstalk even further. The “T” con-

figuration of the SSM2402 switch virtually eliminates crosstalk

between the various input signal sources.

The output amplifier incorporates a buffer amplifier that pro-

vides 4 dB of gain (nominally), with adjustable output level trim

control. The buffer also isolates the switching bus from the bal-

anced output amplifier circuit. The balanced output is designed

to drive 600

Ω loads and utilizes two SSM2134 IC amplifiers.

The differential design increases drive capability, yet increases

the heat dissipation surface area, and keeps IC package tem-

perature well within safe operating limits, even when driving

600

Ω loads. The SSM2134 is recommended due to its low

noise, wide frequency response, and output drive current

capabilities.