SSM2402/SSM2412

REV. A

–9–

DIGITALLY-CONTROLLED ATTENUATOR

Figure 4 shows the usual approach to digitally-controlled at-

tenuator current does not have to flow through the switches.

The disadvantage is that the output is undefined during the

switching period, which can be several milliseconds.

The low distortion characteristics of the SSM2402/SSM2412

enable the alternate arrangement of Figure 5 to be used. Now

only one switch is required to change between two gains, and

typically be in the low kilohm range.

For more gain steps and higher attenuation, the ladder arrange-

ment of Figure 6 can be used. This enables a wide dynamic

range to be achieved without the need for large value resistors,

which would result in degradation of the noise performance.

Figure 4.

Figure 5.

Figure 6.

HIGH PERFORMANCE STEREO ROUTING SWITCHER

The SSM2402 Dual Audio Switch comprises the nucleus for

this 16 channels-to-one high performance stereo audio routing

switcher, which features negligible noise and low distortion over

the frequency range of 20 Hz to 20 kHz. This performance is

achieved even while driving 600

Ω loads at signal levels up to

+30 dBu.

The SSM2402 affords a much simplified electrical design and

printed circuit board layout, along with reduced manufacturing

cost, when compared with discrete JFET circuits of similar per-

formance. The electrical performance of the design described is

vastly superior to CMOS switch designs, which are more prone

to failure resulting from electrical static discharge.

The switching control of the SSM2402 may be activated by

conventional mechanical switches or 5 volt TTL or CMOS logic

circuits. The application shown utilizes a simple mechanical

control switch for illustration purposes only. Many diverse X/Y

control schemes, destination control, or computer controlled

designs can be utilized.

The “T” configuration of the SSM2402 switch provides excel-

lent ON-OFF isolation. The SSM2402 also features ms ramped

turn on and ms ramped turn off for click-free switching. Addi-

tionally, the switch has a break-before-make switching sequence.

Both features become significant in large audio switching sys-

tems where the audio path can pass through multiple switching

elements. Such controlled switching is very important in large

systems used in broadcast program switching or in production

work.

The application circuit design also employs the SSM2015 bal-

anced input amplifier (Figure 7). The input impedance is high

(

≈100 kΩ), balanced or unbalanced. The input circuit incorpo-

rates a single pole RFI filter with a cutoff frequency set at

145 kHz. In addition, the input circuit attenuates the signal by

25 dB and extends the common-mode input voltage range to

±98 volts peak, with common-mode rejection greater than

70 dB from 20 Hz to 20 kHz. The SSM2015 is set to produce a

15 dB gain. The signal drive level into the SSM2402 switch is

then +10 dBu with a +20 dBu input level and +14 dBu peak,

well within ideal operating range. Good signal-to-noise is main-

tained, with generous head-room available by electing to use

±18 V dc power supply voltages.