SSM2164

REV. 0

–9–

30k

500

560pF

POWER SUPPLY

AND BIASING CIRCUITRY

VCA4

30k

500

560pF

VCA3

30k

500

560pF

VCA2

30k

500

560pF

VCA1

V+

GND

V–

MODE

30k

Ω

100pF

OP176

FROM ADDITIONAL SSM2164s

FOR > 4 CHANNELS

If additional SSM2164s are added, the 100 pF capacitor may

need to be increased to ensure stability of the output amplifier.

Most op amps are sensitive to capacitance on their inverting

inputs. The capacitance forms a pole with the feedback resistor,

which reduces the high frequency phase margin. As more

SSM2164’s are added to the mixer circuit, their output capaci-

tance and the parasitic trace capacitance add, increasing the

overall input capacitance. Increasing the feedback capacitor will

maintain the stability of the output amplifier.

Digital Control of the SSM2164

One option for controlling the gain and attenuation of the

SSM2164 is to use a voltage output digital-to-analog converter

such as the DAC8426 (Figure 26), whose 0 V to +10 V output

controls the SSM2164’s attenuation from 0 dB to –100 dB. Its

simple 8-bit parallel interface can easily be connected to a

microcontroller or microprocessor in any digitally controlled

system. The voltage output configuration of the DAC8426

provides a low impedance drive to the SSM2164 so the attenua-

tion can be controlled accurately. The 8-bit resolution of the

DAC and its full-scale voltage of +10 V gives an output of

3.9 mV/bit. Since the SSM2164 has a –33 mV/dB gain con-

stant, the overall control law is 0.12 dB/bit or approximately

8 bits/dB. The input and output configuration for the

SSM2164 is the same as for the basic VCA circuit shown

earlier. The 4-to-1 mixer configuration could also be used.

LATCH A

LATCH B

LATCH C

LATCH D

DAC A

DAC B

DAC C

DAC D

LOGIC

CONTROL

10V

REFERENCE

DATA BUS

3

5

6

AGND

DGND

15

16

17

WR

A1

A0

14

7

LSB

MSB

4

18

+15V

2

1

20

19

DAC8426

POWER SUPPLY

AND BIASING CIRCUITRY

VCA4

VCA3

VCA2

VCA1

V+

GND

V– MODE

+15V

–15V

Figure 26. Digital Control of VCA Gain

Figure 25. Four-Channel Mixer (4 to 1)