DAC8512

–16–

REV. A

A Serial DAC, Audio Volume Control

The DAC8512 is well suited to control digitally the gain or at-

tenuation of a voltage controlled amplifier. In professional audio

mixing consoles, music synthesizers, and other audio processors,

VCAs, such as the SSM2018, adjust audio channel gain and at-

tenuation from front panel potentiometers. The VCA provides a

clean gain transition control of the audio level when the slew

The circuit in Figure 40 illustrates a volume control application

using the DAC8512 to control the attenuation of the SSM2018.

6

2

DAC8512

8

+15V

7

CS

CLR

1

0.1

µF

4

REF02

6

2

18k

Ω

10pF

470k

Ω

P1

100k

Ω

10M

Ω

OFFSET

TRIM

47pF

SYMMETRY

TRIM

P2

500k

Ω

V

OUT

+15V

–15V

30k

Ω

+15V

–15V

0.1

µF

0.1

µF

+15V

18k

Ω

V

IN

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

SSM2018

+5V

0.1

µF

C

CON

1

µF

R6

825

Ω

R7

1k

0V

* – PRECISION RESISTOR

PT146

1k

Ω COMPENSATOR

5

LD

3

SCLK

4

SDI

Figure 40. A Serial DAC, Audio Volume Control

Since the supply voltage available in these systems is typically

±15 V or ±18 V, a REF02 is used to supply the +5 V required

to power the DAC. No trimming of the reference is required be-

cause of the reference’s tight initial tolerance and low supply

current consumption of the DAC8512. The SSM2018 is config-

ured as a unity-gain buffer when its control voltage equals 0

Since the SSM2018 exhibits a gain constant of –28 mV/dB

(typical), the DAC’s full-scale output voltage has to be scaled

down by R6 and R7 to provide 80 dB of attenuation when the

Table IV. SSM-2018 VCA Attenuation vs.

DAC8512 Input Code

Hexadecimal Number

Control

VCA

in DAC Register

Voltage (V)

Attenuation (dB)

000

0

0

400

+0.56

20

800

+1.12

40

C00

+1.68

60

FFF

+2.24

80

sponds to 0.02 dB of attenuation. Table IV illustrates the at-

tenuation vs. digital code of the volume control circuit.

To compensate for the SSM2018’s gain constant temperature

coefficient of –3300 ppm/

°C, a 1 kΩ, temperature-sensitive re-

sistor (R7) manufactured by the Precision Resistor Company

with a temperature coefficient of +3500 ppm/

°C is used. A

µF provides a control transition time of 1 ms which

yields a click-free change in the audio channel attenuation. Sym-

metry and offset trimming details of the VCA can be found in

the SSM2018 data sheet.

Information regarding the PT146 1 k

Ω “Compensator” can be

obtained by contacting:

Precision Resistor Company, Incorporated

10601 75th Street North

Largo, Fl 34647

(813) 541-5771

An Isolated, Programmable, 4-20 mA Process Controller

In many process control system, applications, two-wire current

transmitters are used to transmit analog signals through noisy

environments. These current transmitters use a “zero-scale” sig-

nal current of 4 mA that can be used to power the transmitter’s

signal conditioning circuitry. The “full-scale” output signal in

these transmitters is 20 mA. The converse approach to process

control can also be used; a low-power, programmable current

source can be used to control remotely located sensors or de-

vices in the loop.

A circuit that performs this function is illustrated in Figure 41.

Using the DAC8512 as the controller, the circuit provides a

programmable output current of 4 mA to 20 mA, proportional

to the DAC’s digital code. Biasing for the controller is provided

by the REF02 and requires no external trim for two reasons:

(1) the REF02’s tight initial output voltage tolerance and (2) the

low supply current consumption of both the OP90 and the

DAC8512. The entire circuit, including opto-couplers, con-

sumes less than 3 mA from the total budget of 4 mA. The OP90

regulates the output current to satisfy the current summation at

the noninverting node of the OP-90. The KCL equation at

Pin 3 is given by:

1

R7

×

1 mV

× Digital Code × R3

R1

+

R2







