DAC8512 Datasheet, PDF (16/20 Pages) Analog Devices – % V, Serial Input Complete 12-Bit DAC

DAC8512
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
rate of the analog input control voltage, V_C, is properly chosen.
The circuit in Figure 40 illustrates a volume control application
using the DAC8512 to control the attenuation of the SSM2018.
+15V
P1
100kΩ
OFFSET
TRIM
10M Ω
–15V
P2
500kΩ
SYMMETRY
TRIM
470kΩ
10pF
18kΩ
V_OUT
+15V
0.1µF
18kΩ
V_IN
+15V
0.1µF
2
REF02 6
47pF
1
16
2
15
3
14
4
13
SSM2018
5
12
6
11
7
10
8
9
+5V ^0.1µF
30kΩ
+15V
–15V
0.1µF
4
1
CS
2
CLR
6
LD
5 ^DAC8512
SCLK
3
SDI
4
7
R6
825Ω
8
0V ≤ V_C ≤ +2.24V
R7
1kΩ*
C_CON
1µF
* – PRECISION RESISTOR
PT146
1kΩ COMPENSATOR
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
volt. This corresponds to a 000_H code from the DAC8512.
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
digital code equals FFF_H. Therefore, every DAC LSB corre-
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
C_CON of 1 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 kΩ “Compensator” can be
obtained by contacting:
Precision Resistor Company, Incorporated
10601 75th Street North
Largo, Fl 34647
[phone]
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:
I_OUT
=
R^{17 ×}
1

mV
× Digital
R1
Code
×
R3
+^{VREF ×}
R2
R3

–16–
REV. A