a
FEATURES
Low Cost
Replaces 8 Potentiometers
50 kHz 4-Quadrant Multiplying Bandwidth
Low Zero Output Error
Eight Individual Channels
3-Wire Serial Input
500 kHz Update Data Loading Rate
±3 V Output Swing
Midscale Preset, Zero Volts Out
APPLICATIONS
Automatic Adjustment
Trimmer Replacement
Vertical Deflection Amplitude Adjustment
Waveform Generation and Modulation
GENERAL DESCRIPTION
The AD8842 provides eight general purpose digitally controlled
voltage adjustment devices. The TrimDAC® capability allows
replacement of the mechanical trimmer function in new designs.
The AD8842 is ideal for ac or dc gain control of up to 50 kHz
bandwidth signals. The four-quadrant multiplying capability is
useful for signal inversion and modulation often found in video
vertical deflection circuitry.
Internally the AD8842 contains eight voltage output digital-toanalog converters, each with separate voltage inputs. A new
current conveyor amplifier design performs the four-quadrant
multiplying function with a single amplifier at the output of the
current steering digital-to-analog converter. This approach offers an improved constant input resistance performance versus
previous voltage switched DACs used in TrimDAC circuits,
eliminating the need for additional input buffer amplifiers.
Each DAC has its own DAC register that holds its output state.
These DAC registers are updated from an internal serial-toparallel shift register that is loaded from a standard 3-wire serial
input digital interface. Twelve data bits make up the data word
clocked into the serial input register. This data word is decoded
where the first 4 bits determine the address of the DAC register
to be loaded with the last 8 bits of data. A serial data output pin
at the opposite end of the serial register allows simple daisy
chaining in multiple DAC applications without additional external decoding logic.
TrimDAC is a registered trademark of Analog Devices, Inc.
The current conveyor amplifier is a patented circuit belonging to Analog
Devices, Inc.
8-Bit Octal, 4-Quadrant
Multiplying, CMOS TrimDAC
AD8842
FUNCTIONAL BLOCK DIAGRAM
VIN A
DECODED
ADDRESS
VDD
8
8X 8
4
DAC
R
E
G
I
S
T
E
R
S
LOGIC
LD
DATA
8
SERIAL
REGISTER
SDI
8
G
DAC A
VOUT A
AD8842
VIN H
8
G
DAC H
VOUT H
CLK
GND
SDO
VSS
PR
The AD8842 consumes only 95 mW from ± 5 V power supplies.
For single 5 V supply applications consult the DAC-8841. The
AD8842 is pin compatible with the 1 MHz multiplying bandwidth DAC8840. The AD8842 is available in 24-pin plastic
DIP and surface mount SOL-24 packages.
R
R
VIN
VOUT
VOUT = V IN • (D/128 – 1)
Figure 1. Functional Circuit of One 4-Quadrant
Multiplying Channel
VIN
CURRENT CONVEYOR
AMPLIFIER
REF
D
256
R
VIN
R
(1- D)
256
I1
VIN
R
= V IN
VOUT
(D/128–1)
I2
R
Figure 2. Actual Current Conveyor Implementation of
Multiplying DAC Channel
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
