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PDF ( 数据手册 , 数据表 ) AD7248A

零件编号 AD7248A
描述 LC2MOS 12-Bit DACPORTs
制造商 Analog Devices
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AD7248A 数据手册, 描述, 功能
a
FEATURES
12-Bit CMOS DAC with Output Amplifier and
Reference
Improved AD7245/AD7248:
12 V to 15 V Operation
؎1/2 LSB Linearity Grade
Faster Interface–30 ns typ Data Setup Time
Extended Plastic Temperature Range (–40؇C to +85؇C)
Single or Dual Supply Operation
Low Power–65 mW typ in Single Supply
Parallel Loading Structure: AD7245A
(8+4) Loading Structure: AD7248A
LC2MOS
12-Bit DACPORTs
AD7245A/AD7248A
AD7245A FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The AD7245A/AD7248A is an enhanced version of the industry
standard AD7245/AD7248. Improvements include operation
from 12 V to 15 V supplies, a ± 1/2 LSB linearity grade, faster
interface times and better full scale and reference variations with
VDD. Additional features include extended temperature range
operation for commercial and industrial grades.
The AD7245A/AD7248A is a complete, 12-bit, voltage output,
digital-to-analog converter with output amplifier and Zener volt-
age reference on a monolithic CMOS chip. No external user
trims are required to achieve full specified performance.
Both parts are microprocessor compatible, with high speed data
latches and double-buffered interface logic. The AD7245A ac-
cepts 12-bit parallel data which is loaded into the input latch on
the rising edge of CS or WR. The AD7248A has an 8-bit wide
data bus with data loaded to the input latch in two write opera-
tions. For both parts, an asynchronous LDAC signal transfers
data from the input latch to the DAC latch and updates the ana-
log output. The AD7245A also has a CLR signal on the DAC latch
which allows features such as power-on reset to be implemented.
The on-chip 5 V buried Zener diode provides a low noise, tem-
perature compensated reference for the DAC. For single supply
operation, two output ranges of 0 V to +5 V and 0 V to +10 V
are available, while these two ranges plus an additional ± 5 V
range are available with dual supplies. The output amplifiers are
capable of developing +10 V across a 2 kload to GND.
The AD7245A/AD7248A is fabricated in linear compatible
CMOS (LC2MOS), an advanced, mixed technology process that
combines precision bipolar circuits with low power CMOS logic.
The AD7245A is available in a small, 0.3" wide, 24-pin DIP and
DACPORT is a registered trademark of Analog Devices, Inc.
AD7248A FUNCTIONAL BLOCK DIAGRAM
SOIC and in 28-terminal surface mount packages. The
AD7248A is packaged in a small, 0.3" wide, 20-pin DIP and
SOIC and in 20-terminal surface mount packages.
PRODUCT HIGHLIGHTS
1. The AD7245A/AD7248A is a 12-bit DACPORT® on a single
chip. This single chip design and small package size offer
considerable space saving and increased reliability over
multichip designs.
2. The improved interface times on the part allows easy, direct
interfacing to most modern microprocessors.
3. The AD7245A/AD7248A features a wide power supply range
allowing operation from 12 V supplies.
REV. A
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







AD7248A pdf, 数据表
AD7245A/AD7248A
CIRCUIT INFORMATION
D/A SECTION
The AD7245A/AD7248A contains a 12-bit voltage mode digi-
tal-to-analog converter. The output voltage from the converter
has the same positive polarity as the reference voltage allowing
single supply operation. The reference voltage for the DAC is
provided by an on-chip buried Zener diode.
The DAC consists of a highly stable, thin-film, R–2R ladder and
twelve high-speed NMOS single-pole, double-throw switches.
The simplified circuit diagram for this DAC is shown in Figure 1.
Figure 1. D/A Simplified Circuit Diagram
The input impedance of the DAC is code dependent and can
vary from 8 kto infinity. The input capacitance also varies
with code, typically from 50 pF to 200 pF.
The small signal (200 mV p-p) bandwidth of the output buffer
amplifier is typically 1 MHz. The output noise from the ampli-
fier is low with a figure of 25 nV/Hz at a frequency of 1 kHz.
The broadband noise from the amplifier has a typical peak-to-
peak figure of 150 µV for a 1 MHz output bandwidth. There is
no significant difference in the output noise between single and
dual supply operation.
VOLTAGE REFERENCE
The AD7245A/AD7248A contains an internal low noise buried
Zener diode reference which is trimmed for absolute accuracy
and temperature coefficient. The reference is internally con-
nected to the DAC. Since the DAC has a variable input imped-
ance at its reference input the Zener diode reference is buffered.
This buffered reference is available to the user to drive the cir-
cuitry required for bipolar output ranges. It can be used as a ref-
erence for other parts in the system provided it is externally
buffered. The reference will give long-term stability comparable
with the best discrete Zener reference diodes. The performance
of the AD7245A/AD7248A is specified with internal reference,
and all the testing and trimming is done with this reference. The
reference should be decoupled at the REF OUT pin and recom-
mended decoupling components are 10 µF and 0.1 µF capaci-
tors in series with a 10 resistor. A simplified schematic of the
reference circuitry is shown in Figure 3.
OP AMP SECTION
The output of the voltage mode D/A converter is buffered by a
noninverting CMOS amplifier. The user has access to two gain
setting resistors which can be connected to allow different out-
put voltage ranges (discussed later). The buffer amplifier is ca-
pable of developing up to 10 V across a 2 kload to GND.
The output amplifier can be operated from a single positive
power supply by tying VSS = AGND = 0 V. The amplifier can
also be operated from dual supplies to allow a bipolar output
range of –5 V to +5 V. The advantages of having dual supplies
for the unipolar output ranges are faster settling time to voltages
near 0 V, full sink capability of 2.5 mA maintained over the en-
tire output range and elimination of the effects of negative offset
on the transfer characteristic (outlined previously). Figure 2
shows the sink capability of the amplifier for single supply
operation.
Figure 2. Typical Single Supply Sink Current vs.
Output Voltage
Figure 3. Internal Reference
DIGITAL SECTION
The AD7245A/AD7248A digital inputs are compatible with ei-
ther TTL or 5 V CMOS levels. All data inputs are static pro-
tected MOS gates with typical input currents of less than 1 nA.
The control inputs sink higher currents (150 µA max) as a result
of the fast digital interfacing. Internal input protection of all
logic inputs is achieved by on-chip distributed diodes.
The AD7245A/AD7248A features a very low digital feedthrough
figure of 10 nV-s in a 5 V output range. This is due to the volt-
age mode configuration of the DAC. Most of the impulse is ac-
tually as a result of feedthrough across the package.
INTERFACE LOGIC INFORMATION—AD7245A
Table I shows the truth table for AD7245A operation. The part
contains two 12-bit latches, an input latch and a DAC latch. CS
and WR control the loading of the input latch while LDAC con-
trols the transfer of information from the input latch to the
DAC latch. All control signals are level triggered; and therefore,
either or both latches may be made transparent, the input latch
by keeping CS and WR “LOW”, the DAC latch by keeping
LDAC “LOW.” Input data is latched on the rising edge of WR.
–8– REV. A







AD7248A equivalent, schematic
AD7245A/AD7248A
MECHANICAL INFORMATION —AD7248A
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Pin Plastic DIP (N-20)
20-Pin Cerdip (Q-20)
20-Lead SOIC (R-20)
20-Terminal
Plastic Leaded
Chip Carrier (P-20A)
–16–
REV. A










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