a
FEATURES
12-Bit CMOS DAC with Output Amplifier and
Reference
Improved AD7245/AD7248:
12 V to 15 V Operation
61/2 LSB Linearity Grade
Faster Interface–30 ns typ Data Setup Time
Extended Plastic Temperature Range (–408C to +858C)
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.
AD7248A FUNCTIONAL BLOCK DIAGRAM
The AD7245A/AD7248A is a complete, 12-bit, voltage output,
digital-to-analog converter with output amplifier and Zener voltage 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 accepts 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 operations. For both parts, an asynchronous LDAC signal transfers
data from the input latch to the DAC latch and updates the analog 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, temperature 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 kΩ load 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.
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
AD7245A/AD7248A–SPECIFICATIONS (V = +12 V to +15 V, V
AGND = DGND = O V, R = 2 kV, C = 1OO pF. All specifications T to T unless otherwise noted.)
DD
L
L
Parameter
STATIC PERFORMANCE
Resolution
Relative Accuracy @ +25°C3
TMIN to TMAX
TMIN to TMAX
Differential Nonlinearity3
Unipolar Offset Error @ +25°C3
TMIN to TMAX
Bipolar Zero Error @ +25°C3
TMIN to TMAX
DAC Gain Error3, 6
Full-Scale Output Voltage Error 7 @ +25°C
∆Full Scale/∆VDD
∆Full Scale/∆VSS
Full-Scale Temperature Coefficient8
REFERENCE OUTPUT
REF OUT @ +25°C
∆REF OUT/∆VDD
Reference Temperature Coefficient
Reference Load Change
(∆REF OUT vs. ∆I)
DIGITAL INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current, IIN
Input Capacitance9
ANALOG OUTPUTS
Output Range Resistors
Output Voltage Ranges10
Output Voltage Ranges10
DC Output Impedance
AC CHARACTERISTICS9
Voltage Output Settling Time
Positive Full-Scale Change
Negative Full-Scale Change
Output Voltage Slew Rate
Digital Feedthrough3
Digital-to-Analog Glitch Impulse
POWER REQUIREMENTS
VDD
VSS
IDD @ +25°C
TMlN to TMAX
ISS (Dual Supplies)
MIN
SS
= O V or –12 V to –15 V,1
MAX
A2
Version
B2
Version
T2
Version
12
± 3/4
±1
12
± 1/2
± 3/4
± 1/2
±1
±3
±5
±2
±4
±2
± 0.2
± 0.06
± 0.01
± 30
12
± 1/2
± 3/4
±1
±3
±5
±3
±5
±2
± 0.2
± 0.06
± 0.01
± 30
1
Units
Bits
LSB max
LSB max
LSB max
LSB max
LSB max
LSB max
LSB max
LSB max
LSB max
% of FSR max
% of FSR/V max
% of FSR/V max
ppm of FSR/°C max
±1
±3
±5
±2
±4
±2
± 0.2
± 0.06
± 0.01
± 40
Test Conditions/Comments
VDD = 15 V ± 5%
Guaranteed Monotonic
VSS = 0 V or –12 V to –15 V4
Typical Tempco is ± 3 ppm of FSR5/°C.
ROFS connected to REF OUT; VSS = –12 V to –15 V4
Typical Tempco is ± 3 ppm of FSR5/°C.
VDD = +15 V
VDD = +12 V to +15 V4
VSS = –12 V to –15 V4
VDD = +15 V
4.99/5.01 4.99/5.01
2
2
± 25
± 25
4.99/5.01 V min/V max
2
mV/V max
± 35
ppm/°C typ
VDD = +15 V
VDD = +12 V to +15 V4
–1
–1
–1
mV max
Referenee Load Current Change (0–100 µA)
2.4
0.8
± 10
8
2.4
0.8
± 10
8
2.4
0.8
± 10
8
V min
V max
µA max
pF max
15/30
+5, +10
+5, +10,
±5
0.5
15/30
+5, +10
+5, +10,
±5
0.5
15/30
+5, +10
+5, +10,
±5
0.5
kΩ min/kΩ max
V
7
7
2
10
30
7
7
2
10
30
10
10
1.5
10
30
µs max
µs max
V/µs min
nV-s typ
nV-s typ
+10.8/
+16.5
–10.8/
–16.5
9
10
3
+11.4/
+15.75
–11.4/
–15.75
9
10
3
+11.4/
+15.75
–11.4/
–15.75
9
12
5
V min/
V max
V min/
V max
mA max
mA max
mA max
VIN = 0 V to VDD
VSS = 0 V; Pin Strappable
VSS = –12 V to –15 V;4 Pin Strappable
V
Ω typ
Settling Time to Within ± 1/2 LSB of Final Value
DAC Latch All 0s to All 1s
DAC Latch All 1s to All 0s; VSS = –12 V to –15 V4
For Specified Performance Unless Otherwise Stated
For Specified Performance Unless Otherwise Stated
Output Unloaded; Typically 5 mA
Output Unloaded
Output Unloaded; Typically 2 mA
NOTES
1
Power supply tolerance is ± 10% for A Version and ± 5% for B and T Versions.
2
Temperature ranges are as follows: A/B Versions; –40°C to +85°C; T Version; –55°C to +125°C.
3
See Terminology.
4
With appropriate power supply tolerances.
5
FSR means Full-Scale Range and is 5 V for the 0 V to +5 V output range and 10 V for both the 0 V to +10 V and ± 5 V output ranges.
6
This error is calculated with respect to the reference voltage and is measured after the offset error has been allowed for.
7
This error is calculated with respect to an ideal 4.9988 V on rhe 0 V to +5 V and ± 5 V ranges; it is calculated with respect to an ideal 9.9976 V on the
0 V to +10 V range. It includes the effects of internal voltage reference, gain and offset errors.
8
Full-Scale TC = ∆FS/∆T, where ∆FS is the full-scale change from T A = +25°C to TMIN or TMAX.
9
Sample tested at +25°C to ensure compliance.
10
0 V to +10 V output range is available only when V DD ≥ +14.25 V.
Specifications subject to change without notice.
–2–
REV. A
AD7245A/AD7248A
SWITCHING CHARACTERISTICS1 (V
Parameter
DD
= +12 V to +15 V;2 VSS = O V or –12 V to –15 V;2 See Figures 5 and 7.)
A, B Versions
T Version
Units
Conditions
@ +25°C
TMIN to TMAX
55
80
55
100
ns typ
ns min
Chip Select Pulse Width
@ +25°C
TMIN to TMAX
40
80
40
100
ns typ
ns min
Write Pulse Width
@ +25°C
TMIN to TMAX
0
0
0
0
ns min
ns min
Chip Select to Write Setup Time
@ +25°C
TMIN to TMAX
0
0
0
0
ns min
ns min
Chip Select to Write Hold Time
@ +25°C
TMIN to TMAX
40
80
40
80
ns typ
ns min
Data Valid to Write Setup Time
@ +25°C
TMIN to TMAX
10
10
10
10
ns min
ns min
Data Valid to Write Hold Time
@ +25°C
TMIN to TMAX
40
80
40
100
ns typ
ns min
Load DAC Pulse Width
t8 (AD7245A only)
@ +25°C
TMIN to TMAX
40
80
40
100
ns typ
ns min
Clear Pulse Width
t1
t2
t3
t4
t5
t6
t7
NOTES
1
Sample tested at +25°C to ensure compliance.
2
Power supply tolerance is ± 10% for A Version and ± 5% for B and T Versions.
ABSOLUTE MAXIMUM RATINGS 1
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V to +17 V
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V to +17 V
VDD to VSS . . . . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V to +34 V
AGND to DGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, VDD
Digital Input Voltage to DGND . . . . . . . . –0.3 V, VDD +0.3 V
VOUT to AGND2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . VSS, VDD
VOUT to VSS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V, +24 V
VOUT to VDD2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –32 V, 0 V
REF OUT2 to AGND . . . . . . . . . . . . . . . . . . . . . . . . 0 V, VDD
Power Dissipation (Any Package) to +75°C . . . . . . . . 450 mW
Derates above +75°C by . . . . . . . . . . . . . . . . . . . . 6 mW/°C
Operating Temperature
Commercial (A, B Versions) . . . . . . . . . . . –40°C to +85°C
Extended (S Version) . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 secs) . . . . . . . . . . . . +300°C
NOTES
1
Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those listed in the
operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
2
The output may be shorted to voltages in this range provided the power dissipation of the package is not exceeded. V OUT short circuit current is typically
80 mA.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD7245A/AD7248A features proprietary ESD protection circuitry, permanent
damage may occur on devices subjected to high energy electrostatic discharges. Therefore,
proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
REV. A
–3–
WARNING!
ESD SENSITIVE DEVICE
AD7245A/AD7248A
AD7245A ORDERING GUIDE
DAC GAIN ERROR
Modell
Temperature
Range
Relative
Accuracy
Package
Option2
AD7245AAN
AD7245ABN
AD7245AAQ
AD7245ATQ3
AD7245AAP
AD7245AAR
AD7245ABR
AD7245ATE3
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–55°C to +125°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–55°C to +125°C
± 3/4 LSB
± 1/2 LSB
± 3/4 LSB
± 3/4 LSB
± 3/4 LSB
± 3/4 LSB
± 1/2 LSB
± 3/4 LSB
N-24
N-24
Q-24
Q-24
P-28A
R-24
R-24
E-28A
DAC Gain Error is a measure of the output error between an
ideal DAC and the actual device output with all 1s loaded after
offset error has been allowed for. It is, therefore defined as:
Measured Value—Offset—Ideal Value
where the ideal value is calculated relative to the actual reference value.
UNIPOLAR OFFSET ERROR
Unipolar Offset Error is a combination of the offset errors of the
voltage mode DAC and the output amplifier and is measured
when the part is configured for unipolar outputs. It is present
for all codes and is measured with all 0s in the DAC register.
NOTES
1
To order MIL-STD-883, Class B. processed parts, add /883B to part number.
Contact our local sales office for military data sheet and availability.
2
E = Leadless Ceramic Chip Carrier; N = Plastic DIP; P = Plastic Leaded Chip
Carrier; Q = Cerdip; R = SOIC.
3
This grade will be available to /883B processing only.
BIPOLAR ZERO OFFSET ERROR
Bipolar Zero Offset Error is measured when the part is configured for bipolar output and is a combination of errors from the
DAC and output amplifier. It is present for all codes and is
measured with a code of 2048 (decimal) in the DAC register.
SINGLE SUPPLY LINEARITY AND GAIN ERROR
AD7248A ORDERING GUIDE
Modell
Temperature
Range
Relative
Accuracy
Package
Option2
AD7248AAN
AD7248ABN
AD7248AAQ
AD7248ATQ3
AD7248AAP
AD7248AAR
AD7248ABR
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–55°C to +125°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
± 3/4 LSB
± 1/2 LSB
± 3/4 LSB
± 3/4 LSB
± 3/4 LSB
± 3/4 LSB
± 1/2 LSB
N-20
N-20
Q-20
Q-20
P-20A
R-20
R-20
The output amplifier of the AD7245A/AD7248A can have a
true negative offset even when the part is operated from a single
positive power supply. However, because the lower supply rail
to the part is 0 V, the output voltage cannot actually go negative. Instead the output voltage sits on the lower rail and this results in the transfer function shown. This is an offset effect and
the transfer function would have followed the dotted line if the
output voltage could have gone negative. Normally, linearity is
measured after offset and full scale have been adjusted or allowed for. On the AD7245A/AD7248A the negative offset is allowed for by calculating the linearity from the code which the
amplifier comes off the lower rail. This code is given by the
negative offset specification. For example, the single supply linearity specification applies between Code 3 and Code 4095 for
the 25°C specification and between Code 5 and Code 4095 over
the TMIN to TMAX temperature range. Since gain error is also
measured after offset has been allowed for, it is calculated between
the same codes as the linearity error. Bipolar linearity and gain error are measured between Code 0 and Code 4095.
NOTES
1
To order MIL-STD-883, Class B, processed parts, add /883B to part number.
Contact our local sales office for military data sheet and availability.
2
N = Plastic DIP; P = Plastic Leaded Chip Carrier; Q = Cerdip; R = SOIC.
3
This grade will be available to /883B processing only.
TERMINOLOGY
RELATIVE ACCURACY
Relative Accuracy, or end-point nonlinearity, is a measure of the
actual deviation from a straight line passing through the endpoints of the DAC transfer function. It is measured after allowing for zero and full scale and is normally expressed in LSBs or
as a percentage of full-scale reading.
OUTPUT
VOLTAGE
DIFFERENTIAL NONLINEARITY
Differential Nonlinearity is the difference between the measured
change and the ideal 1 LSB change between any two adjacent
codes. A specified differential nonlinearity of ± 1 LSB max over
the operating temperature range ensures monotonicity.
0V
NEGATIVE
OFFSET
DIGITAL FEEDTHROUGH
{
DAC CODE
Digital Feedthrough is the glitch impulse injected from the digital inputs to the analog output when the inputs change state. It
is measured with LDAC high and is specified in nV-s.
–4–
REV. A
AD7245A/AD7248A
AD7248A PIN FUNCTION DESCRIPTION
(DIP PIN NUMBERS)
Pin
Mnemonic
Description
Pin
Mnemonic
Description
l
VSS
Negative Supply Voltage (0 V for single
supply operation).
19
WR
2
ROFS
Bipolar Offset Resistor. This provides
access to the on-chip application resistors
and allows different output voltage ranges.
Write Input (Active LOW). This is used in
conjunction with CS to write data into the
input latch of the AD7245A.
20
LDAC
Load DAC Input (Active LOW). This is
an asynchronous input which when active
transfers data from the input latch to the
DAC latch.
21
CLR
Clear Input (Active LOW). When this input is active the contents of the DAC latch
are reset to all 0s.
22
VDD
Positive Supply Voltage.
23
RFB
Feedback Resistor. This allows access to
the amplifier’s feedback loop.
24
VOUT
Output Voltage. Three different output
voltage ranges can be chosen: 0 V to +5 V,
0 V to +10 V or –5 V to +5 V.
3
REF OUT
Reference Output. The on-chip reference
is provided at this pin and is used when
configuring the part for bipolar outputs.
4
AGND
Analog Ground.
5
DB11
Data Bit 11. Most Significant Bit (MSB).
6-11
DB10-DB5
Data Bit 10 to Data Bit 5.
12
DGND
Digital Ground.
13-16 DB4-DB1
Data Bit 4 to Data Bit 1.
17
DB0
Data Bit 0. Least Significant Bit (LSB).
18
CS
Chip Select Input (Active LOW). The device is selected when this input is active.
AD7245A PIN CONFIGURATIONS
DIP and SOIC
LCCC
PLCC
REV. A
–5–
AD7245A/AD7248A
AD7248A PIN FUNCTION DESCRIPTION
(ANY PACKAGE)
Pin
Mnemonic
Description
Pin
Mnemonic
Description
l
VSS
Negative Supply Voltage (0 V for single
supply operation).
14
CSMSB
2
ROFS
Bipolar Offset Resistor. This provides
access to the on-chip application resistors
and allows different output voltage ranges.
Chip Select Input for MS Nibble. (Active
LOW). This selects the upper 4 bits of the
input latch. Input data is right justified.
15
CSLSB
Reference Output. The on-chip reference
is provided at this pin and is used when
configuring the part for bipolar outputs.
Chip Select Input for LS byte. (Active
LOW). This selects the lower 8 bits of the
input latch.
16
WR
Write Input. This is used in conjunction
with CSMSB and CSLSB to load data
into the input latch of the AD7248A.
17
LDAC
Load DAC Input (Active LOW). This is
an asynchronous input which when active
transfers data from the input latch to the
DAC latch.
18
VDD
Positive Supply Voltage.
19
RFB
Feedback Resistor. This allows access to
the amplifier’s feedback loop.
20
VOUT
Output Voltage. Three different output
voltage ranges can be chosen: 0 V to +5 V,
0 V to +10 V or –5 V to +5 V.
3
REF OUT
4
AGND
Analog Ground.
5
DB7
Data Bit 7.
6
DB6
Data Bit 6.
7
DB5
Data Bit 5.
8
DB4
Data Bit 4.
9
DB3
Data Bit 3.
10
DGND
Digital Ground.
11
DB2
Data Bit 2/Data Bit 10.
12
DB1
Data Bit 1/Data Bit 9.
13
DB0
Data Bit 0 (LSB)/Data Bit 8.
AD7248A PIN CONFIGURATIONS
DIP and SOIC
LCCC
–6–
PLCC
REV. A
Typical Performance–AD7245A/AD7248A
Power Supply Current vs. Temperature
Reference Voltage vs. Temperature
Noise Spectral Density vs. Frequency
Power Supply Rejection Ration vs. Frequency
Positive-Going Settling Time
(VDD = +15 V, VSS = –15 V)
REV. A
Negative Going Settling Time
(VDD = +15 V, VSS = –15 V)
–7–
AD7245A/AD7248A
CIRCUIT INFORMATION
The small signal (200 mV p-p) bandwidth of the output buffer
amplifier is typically 1 MHz. The output noise from the amplifier 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-topeak 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.
D/A SECTION
The AD7245A/AD7248A contains a 12-bit voltage mode digital-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.
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 connected to the DAC. Since the DAC has a variable input impedance at its reference input the Zener diode reference is buffered.
This buffered reference is available to the user to drive the circuitry required for bipolar output ranges. It can be used as a reference 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 recommended decoupling components are 10 µF and 0.1 µF capacitors in series with a 10 Ω resistor. A simplified schematic of the
reference circuitry is shown in Figure 3.
Figure 1. D/A Simplified Circuit Diagram
The input impedance of the DAC is code dependent and can
vary from 8 kΩ to infinity. The input capacitance also varies
with code, typically from 50 pF to 200 pF.
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 output voltage ranges (discussed later). The buffer amplifier is capable of developing up to 10 V across a 2 kΩ load 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 entire 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 3. Internal Reference
DIGITAL SECTION
The AD7245A/AD7248A digital inputs are compatible with either TTL or 5 V CMOS levels. All data inputs are static protected 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 voltage mode configuration of the DAC. Most of the impulse is actually 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 controls 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.
Figure 2. Typical Single Supply Sink Current vs.
Output Voltage
–8–
REV. A
AD7245A/AD7248A
The data held in the DAC latch determines the analog output of
the converter. Data is latched into the DAC latch on the rising
edge of LDAC. This LDAC signal is an asynchronous signal
and is independent of WR. This is useful in many applications.
However, in systems where the asynchronous LDAC can occur
during a write cycle (or vice versa) care must be taken to ensure
that incorrect data is not latched through to the output. For example, if LDAC goes LOW while WR is “LOW”, then the
LDAC signal must stay LOW for t7 or longer after WR goes
high to ensure correct data is latched through to the output.
Table I. AD7245A Truth Table
CLR
LDAC WR
CS
Function
H
H
H
H
H
H
H
L
g
L
H
H
H
H
L
g
X
H
L
H
X
L
g
H
H
X
H
L
X
H
L
L
H
H
X
H
g
L
L
L
Both Latches are Transparent
Both Latches are Latched
Both Latches are Latched
Input Latches Transparent
Input Latches Latched
DAC Latches Transparent
DAC Latches Latched
DAC Latches Loaded with all 0s
DAC Latches Latched with All
0s and Output Remains at
0 V or –5 V
Both Latches are Transparent
and Output Follows Input Data
Figure 5. AD7245A Write Cycle Timing Diagram
INTERFACE LOGIC INFORMATION—AD7248A
The input loading structure on the AD7248A is configured for
interfacing to microprocessors with an 8-bit wide data bus. The
part contains two 12-bit latches—an input latch and a DAC
latch. Only the data held in the DAC latch determines the analog output from the converter. The truth table for AD7248A
operation is shown in Table II, while the input control logic diagram is shown in Figure 6.
H = High State L = Low State X = Don’t Care
The contents of the DAC latch are reset to all 0s by a low level
on the CLR line. With both latches transparent, the CLR line
functions like a zero override with the output brought to 0 V in
the unipolar mode and –5 V in the bipolar mode for the duration of the CLR pulse. If both latches are latched, a “LOW”
pulse on the CLR input latches all 0s into the DAC latch and
the output remains at 0 V (or –5 V) after the CLR line has returned “HIGH.” The CLR line can be used to ensure powerup
to 0 V on the AD7245A output in unipolar operation and is also
useful, when used as a zero override, in system calibration
cycles.
Figure 4 shows the input control logic for the AD7245A and the
write cycle timing for the part is shown in Figure 5.
Figure 4. AD7245A Input Control Logic
Figure 6. AD7248A Input Control Logic
CSMSB, CSLSB and WR control the loading of data from the
external data bus to the input latch. The eight data inputs on
the AD7248A accept right justified data. This data is loaded to
the input latch in two separate write operations. CSLSB and
WR control the loading of the lower 8-bits into the 12-bit wide
latch. The loading of the upper 4-bit nibble is controlled by
CSMSB and WR. All control inputs are level triggered, and input data for either the lower byte or upper 4-bit nibble is latched
into the input latches on the rising edge of WR (or either
CSMSB or CSLSB). The order in which the data is loaded to
the input latch (i.e., lower byte or upper 4-bit nibble first) is not
important.
The LDAC input controls the transfer of 12-bit data from the
input latch to the DAC latch. This LDAC signal is also level
triggered, and data is latched into the DAC latch on the rising
edge of LDAC. The LDAC input is asynchronous and independent of WR. This is useful in many applications especially in
REV. A
–9–
AD7245A/AD7248A
the simultaneous updating of multiple AD7248A outputs. However, in systems where the asynchronous LDAC can occur during a write cycle (or vice versa) care must be taken to ensure
that incorrect data is not latched through to the output. In other
words, if LDAC goes low while WR and either CS input are low
(or WR and either CS go low while LDAC is low), then the
LDAC signal must stay low for t7 or longer after WR returns
high to ensure correct data is latched through to the output.
The write cycle timing diagram for the AD7248A is shown in
Figure 7.
UNIPOLAR (0 V TO +10 V) CONFIGURATION
The first of the configurations provides an output voltage range
of 0 V to +10 V. This is achieved by connecting the bipolar offset resistor, ROFS, to AGND and connecting RFB to VOUT. In
this configuration the AD7245A/AD7248A can be operated
single supply (VSS = 0 V = AGND). If dual supply performance
is required, a VSS of –12 V to –15 V should be applied. Figure 8
shows the connection diagram for unipolar operation while the
table for output voltage versus the digital code in the DAC latch
is shown in Table III.
Figure 7. AD7248A Write Cycle Timing Diagram
An alternate scheme for writing data to the AD7248A is to tie
the CSMSB and LDAC inputs together. In this case exercising
CSLSB and WR latches the lower 8 bits into the input latch.
The second write, which exercises CSMSB, WR and LDAC
loads the upper 4-bit nibble to the input latch and at the same
time transfers the 12-bit data to the DAC latch. This automatic
transfer mode updates the output of the AD7248A in two write
operations. This scheme works equally well for CSLSB and
LDAC tied together provided the upper 4-bit nibble is loaded to
the input latch followed by a write to the lower 8 bits of the input latch.
Figure 8. Unipolar (0 to +10 V) Configuration
Table III. Unipolar Code Table (0 V to +10 V Range)
DAC Latch Contents
MSB
LSB
Analog Output, VOUT
4095
1111
1111
1111
+2 VREF 3
4096
1000
0000
0001
+2 VREF 3
4096
1000
0000
0000
+2 VREF 3
= +VREF
4096
0111
1111
1111
+2 VREF 3
4096
0000
0000
0001
0000
0000
0000
+2 VREF 3
4096
0V
2049
Table II. AD7248A Truth Table
CSLSB CSMSB WR LDAC Function
L
L
g
H
H
H
H
H
H
H
H
H
H
L
L
g
H
H
L
H
L
g
L
L
g
L
H
H
L
H
H
H
H
H
H
H
L
g
L
H
I.oad LS Byte into Input Latch
Latches LS Byte into Input Latch
Latches LS Byte into Input Latch
Loads MS Nibble into Input Latch
Latches MS Nibble into Input Latch
Latches MS Nibble into Input Latch
Loads Input Latch into DAC Latch
Latches Input Latch into DAC Latch
Loads MS Nibble into Input Latch and
Loads Input Latch into DAC Latch
No Data Transfer Operation
2048
2047
1
1
NOTE: 1 LSB = 2 3 VREF(2–12) = VREF
2048
UNIPOLAR (0 V TO +5 V) CONFIGURATION
H = High State L = Low State
APPLYING THE AD7245A/AD7248A
The internal scaling resistors provided on the AD7245A/
AD7248A allow several output voltage ranges. The part can
produce unipolar output ranges of 0 V to +5 V or 0 V to +10 V
and a bipolar output range of –5 V to +5 V. Connections for the
various ranges are outlined below.
The 0 V to +5 V output voltage range is achieved by tying ROFS,
RFB and VOUT together. For this output range the AD7245A/
AD7248A can be operated single supply (VSS = 0 V) or dual
supply. The table for output voltage versus digital code is as in
Table III, with 2 • VREF replaced by VREF. Note that for this
range
1
.
1 LSB = V (2–12) = V
3
–10–
REF
REF
4096
REV. A
AD7245A/AD7248A
BIPOLAR CONFIGURATION
The bipolar configuration for the AD7245A/AD7248A, which
gives an output voltage range from –5 V to +5 V, is achieved by
connecting the ROFS input to REF OUT and connecting RFB
and VOUT. The AD7245A/AD7248A must be operated from
dual supplies to achieve this output voltage range. The code
table for bipolar operation is shown in Table IV.
Table IV. Bipolar Code Table
DAC Latch Contents
MSB
LSB
Analog Output, VOUT
2047
1111
1111
1111
+VREF ×
2048
1000
0000
0001
1000
0000
0000
+VREF ×
2048
0V
1
0111
1111
1111
1
–VREF ×
2048
0000
0000
0001
–VREF ×
2048
0000
0000
0000
–VREF ×
= –VREF
2048
2047
2048
Figure 9. AGND Bias Circuit
PROGRAMMABLE CURRENT SINK
Figure 10 shows how the AD7245A/AD7248A can be configured with a power MOSFET transistor, the VN0300M, to provide a programmable current sink from VDD or VSOURCE. The
VN0300M is placed in the feedback of the AD7245A/
AD7248A amplifier. The entire circuit can be operated in single
supply by tying the VSS of the AD7245A/AD7248A to AGND.
The sink current, ISINK, can be expressed as:
ISINK =
D ×VREF
R1
1
NOTE: 1 LSB = 2 × VREF(2–11) = VREF
2048
AGND BIAS
The AD7245A/AD7248A AGND pin can be biased above system GND (AD7245A/AD7248A DGND) to provide an offset
“zero” analog output voltage level. With unity gain on the amplifier (ROFS = VOUT = RFB) the output voltage, VOUT is expressed as:
VOUT = VBIAS + D 3 VREF
where D is a fractional representation of the digital word in the
DAC latch and VBIAS is the voltage applied to the AD7245A/
AD7248A AGND pin.
Because the current flowing out of the AGND pin varies with
digital code, the AGND pin should be driven from a low impedance source. A circuit configuration is outlined for AGND bias
in Figure 9 using the AD589, a +1.23 V bandgap reference.
If a gain of 2 is used on the buffer amplifier the output voltage,
VOUT is expressed as
VOUT = 2(VBIAS + D 3 VREF)
In this case care must be taken to ensure that the maximum output voltage is not greater than VDD –3 V. The VDD–VOUT overhead must be greater than 3 V to ensure correct operation of the
part. Note that VDD and VSS for the AD7245A/AD7248A must
be referenced to DGND (system GND). The entire circuit can
be operated in single supply with the VSS pin of the AD7245A/
AD7248A connected to system GND.
REV. A
Figure 10. Programmable Current Sink
Using the VN0300M, the voltage drop across the load can typically be as large as VSOURCE –6 V) with VOUT of the DAC at
+5 V. Therefore, for a current of 50 mA flowing in the R1 (with
all 1s in the DAC register) the maximum load is 200 Ω with
VSOURCE = +15 V. The VN0300M can actually handle currents
up to 500 mA and still function correctly in the circuit, but in
practice the circuit must be used with larger values of VSOURCE
otherwise it requires a very small load.
Since the tolerance value on the reference voltage of the
AD7245A/AD7248A is ± 0.2%, then the absolute value of ISINK
can vary by ± 0.2% from device to device for a fixed value of R1.
Because the input bias current of the AD7245A/AD7248A’s op
amp is only of the order of picoamps, its effect on the sink current is negligible. Tying the ROFS input to RFB input reduces this
effect even further and prevents noise pickup which could occur
if the ROFS pin was left unconnected.
–11–
AD7245A/AD7248A
The circuit of Figure 10 can be modified to provide a programmable current source to AGND or –VSINK (for –VSINK, dual supplies are required on the AD7245A/AD7248A). The AD7245A/
AD7248A is configured as before. The current through R1 is
mirrored with a current mirror circuit to provide the programmable source current (see CMOS DAC Application Guide,
Publication No. G872-30-10/84, for suitable current mirror circuit). As before the absolute value of the source current will be
affected by the ± 0.2% tolerance on VREF. In this case the performance of the current mirror will also affect the value of the
source current.
MICROPROCESSOR INTERFACING—AD7245
AD7245A—8086A INTERFACE
Figure 12 shows the 8086 16-bit processor interfacing to the
AD7245A. In the setup shown the double buffering feature of
the DAC is not used and the LDAC input is tied LOW. AD0–
AD11 of the 16-bit data bus are connected to the AD7245A
data bus (DB0-DB11). The 12-bit word is written to the
AD7245A in one MOV instruction and the analog output responds immediately. In this example the DAC address is D000.
A software routine for Figure 12 is given in Table V.
FUNCTION GENERATOR WITH PROGRAMMABLE
FREQUENCY
Figure 11 shows how the AD7245A/AD7248A with the AD537,
voltage-to-frequency converter and the AD639, trigonometric
function generator to provide a complete function generator
with programmable frequency. The circuit provides square
wave, triwave and sine wave outputs, each output of ± 10 V
amplitude.
The AD7245A/AD7248A provides a programmable voltage to
the AD537 input. Since both the AD7245A/AD7248A and
AD537 are guaranteed monotonic, the output frequency will always increase with increasing digital code. The AD537 provides
a square wave output which is conditioned for ± 10 V by amplifier A1. The AD537 also provides a differential triwave output.
This is conditioned by amplifiers A2 and A3 to provide the
± 1.8 V triwave required at the input of the AD639. The triwave
is further scaled by amplifier A4 to provide a ± 10 V output.
Adjusting the triwave applied to the AD639 adjust the distortion
performance of the sine wave output, (+10 V in configuration
shown). Amplitude, offset and symmetry of the triwave can affect the distortion. By adjusting these, via VR1 and VR2, an
output sine wave with harmonic distortion of better than –50 dB
can be achieved at low and intermediate frequencies.
Using the capacitor value shown in Figure 11 for CF (i.e.,
680 pF) the output frequency range is 0 to 100 kHz over the
digital input code range. The step size for frequency increments
is 25 Hz. The accuracy of the output frequency is limited to 8 or
9 bits by the AD537, but is guaranteed monotonic to 12 bits.
Figure 12. AD7245A to 8086 Interface
Table V. Sample Program for Loading AD7245A from 8086
ASSUME DS: DACLOAD, CS: DACLOAD
DACLOAD SEGMENT AT 000
00 8CC9
MOV CS,
CS
: DEFINE DATA SEGMENT
REGISTER
02 8ED9
MOV DS,
CX
: EQUAL TO CODE
SEGMENT REGISTER
04 BF00D0
0MOV DI,
#D000
: LOAD DI WITH D000
07 C705
MOV MEM, : DAC LOADED WITH WXYZ
“YZWX” #YZWX
0B EA00 00
0E 00 FF
: CONTROL IS RETURNED TO
THE MONITOR PROGRAM
Figure 11. Programmable Function Generator
–12–
REV. A
AD7245A/AD7248A
In a multiple DAC system the double buffering of the AD7245A
allows the user to simultaneously update all DACs. In Figure
13, a 12-bit word is loaded to the input latches of each of the
DACs in sequence. Then, with one instruction to the appropriate address, CS4 (i.e., LDAC) is brought LOW, updating all the
DACs simultaneously.
Table Vl. Sample Routine for Loading AD7245A from 68000
01000
MOVE.W
#X,D0
The desired DAC data, X,
is loaded into Data Register 0. X may be any
value between 0 and 4094
(decimal) or 0 and OFFF
(hexadecimal).
MOVE.W
D0,$E000
The Data X is transferred
between D0 and the
DAC Latch.
MOVE.B
#228,D7
TRAP
#14
Control is returned to the
System Monitor Program
using these two
instructions.
MICROPROCESSOR INTERFACE—AD7248A
Figure 13. AD7245A to 8086 Multiple DAC Interface
AD7245A—MC68000 INTERFACE
Interfacing between the MC68000 and the AD7245A is accomplished using the circuit of Figure 14. Once again the AD7245A
is used in the single buffered mode. A software routine for loading data to the AD7245A is given in Table VI. In this example
the AD7245A is located at address E000, and the 12-bit word is
written to the DAC in one MOVE instruction.
Figure 15 shows the connection diagram for interfacing the
AD7248A to both the 8085A and 8088 microprocessors. This
scheme is also suited to the Z80 microprocessor, but the Z80
address/data bus does not have to be demultiplexed. Data to be
loaded to the AD7248A is right justified. The AD7248A is
memory mapped with a separate memory address for the input
latch high byte, the input latch low byte and the DAC latch.
Data is first written to the AD7248A input latch in two write
operations. Either the high byte or the low byte data can be
written first to the AD7248A input latch. A write to the
AD7248A DAC latch address transfers the input latch data to
the DAC latch and updates the output voltage. Alternatively,
the LDAC input can be asynchronous or can be common to a
number of AD7248As for simultaneous updating of a number of
voltage channels.
Figure 15. AD7248A to 8085A/8088 Interface
Figure 14. AD7245A to 68000 Interface
REV. A
A connection diagram for the interface between the AD7248A
and 68008 microprocessor is shown in Figure 16. Once again
the AD7248A acts as a memory mapped device and data is right
justified. In this case the AD7248A is configured in the automatic transfer mode which means that the high byte of the input
latch has the same address as the DAC latch. Data is written to
the AD7248A by first writing data to the AD7248A low byte.
Writing data to the high byte of the input latch also transfers the
input latch contents to the DAC latch and updates the output.
–13–
AD7245A/AD7248A
Figure 18 shows a connection diagram between the AD7248A
and the 8051 microprocessor. The AD7248A is port mapped in
this interface and is configured in the automatic transfer mode.
Data to be loaded to the input latch low byte is output to Port 1.
Output Line P3.0, which is connected to CSLSB of the
AD7248A, is pulsed to load data into the low byte of the input
latch. Pulsing the P3.1 line, after the high byte data has been set
up on Port 1, updates the output of the AD7248A. The WR input of the AD7248A can be hardwired low in this application
because spurious address strobes on CSLSB and CSMSB do not
occur.
Figure 16. AD7248A to 68008 Interface
An interface circuit for connections to the 6502 or 6809 microprocessors is shown in Figure 17. Once again, the AD7248A is
memory mapped and data is right justified. The procedure for
writing data to the AD7248A is as outlined for the 8085A/8088.
For the 6502 microprocessor the φ2 clock is used to generate
the WR, while for the 6809 the E signal is used.
Figure 18. AD7248A to MCS-51 Interface
Figure 17. AD7248A to 6502/6809 Interface
–14–
REV. A
AD7245A/AD7248A
MECHANICAL INFORMATION—AD7245A
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Pin Plastic DIP (N-24)
24-Pin SOIC (R-24) Package
24-Pin Cerdip (Q-24)
28-Terminal
Plastic Leaded
Chip Carrier (P-28A)
28-Terminal
Leadless Ceramic Chip
Carrier (E-28A)
REV. A
–15–
AD7245A/AD7248A
MECHANICAL INFORMATION —AD7248A
OUTLINE DIMENSIONS
C1461–24–4/92
Dimensions shown in inches and (mm).
20-Pin Plastic DIP (N-20)
20-Pin Cerdip (Q-20)
20-Terminal
Plastic Leaded
Chip Carrier (P-20A)
PRINTED IN U.S.A.
20-Lead SOIC (R-20)
–16–
REV. A