REV. B
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.
a
LC
2
MOS Quad 8-Bit DAC
with Separate Reference Inputs
AD7225
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 Fax: 617/326-8703
FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The AD7225 contains four 8-bit voltage output digital-to-
analog converters, with output buffer amplifiers and interface
logic on a single monolithic chip. Each D/A converter has a
separate reference input terminal. No external trims are re-
quired to achieve full specified performance for the part.
The double-buffered interface logic consists of two 8-bit regis-
ters per channel–an input register and a DAC register. Control
inputs A0 and A1 determine which input register is loaded when
WR goes low. Only the data held in the DAC registers deter-
mines the analog outputs of the converters. The double-
buffering allows simultaneous update of all four outputs under
control of LDAC. All logic inputs are TTL and CMOS (5 V)
level compatible and the control logic is speed compatible with
most 8-bit microprocessors.
Specified performance is guaranteed for input reference voltages
from +2 V to +12.5 V when using dual supplies. The part is also
specified for single supply operation using a reference of +10 V.
Each output buffer amplifier is capable of developing +10 V
across a 2 kΩ load.
The AD7225 is fabricated on an all ion-implanted high-speed
Linear Compatible CMOS (LC
2
MOS) process which has been
specifically developed to integrate high speed digital logic cir-
cuits and precision analog circuitry on the same chip.
FEATURES
Four 8-Bit DACs with Output Amplifiers
Separate Reference Input for Each DAC
mP Compatible with Double-Buffered Inputs
Simultaneous Update of All Four Outputs
Operates with Single or Dual Supplies
Extended Temperature Range Operation
No User Trims Required
Skinny 24-Pin DIP, SOIC and 28-Terminal Surface
Mount Packages
PRODUCT HIGHLIGHTS
1. DACs and Amplifiers on CMOS Chip
The single-chip design of four 8-bit DACs and amplifiers al-
lows a dramatic reduction in board space requirements and
offers increased reliability in systems using multiple convert-
ers. Its pinout is aimed at optimizing board layout with all
analog inputs and outputs at one end of the package and all
digital inputs at the other.
2. Single or Dual Supply Operation
The voltage-mode configuration of the AD7225 allows single
supply operation. The part can also be operated with dual
supplies giving enhanced performance for some parameters.
3. Versatile Interface Logic
The AD7225 has a common 8-bit data bus with individual
DAC latches, providing a versatile control architecture for
simple interface to microprocessors. The double-buffered in-
terface allows simultaneous update of the four outputs.
4. Separate Reference Input for Each DAC
The AD7225 offers great flexibility in dealing with input sig-
nals with a separate reference input provided for each DAC
and each reference having variable input voltage capability.
REV. B
–2–
AD7225–SPECIFICATIONS
DUAL SUPPLY
K, B L, C
Parameter Versions
2
Versions
2
T Version U Version Units Conditions/Comments
STATIC PERFORMANCE
Resolution 8 8 8 8 Bits
Total Unadjusted Error ±2±1±2±1 LSB max V
DD
= +15 V ± 5%, V
REF
= +10 V
Relative Accuracy ±1±1/2 ±1±1/2 LSB max
Differential Nonlinearity ±1±1±1±1 LSB max Guaranteed Monotonic
Full-Scale Error ±1±1/2 ±1±1/2 LSB max
Full-Scale Temp. Coeff. ±5±5±5±5 ppm/°C typ V
DD
= 14 V to 16.5 V, V
REF
= +10 V
Zero Code Error @ 25 °C±25 ±15 ±25 ±15 mV max
T
MIN
to T
MAX
±30 ±20 ±30 ±20 mV max
Zero Code Error Temp Coeff. ±30 ±30 ±30 ±30 µV/°C typ
REFERENCE INPUT
Voltage Range 2 to (V
DD
– 4) 2 to (V
DD
– 4) 2 to (V
DD
– 4) 2 to (V
DD
– 4) V min to V max
Input Resistance 11 11 11 11 kΩ min
Input Capacitance
3
100 100 100 100 pF max Occurs when each DAC is loaded with all 1s.
Channel-to-Channel Isolation
3
60 60 60 60 dB min V
REF
= 10 V p-p Sine Wave @ 10 kHz
AC Feedthrough
3
–70 –70 –70 –70 dB max V
REF
= 10 V p-p Sine Wave @ 10 kHz
DIGITAL INPUTS
Input High Voltage, V
INH
2.4 2.4 2.4 2.4 V min
Input Low Voltage, V
INL
0.8 0.8 0.8 0.8 V max
Input Leakage Current ±1±1±1±1µA max V
IN
= 0 V or V
DD
Input Capacitance
3
8 8 8 8 pF max
Input Coding Binary Binary Binary Binary
DYNAMIC PERFORMANCE
Voltage Output Slew Rate
3
2.5 2.5 2.5 2.5 V/µs min
Voltage Output Settling Time
3
Positive Full-Scale Change 5 5 5 5 µs max V
REF
= +10 V; Settling Time to ±1/2 LSB
Negative Full-Scale Change 5 5 5 5 µs max V
REF
= +10 V; Settling Time to ±1/2 LSB
Digital Feedthrough
3
50 50 50 50 nV secs typ Code transition all 0s to all 1s.
Digital Crosstalk
3
50 50 50 50 nV secs typ Code transition all 0s to all 1s.
Minimum Load Resistance 2 2 2 2 kΩ min V
OUT
= +10 V
POWER SUPPLIES
V
DD
Range 11.4/16.5 11.4/16.5 11.4/16.5 11.4/16.5 V min to V max For Specified Performance
I
DD
10 10 12 12 mA max Outputs Unloaded; V
IN
= V
INL
or V
INH
I
SS
9 9 10 10 mA max Outputs Unloaded; V
IN
= V
INL
or V
INH
SWITCHING CHARACTERISTICS
3, 4
t
1
@ 25°C 95 95 95 95 ns min Write Pulse Width
T
MIN
to T
MAX
120 120 150 150 ns min
t
2
@ 25°C 0 0 0 0 ns min Address to Write Setup Time
T
MIN
to T
MAX
0 0 0 0 ns min
t
3
@ 25°C 0 0 0 0 ns min Address to Write Hold Time
T
MIN
to T
MAX
0 0 0 0 ns min
t
4
@ 25°C 70 70 70 70 ns min Data Valid to Write Setup Time
T
MIN
to T
MAX
90 90 90 90 ns min
t
5
@ 25°C 10 10 10 10 ns min Data Valid to Write Hold Time
T
MIN
to T
MAX
10 10 10 10 ns min
t
6
@ 25°C 95 95 95 95 ns min Load DAC Pulse Width
T
MIN
to T
MAX
120 120 150 150 ns min
NOTES
1
Maximum possible reference voltage.
2
Temperature ranges are as follows:
K, L Versions: –40°C to +85°C
B, C Versions: –40°C to +85°C
T, U Versions: –55°C to +125°C
3
Sample Tested at 25°C to ensure compliance.
4
Switching characteristics apply for single and dual supply operation.
Specifications subject to change without notice.
(VDD = 11.4 V to 16.5 V, VSS = –5 V 6 10%; AGND = DGND = O V; VREF = +2 V to (VDD – 4 V)1 unless otherwise noted.
All specifications TMIN to TMAX unless otherwise noted.)
ORDERING GUIDE
Total
Temperature Unadjusted Package
Model
1
Range Error Option
2
AD7225KN –40°C to +85°C±2 LSB N-24
AD7225LN –40°C to +85°C±1 LSB N-24
AD7225KP –40°C to +85°C±2 LSB P-28A
AD7225LP –40°C to +85°C±1 LSB P-28A
AD7225KR –40°C to +85°C±2 LSB R-24
AD7225LR –40°C to +85°C±1 LSB R-24
AD7225BQ –40°C to +85°C±2 LSB Q-24
AD7225CQ –40°C to +85°C±1 LSB Q-24
Total
Temperature Unadjusted Package
Model
1
Range Error Option
2
AD7225TQ –55°C to +125°C±2 LSB Q-24
AD7225UQ –55°C to +125°C±1 LSB Q-24
AD7225TE –55°C to +125°C±2 LSB E-28A
AD7225UE –55°C to +125°C±1 LSB E-28A
NOTES
1
To order MIL-STD-883 processed parts, add /883B to part number. Contact your
local sales office for military data sheet.
2
E = Leadless Ceramic Chip Carrier; N = Plastic DIP;
P = Plastic Leaded Chip Carrier; Q = Cerdip; R = SOIC.
AD7225
REV. B –3–
SINGLE SUPPLY
K, B L, C
Parameter Versions
2
Versions
2
T Version U Version Units Conditions/Comments
STATIC PERFORMANCE
Resolution 8888Bits
Total Unadjusted Error
3
±2±1±2±1 LSB max
Differential Nonlinearity
3
±1±1±1±1 LSB max Guaranteed Monotonic
REFERENCE INPUT
Input Resistance 11 11 11 11 kΩ min
Input Capacitance
4
100 100 100 100 pF max Occurs when each DAC is loaded with all 1s.
Channel-to-Channel Isolation
3, 4
60 60 60 60 dB min V
REF
= 10 V p-p Sine Wave @ 10 kHz
AC Feedthrough
3, 4, 5
–70 –70 –70 –70 dB max V
REF
= 10 V p-p Sine Wave @ 10 kHz
DIGITAL INPUTS
Input High Voltage, V
INH
2.4 2.4 2.4 2.4 V min
Input Low Voltage, V
INL
0.8 0.8 0.8 0.8 V max
Input Leakage Current ±1±1±1±1µA max V
IN
= 0 V or V
DD
Input Capacitance
4
8888pF max
Input Coding Binary Binary Binary Binary
DYNAMIC PERFORMANCE
Voltage Output Slew Rate
4
2222V/µs min
Voltage Output Settling Time
4
Positive Full-Scale Change 5555µs max Settling Time to ±1/2 LSB
Negative Full-Scale Change 7777µs max Settling Time to ±1/2 LSB
Digital Feedthrough
3, 4
50 50 50 50 nV secs typ Code transition all 0s to all 1s.
Digital Crosstalk
3, 4
50 50 50 50 nV secs typ Code transition all 0s to all 1s.
Minimum Load Resistance 2222kΩ min V
OUT
= +10 V
POWER SUPPLIES
V
DD
Range 14.25/15.75 14.25/15.75 14.25/15.75 14.25/15.75 V min to V
max For Specified Performance
I
DD
10 10 12 12 mA max Outputs Unloaded; V
IN
= V
INL
or V
INH
SWITCHING CHARACTERISTICS
4
t
1
@ 25°C 95 95 95 95 ns min Write Pulse Width
T
MIN
to T
MAX
120 120 150 150 ns min
t
2
@ 25°C 0000ns minAddress to Write Setup Time
T
MIN
to T
MAX
0000ns min
t
3
@ 25°C 0000ns minAddress to Write Hold Time
T
MIN
to T
MAX
0000ns min
t
4
@ 25°C 70 70 70 70 ns min Data Valid to Write Setup Time
T
MIN
to T
MAX
90 90 90 90 ns min
t
5
@ 25°C 10 10 10 10 ns min Data Valid to Write Hold Time
T
MIN
to T
MAX
10 10 10 10 ns min
t
6
@ 25°C 95 95 95 95 ns min Load DAC Pulse Width
T
MIN
to T
MAX
120 120 150 150 ns min
NOTES
1
Maximum possible reference voltage.
2
Temperature ranges are as follows:
K, L Versions: –40°C to +85°C
B, C Versions: –40°C to +85°C
T, U Versions: –55°C to +125°C
(VDD = +15 V 6 5%; VSS = AGND = DGND = O V; VREF = +10 V1 unless otherwise noted.
All specifications TMIN to TMAX unless otherwise noted.)
3
Sample Tested at 25°C to ensure compliance.
4
Switching characteristics apply for single and dual supply operation.
Specifications subject to change without notice.
AD7225
REV. B
–4–
ABSOLUTE MAXIMUM RATINGS
1
V
DD
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V, +17 V
V
DD
to DGND . . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V, +17 V
V
DD
to V
SS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .–0.3 V, +24 V
AGND to DGND . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, V
DD
Digital Input Voltage to DGND . . . . . . . –0.3 V, V
DD
+ 0.3 V
V
REF
to AGND . . . . . . . . . . . . . . . . . . . . –0.3 V, V
DD
+ 0.3 V
V
OUT
to AGND
2
. . . . . . . . . . . . . . . . . . . . . . . . . . . . V
SS
, V
DD
Power Dissipation (Any Package) to +75°C . . . . . . . . 500 mW
Derates above 75°C by . . . . . . . . . . . . . . . . . . . . . 2.0 mW/°C
Operating Temperature
Commercial (K, L Versions) . . . . . . . . . . . –40°C to +85°C
Industrial (B, C Versions) . . . . . . . . . . . . . –40°C to +85°C
Extended (T, U Versions) . . . . . . . . . . . . –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 indicated in
the operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
2
Outputs may be shorted to any voltage in the range V
SS
to V
DD
provided that the
power dissipation of the package is not exceeded. Typical short circuit current for
a short to AGND or V
SS
is 50 mA.
WARNING!
ESD SENSITIVE DEVICE
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 AD7225 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.
PIN CONFIGURATIONS
DIP and SOIC LCCC PLCC
TERMINOLOGY
TOTAL UNADJUSTED ERROR
Total Unadjusted Error is a comprehensive specification which
includes full-scale error, relative accuracy, and zero code error.
Maximum output voltage is V
REF
– 1 LSB (ideal), where 1 LSB
(ideal) is V
REF
/256. The LSB size will vary over the V
REF
range.
Hence the zero code error will, relative to the LSB size, increase
as V
REF
decreases. Accordingly, the total unadjusted error,
which includes the zero code error, will also vary in terms of
LSBs over the V
REF
range. As a result, total unadjusted error is
specified for a fixed reference voltage of +10 V.
RELATIVE ACCURACY
Relative Accuracy or endpoint nonlinearity is a measure of the
maximum deviation from a straight line passing through the
endpoints of the DAC transfer function. It is measured after al-
lowing for zero code error and full-scale error and is normally
expressed in LSBs or as a percentage of full-scale reading.
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.
DIGITAL FEEDTHROUGH
Digital Feedthrough is the glitch impulse transferred to the out-
put of the DAC due to a change in its digital input code. It is
specified in nV secs and is measured at V
REF
= 0 V.
DIGITAL CROSSTALK
Digital Crosstalk is the glitch impulse transferred to the output
of one converter (not addressed) due to a change in the digital
input code to another addressed converter. It is specified in
nV secs and is measured at V
REF
= 0 V.
AC FEEDTHROUGH
AC Feedthrough is the proportion of reference input signal
which appears at the output of a converter when that DAC is
loaded with all 0s.
CHANNEL-TO-CHANNEL ISOLATION
Channel-to-channel isolation is the proportion of input signal
from the reference of one DAC (loaded with all 1s) which ap-
pears at the output of one of the other three DACs (loaded with
all 0s) The figure given is the worst case for the three other out-
puts and is expressed as a ratio in dBs.
FULL-SCALE ERROR
Full-Scale Error is defined as:
Measured Value – Zero Code Error – Ideal Value
AD7225
REV. B –5–
Typical Performance Characteristics–
TA = 258C, VDD = +15 V, VSS = –5 V unless otherwise noted.
Figure 1. Channel-to-Channel Matching
Figure 3. Differential Nonlinearity vs. V
REF
Figure 5. Zero Code Error vs. Temperature
Figure 2. Relative Accuracy vs. V
REF
Figure 4. Power Supply Current vs. Temperature
Figure 6. Broadband Noise
AD7225
REV. B
–6–
CIRCUIT INFORMATION
D/A SECTION
The AD7225 contains four, identical, 8-bit voltage mode
digital-to-analog converters. Each D/A converter has a separate
reference input. The output voltages from the converters have
the same polarity as the reference voltages, allowing single sup-
ply operation. A novel DAC switch pair arrangement on the
AD7225 allows a reference voltage range from +2 V to +12.5 V
on each reference input.
Each DAC consists of a highly stable, thin-film, R-2R ladder
and eight high speed NMOS, single-pole, double-throw
switches. The simplified circuit diagram for channel A is shown
in Figure 7. Note that AGND (Pin 6) is common to all four
DACs.
Figure 7. D/A Simplified Circuit Diagram
The input impedance at any of the reference inputs is code de-
pendent and can vary from 11 kΩ minimum to infinity. The
lowest input impedance at any reference input occurs when that
DAC is loaded with the digital code 01010101. Therefore, it is
important that the reference presents a low output impedance
under changing load conditions. The nodal capacitance at the
reference terminals is also code dependent and typically varies
from 15 pF to 35 pF.
Each V
OUT
pin can be considered as a digitally programmable
voltage source with an output voltage of:
V
OUTX
= D
X
• V
REFX
where D
X
is fractional representation of the digital input code
and can vary from 0 to 255/256.
The output impedance is that of the output buffer amplifier.
OP-AMP SECTION
Each voltage mode D/A converter output is buffered by a unity
gain noninverting CMOS amplifier. This buffer amplifier is ca-
pable of developing +10 V across a 2 kΩ load and can drive ca-
pacitive loads of 3300 pF.
The AD7225 can be operated single or dual supply; operating
with dual supplies results in enhanced performance in some pa-
rameters which cannot be achieved with single supply operation.
In single supply operation (V
SS
= 0 V = AGND) the sink capa-
bility of the amplifier, which is normally 400 µA, is reduced as
the output voltage nears AGND. The full sink capability of
400 µA is maintained over the full output voltage range by tying
V
SS
to –5 V. This is indicated in Figure 8.
Settling-time for negative-going output signals approaching
AGND is similarly affected by V
SS
. Negative-going settling-time
for single supply operation is longer than for dual supply opera-
tion. Positive-going settling-time is not affected by V
SS
.
Figure 8. Variation of I
SINK
with V
OUT
Additionally, the negative V
SS
gives more headroom to the out-
put amplifiers which results in better zero code performance and
improved slew rate at the output, than can be obtained in the
single supply mode.
DIGITAL SECTION
The AD7225 digital inputs are compatible with either TTL or
5 V CMOS levels. All logic inputs are static protected MOS
gates with typical input currents of less than 1 nA. Internal in-
put protection is achieved by an on-chip distributed diode be-
tween DGND and each MOS gate. To minimize power supply
currents, it is recommended that the digital input voltages be
driven as close to the supply rails (V
DD
and DGND) as practi-
cally possible.
INTERFACE LOGIC INFORMATION
The AD7225 contains two registers per DAC, an input register
and a DAC register. Address lines A0 and A1 select which input
register will accept data from the input port. When the WR sig-
nal is LOW, the input latches of the selected DAC are transpar-
ent. The data is latched into the addressed input register on the
rising edge of WR. Table I shows the addressing for the input
registers on the AD7225.
Table I. AD7225 Addressing
A1 A0 Selected Input Register
L L DAC A Input Register
L H DAC B Input Register
H L DAC C Input Register
H H DAC D Input Register
AD7225
REV. B –7–
Only the data held in the DAC register determines the analog
output of the converter. The LDAC signal is common to all four
DACs and controls the transfer of information from the input
registers to the DAC registers. Data is latched into all four DAC
registers simultaneously on the rising edge of LDAC. The
LDAC signal is level triggered and therefore the DAC registers
may be made transparent by tying LDAC LOW (in this case the
outputs of the converters will respond to the data held in their
respective input latches). LDAC 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. In other
words, if LDAC is activated prior to the rising edge of WR (or
WR occurs during LDAC), then LDAC must stay LOW for t
6
or longer after WR goes HIGH to ensure correct data is latched
through to the output. Table II shows the truth table for AD7225
operation. Figure 9 shows the input control logic for the part
and the write cycle timing diagram is given in Figure 10.
Table II. AD7225 Truth Table
WR LDAC Function
H H No Operation. Device not selected
L H Input Register of Selected DAC Transparent
g
H Input Register of Selected DAC Latched
H L All Four DAC Registers Transparent
(i.e. Outputs respond to data held in respective
input registers)
Input Registers are Latched
H
g
All Four DAC Registers Latched
L L DAC Registers and Selected Input Register
Transparent Output follows Input Data for
Selected Channel.
Figure 9. Input Control Logic
Figure 10. Write Cycle Timing Diagram
GROUND MANAGEMENT AND LAYOUT
Since the AD7225 contains four reference inputs which can be
driven from ac sources (see AC REFERENCE SIGNAL sec-
tion) careful layout and grounding is important to minimize
analog crosstalk between the four channels. The dynamic per-
formance of the four DACs depends upon the optimum choice
of board layout. Figure 11 shows the relationship between input
Figure 11. Channel-to-Channel Isolation
Figure 12. Suggested PCB Layout for AD7225.
Layout Shows Component Side (Top View)
frequency and channel-to-channel isolation. Figure 12 shows a
printed circuit board layout which is aimed at minimizing
crosstalk and feedthrough. The four input signals are screened
by AGND. V
REF
was limited to between 2 V and 3.24 V to
avoid slew rate limiting effects from the output amplifier during
measurements.
AD7225
REV. B
–8–
SPECIFICATION RANGES
For the AD7225 to operate to rated specifications, its input ref-
erence voltage must be at least 4 V below the V
DD
power supply
voltage. This voltage differential is the overhead voltage re-
quired by the output amplifiers.
The AD7225 is specified to operate over a V
DD
range from
+12 V ± 5% to +15 V ±10% (i.e., from +11.4 V to +16.5 V)
with a V
SS
of –5 V ±10%. Operation is also specified for a single
+15 V ± 5% V
DD
supply. Applying a V
SS
of –5 V results in im-
proved zero code error, improved output sink capability with
outputs near AGND and improved negative going settling time.
Performance is specified over a wide range of reference voltages
from 2 V to (V
DD
– 4 V) with dual supplies. This allows a range
of standard reference generators to be used such as the AD580,
a +2.5 V bandgap reference and the AD584, a precision +10 V
reference. Note that an output voltage range of 0 V to +10 V re-
quires a nominal +15 V ± 5% power supply voltage.
UNIPOLAR OUTPUT OPERATION
This is the basic mode of operation for each channel of the
AD7225, with the output voltage having the same positive
polarity as V
REF
. The AD7225 can be operated single supply
(V
SS
= AGND) or with positive/negative supplies (see op-amp
section which outlines the advantages of having negative V
SS
).
Connections for the unipolar output operation are shown in Fig-
ure 13. The voltage at any of the reference inputs must never be
negative with respect to DGND. Failure to observe this precau-
tion may cause parasitic transistor action and possible device de-
struction. The code table for unipolar output operation is shown
in Table III.
Figure 13. Unipolar Output Circuit
Table III. Unipolar Code Table
DAC Latch Contents
MSB LSB Analog Output
1 1 1 1 1 1 1 1
+V
REF
255
256
1 0 0 0 0 0 0 1
+V
REF
129
256
1 0 0 0 0 0 0 0
+V
REF
128
256
=+V
REF
2
0 1 1 1 1 1 1 1
+V
REF
127
256
0 0 0 0 0 0 0 1
+V
REF
1
256
0 0 0 0 0 0 0 0 0 V
Note: 1 LSB =V
REF
()
2
−8
()
=V
REF
1
256
BIPOLAR OUTPUT OPERATION
Each of the DACs of the AD7225 can be individually config-
ured to provide bipolar output operation. This is possible using
one external amplifier and two resistors per channel. Figure 14
shows a circuit used to implement offset binary coding (bipolar
operation) with DAC A of the AD7225. In this case
V
OUT
=1+R2
R1
⋅D
A
V
REF
()
–
R2
R1
⋅V
REF
()
With R1 = R2V
OUT
= (2 D
A
– 1) • V
REF
where D
A
is a fractional representation of the digital word in
latch A. (0 ≤ D
A
≤ 255/256)
Mismatch between R1 and R2 causes gain and offset errors and,
therefore, these resistors must match and track over tempera-
ture. Once again the AD7225 can be operated in single supply
or from positive/negative supplies. Table IV shows the digital
code versus output voltage relationship for the circuit of Figure
14 with R1 = R2.
AD7225
REV. B –9–
Figure 14. AD7225 Bipolar Output Circuit
Table IV. Bipolar (Offset Binary) Code Table
DAC Latch Contents
MSB LSB Analog Output
1 1 1 1 1 1 1 1
+V
REF
127
128
1 0 0 0 0 0 0 1
+V
REF
1
128
1 0 0 0 0 0 0 0 0 V
0 1 1 1 1 1 1 1
–V
REF
1
128
0 0 0 0 0 0 0 1
–V
REF
127
128
0 0 0 0 0 0 0 0
–V
REF
128
128
=–V
REF
AGND BIAS
The AD7225 AGND pin can be biased above system GND
(AD7225 DGND) to provide an offset “zero” analog output
voltage level. Figure 15 shows a circuit configuration to achieve
this for channel A of the AD7225. The output voltage, V
OUT
A,
can be expressed as:
V
OUT
A = V
BIAS
+ D
A
(V
IN
)
where D
A
is a fractional representation of the digital word in
DAC latch A. (0 ≤ D
A
≤ 255/256).
Figure 15. AGND Bias Circuit
For a given V
IN
, increasing AGND above system GND will re-
duce the effective V
DD
–V
REF
which must be at least 4 V to en-
sure specified operation. Note that because the AGND pin is
common to all four DACs, this method biases up the output
voltages of all the DACs in the AD7225. Note that V
DD
and V
SS
of the AD7225 should be referenced to DGND.
AC REFERENCE SIGNAL
In some applications it may be desirable to have ac reference
signals. The AD7225 has multiplying capability within the up-
per (V
DD
– 4 V) and lower (2 V) limits of reference voltage when
operated with dual supplies. Therefore ac signals need to be ac
coupled and biased up before being applied to the reference in-
puts. Figure 16 shows a sine wave signal applied to V
REF
A. For
input signal frequencies up to 50 kHz the output distortion typi-
cally remains less than 0.1%. The typical 3 dB bandwidth figure
for small signal inputs is 800 kHz.
Figure 16. Applying an AC Signal to the AD7225
APPLICATIONS
PROGRAMMABLE TRANSVERSAL FILTER
A discrete-time filter may be described by either multiplication
in the frequency domain or convolution in the time domain i.e.
Yω
()
=Hω
()
Xω
()
or y
n
=∑
k=1
N
h
kXn–k+1
The convolution sum may be implemented using the special
structure known as the transversal filter (Figure 17). Basically, it
consists of an N-stage delay line with N taps weighted by N co-
efficients, the resulting products being accumulated to form the
output. The tap weights or coefficients h
k
are actually the non-
zero elements of the impulse response and therefore determine
the filter transfer function. A particular filter frequency response
is realized by setting the coefficients to the appropriate values.
This property leads to the implementation of transversal filters
whose frequency response is programmable.
Figure 17. Transversal Filter
AD7225
REV. B
–10–
FILTER
I/P I/P
SAMPLES
Am29520
TLD
AD7820
ADC
SAMPLES
AD7225
QUAD DAC
DELAYED
I/P
AD7226
QUAD DAC
VREF A
h1h2
VOUT A
VOUT A
VOUT B
VOUT C
VOUT D
ACCUMULATOR
O/P
AD585
SHA
FILTER
O/P
TAP WEIGHTS
Am7224
DAC
AD584
REF
GAIN SET
+10V VOUT
VREF
VREF
+
T T T
1234
h
4
h
3
h
2
h
1
+
FILTER
O/P Yn
FILTER
I/P
Xn–1 Xn–2
XnXn–3
VREF A
h3
VOUT A
VREF A
h4
VOUT A
VREF A
VOUT A
Figure 18. Programmable Transversal Filter
A 4-tap programmable transversal filter may be implemented
using the AD7225 (Figure 18). The input signal is first sampled
and converted to allow the tapped delay line function to be pro-
vided by the Am29520. The multiplication of delayed input
samples by fixed, programmable up weights is accomplished by
the AD7225, the four coefficients or reference inputs being set
by the digital codes stored in the AD7226. The resultant prod-
ucts are accumulated to yield the convolution sum output
sample which is held by the AD585.
0
–100 0.5
–70
–90
0.05
–80
0
–40
–60
–50
–30
–20
–10
0.450.40.350.30.250.20.150.1
NORMALIZED FREQUENCY – f/fs
GAIN – dB
h
1
= 0.117
h
2
= 0.417
h
3
= 0.417
h
4
= 0.417
Figure 19. Predicted (Theoretical) Response
Figure 20. Actual Response
Low pass, bandpass and high pass filters may be synthesized us-
ing this arrangement. The particular up weights needed for any
desired transfer function may be obtained using the standard
Remez Exchange Algorithm. Figure 19 shows the theoretical
low pass frequency response produced by a 4-tap transversal
filter with the coefficients indicated. Although the theoretical
prediction does not take into account the quantization of the in-
put samples and the truncation of the coefficients, nevertheless,
there exists a good correlation with the actual performance of
the transversal filter (Figure 20).
DIGITAL WORD MULTIPLICATION
Since each DAC of the AD7225 has a separate reference input,
the output of one DAC can be used as the reference input for
another. This means that multiplication of digital words can be
performed (with the result given in analog form). For example,
if the output from DACA is applied to V
REF
B then the output
from DACB, V
OUT
B, can be expressed as:
V
OUT
B = D
A
• D
B
• V
REF
A
where D
A
and D
B
are the fractional representations of the
digital words in DAC latches A and B respectively.
If D
A
= D
B
= D then the result is D
2
• V
REF
A
In this manner, the four DACs can be used on their own or in
conjunction with an external summing amplifier to generate
complex waveforms. Figure 21 shows one such application. In
this case the output waveform, Y, is represented by:
Y = –(x
4
+ 2x
3
+ 3x
2
+ 2x + 4) • V
IN
where x is the digital code which is applied to all four DAC
latches.
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
V
REF
A
V
REF
B
V
REF
C
V
REF
D
+15V
V
DD
AD7225*
DGNDAGND V
SS
V
IN
25kΩ
50kΩ
33kΩ
50kΩ
100kΩ
100kΩ
Y
*DIGITAL INPUTS OMITTED
FOR CLARITY
Figure 21. Complex Waveform Generation
AD7225
REV. B –11–
MICROPROCESSOR INTERFACE
8085A/
8088
A15
A8
ALE
AD0
AD7
ADDRESS
DECODE
LATCH
EN
AD7225*
WR
A0
A1
DB7
DB0
LDAC
WR
ADDRESS BUS
ADDRESS DATA BUS
*
LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 22. AD7225 to 8085A/8088 Interface,
Double-Buffered Mode
6809/
6502
A15
A0
E OR φ2
D0
D7
AD7225*
R/W
A0
A1
DB7
DB0
LDAC
WR
ADDRESS BUS
DATA BUS
*
LINEAR CIRCUITRY OMITTED FOR CLARITY
ADDRESS
DECODE
EN
Figure 23. AD7225 to 6809/6502 Interface,
Single-Buffered Mode
Z-80
A15
A8
D0
D7
AD7225*
WR
A0
A1
DB7
DB0
LDAC
WR
ADDRESS BUS
DATA BUS
*
LINEAR CIRCUITRY OMITTED FOR CLARITY
ADDRESS
DECODE
EN
MREQ
Figure 24. AD7225 to Z-80 Interface,
Double-Buffered Mode
68008
A23
A1
D0
D7
AD7225*
R/W
A0
A1
DB7
DB0
LDAC
WR
ADDRESS BUS
DATA BUS
*LINEAR CIRCUITRY OMITTED FOR CLARITY
ADDRESS
DECODE
EN
AS
DTACK
Figure 25. AD7225 to 68008 Interface,
Single-Buffered Mode
V
SS
GENERATION
Operating the AD7225 from dual supplies results in enhanced
performance over single supply operation on a number of pa-
rameters as previously outlined. Some applications may require
this enhanced performance, but may only have a single power
supply rail available. The circuit of Figure 26 shows a method of
generating a negative voltage using one CD4049, operated from
a V
DD
of +15 V. Two inverters of the hex inverter chip are used
as an oscillator. The other four inverters are in parallel and used
as buffers for higher output current. The square-wave output is
level translated to a negative-going signal, then rectified and fil-
tered. The circuit configuration shown will provide an output
voltage of –5.1 V for current loadings in the range 0.5 mA to
9 mA. This will satisfy the AD7225 I
SS
requirement over the
commercial operating temperature range.
1/6
CD4049AE
1/6
CD4049AE
1/6
CD4049AE
1/6
CD4049AE
1N4001
510Ω
1/6
CD4049AE
5.1k
1/6
CD4049AE
510k
+
+
–V
OUT
5V1
47µF
1N4001
47µF
0.02µF
Figure 26. V
SS
Generation Circuit
AD7225
REV. B
–12–
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C927a–5–5/86
PRINTED IN U.S.A.
24-Pin Plastic (N-24)
24-Pin Cerdip (Q-24)
28-Lead PLCC (P-28A)
24-Lead SOIC (R-24)
28-Terminal Leadless
Ceramic Chip Carrier (E-28A)
