AD AD7225TQ

a
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
LC2MOS Quad 8-Bit DAC
with Separate Reference Inputs
AD7225
FUNCTIONAL BLOCK DIAGRAM
GENERAL DESCRIPTION
The AD7225 contains four 8-bit voltage output digital-toanalog 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 required to achieve full specified performance for the part.
The double-buffered interface logic consists of two 8-bit registers 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 determines the analog outputs of the converters. The doublebuffering 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.
PRODUCT HIGHLIGHTS
1. DACs and Amplifiers on CMOS Chip
The single-chip design of four 8-bit DACs and amplifiers allows a dramatic reduction in board space requirements and
offers increased reliability in systems using multiple converters. 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.
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.
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 interface allows simultaneous update of the four outputs.
The AD7225 is fabricated on an all ion-implanted high-speed
Linear Compatible CMOS (LC2MOS) process which has been
specifically developed to integrate high speed digital logic circuits and precision analog circuitry on the same chip.
4. Separate Reference Input for Each DAC
The AD7225 offers great flexibility in dealing with input signals with a separate reference input provided for each DAC
and each reference having variable input voltage capability.
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
AD7225–SPECIFICATIONS
DUAL SUPPLY
(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.)
Parameter
K, B
Versions2
L, C
Versions2
T Version
U Version
Units
STATIC PERFORMANCE
Resolution
Total Unadjusted Error
Relative Accuracy
Differential Nonlinearity
Full-Scale Error
Full-Scale Temp. Coeff.
Zero Code Error @ 25°C
TMIN to TMAX
Zero Code Error Temp Coeff.
8
±2
±1
±1
±1
±5
± 25
± 30
± 30
8
±1
± 1/2
±1
± 1/2
±5
± 15
± 20
± 30
8
±2
±1
±1
±1
±5
± 25
± 30
± 30
8
±1
± 1/2
±1
± 1/2
±5
± 15
± 20
± 30
Bits
LSB max
LSB max
LSB max
LSB max
ppm/°C typ
mV max
mV max
µV/°C typ
REFERENCE INPUT
Voltage Range
Input Resistance
Input Capacitance 3
Channel-to-Channel Isolation 3
AC Feedthrough 3
2 to (VDD – 4)
11
100
60
–70
2 to (VDD – 4)
11
100
60
–70
2 to (VDD – 4)
11
100
60
–70
2 to (VDD – 4)
11
100
60
–70
V min to V max
kΩ min
pF max
Occurs when each DAC is loaded with all 1s.
dB min
VREF = 10 V p-p Sine Wave @ 10 kHz
dB max
VREF = 10 V p-p Sine Wave @ 10 kHz
DIGITAL INPUTS
Input High Voltage, V INH
Input Low Voltage, V INL
Input Leakage Current
Input Capacitance 3
Input Coding
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
V min
V max
µA max
pF max
Conditions/Comments
VDD = +15 V ± 5%, VREF = +10 V
Guaranteed Monotonic
VDD = 14 V to 16.5 V, V REF = +10 V
VIN = 0 V or VDD
DYNAMIC PERFORMANCE
Voltage Output Slew Rate 3
Voltage Output Settling Time 3
Positive Full-Scale Change
Negative Full-Scale Change
Digital Feedthrough 3
Digital Crosstalk 3
Minimum Load Resistance
2.5
2.5
2.5
2.5
V/µs min
5
5
50
50
2
5
5
50
50
2
5
5
50
50
2
5
5
50
50
2
µs max
µs max
nV secs typ
nV secs typ
kΩ min
POWER SUPPLIES
VDD Range
IDD
ISS
11.4/16.5
10
9
11.4/16.5
10
9
11.4/16.5
12
10
11.4/16.5
12
10
V min to V max For Specified Performance
mA max
Outputs Unloaded; V IN = VINL or VINH
mA max
Outputs Unloaded; V IN = VINL or VINH
95
120
95
150
95
150
ns min
ns min
Write Pulse Width
0
0
0
0
0
0
ns min
ns min
Address to Write Setup Time
0
0
0
0
0
0
ns min
ns min
Address to Write Hold Time
70
90
70
90
70
90
ns min
ns min
Data Valid to Write Setup Time
10
10
10
10
10
10
ns min
ns min
Data Valid to Write Hold Time
95
120
95
150
95
150
ns min
ns min
Load DAC Pulse Width
SWITCHING CHARACTERISTICS 3, 4
t1
@ 25°C
95
TMIN to TMAX
120
t2
@ 25°C
0
TMIN to TMAX
0
t3
@ 25°C
0
TMIN to TMAX
0
t4
@ 25°C
70
TMIN to TMAX
90
t5
@ 25°C
10
TMIN to TMAX
10
t6
@ 25°C
95
TMIN to TMAX
120
VREF = +10 V; Settling Time to ± 1/2 LSB
VREF = +10 V; Settling Time to ± 1/2 LSB
Code transition all 0s to all 1s.
Code transition all 0s to all 1s.
VOUT = +10 V
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.
–2–
REV. B
AD7225
SINGLE SUPPLY
(VDD = +15 V 6 5%; VSS = AGND = DGND = O V; VREF = +10 V1 unless otherwise noted.
All specifications TMIN to TMAX unless otherwise noted.)
Parameter
K, B
Versions2
L, C
Versions2
T Version
U Version
Units
Conditions/Comments
STATIC PERFORMANCE
Resolution
Total Unadjusted Error 3
Differential Nonlinearity 3
8
±2
±1
8
±1
±1
8
±2
±1
8
±1
±1
Bits
LSB max
LSB max
Guaranteed Monotonic
REFERENCE INPUT
Input Resistance
Input Capacitance 4
Channel-to-Channel Isolation 3, 4
AC Feedthrough3, 4, 5
11
100
60
–70
11
100
60
–70
11
100
60
–70
11
100
60
–70
kΩ min
pF max
dB min
dB max
Occurs when each DAC is loaded with all 1s.
VREF = 10 V p-p Sine Wave @ 10 kHz
VREF = 10 V p-p Sine Wave @ 10 kHz
DIGITAL INPUTS
Input High Voltage, V INH
Input Low Voltage, V INL
Input Leakage Current
Input Capacitance 4
Input Coding
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
2.4
0.8
±1
8
Binary
V min
V max
µA max
pF max
2
2
2
2
V/µs min
5
7
50
50
2
5
7
50
50
2
5
7
50
50
2
5
7
50
50
2
µs max
µs max
nV secs typ
nV secs typ
kΩ min
14.25/15.75
10
14.25/15.75
10
14.25/15.75
12
14.25/15.75
12
V min to V max For Specified Performance
mA max
Outputs Unloaded; V IN = VINL or VINH
95
120
95
150
95
150
ns min
ns min
Write Pulse Width
0
0
0
0
0
0
ns min
ns min
Address to Write Setup Time
0
0
0
0
0
0
ns min
ns min
Address to Write Hold Time
70
90
70
90
70
90
ns min
ns min
Data Valid to Write Setup Time
10
10
10
10
10
10
ns min
ns min
Data Valid to Write Hold Time
95
120
95
150
95
150
ns min
ns min
Load DAC Pulse Width
DYNAMIC PERFORMANCE
Voltage Output Slew Rate 4
Voltage Output Settling Time 4
Positive Full-Scale Change
Negative Full-Scale Change
Digital Feedthrough 3, 4
Digital Crosstalk 3, 4
Minimum Load Resistance
POWER SUPPLIES
VDD Range
IDD
SWITCHING CHARACTERISTICS 4
t1
@ 25°C
95
TMIN to TMAX
120
t2
@ 25°C
0
TMIN to TMAX
0
t3
@ 25°C
0
TMIN to TMAX
0
t4
@ 25°C
70
TMIN to TMAX
90
t5
@ 25°C
10
TMIN to TMAX
10
t6
@ 25°C
95
TMIN to TMAX
120
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
VIN = 0 V or VDD
Settling Time to ± 1/2 LSB
Settling Time to ± 1/2 LSB
Code transition all 0s to all 1s.
Code transition all 0s to all 1s.
VOUT = +10 V
3
Sample Tested at 25°C to ensure compliance.
Switching characteristics apply for single and dual supply operation.
Specifications subject to change without notice.
4
ORDERING GUIDE
Model1
Temperature
Range
Total
Unadjusted
Error
Package
Option2
AD7225KN
AD7225LN
AD7225KP
AD7225LP
AD7225KR
AD7225LR
AD7225BQ
AD7225CQ
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
± 2 LSB
± 1 LSB
± 2 LSB
± 1 LSB
± 2 LSB
± 1 LSB
± 2 LSB
± 1 LSB
N-24
N-24
P-28A
P-28A
R-24
R-24
Q-24
Q-24
REV. B
Model1
Temperature
Range
Total
Unadjusted
Error
Package
Option2
AD7225TQ
AD7225UQ
AD7225TE
AD7225UE
–55°C to +125°C
–55°C to +125°C
–55°C to +125°C
–55°C to +125°C
± 2 LSB
± 1 LSB
± 2 LSB
± 1 LSB
Q-24
Q-24
E-28A
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.
–3–
AD7225
ABSOLUTE MAXIMUM RATINGS 1
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
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, +17 V
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, +17 V
VDD to VSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, +24 V
AGND to DGND . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V, VDD
Digital Input Voltage to DGND . . . . . . . –0.3 V, VDD + 0.3 V
VREF to AGND . . . . . . . . . . . . . . . . . . . . –0.3 V, VDD + 0.3 V
VOUT to AGND2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . VSS, VDD
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
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 VDD 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.
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.
DIP and SOIC
PIN CONFIGURATIONS
LCCC
WARNING!
ESD SENSITIVE DEVICE
PLCC
DIGITAL FEEDTHROUGH
TERMINOLOGY
TOTAL UNADJUSTED ERROR
Digital Feedthrough is the glitch impulse transferred to the output of the DAC due to a change in its digital input code. It is
specified in nV secs and is measured at VREF = 0 V.
Total Unadjusted Error is a comprehensive specification which
includes full-scale error, relative accuracy, and zero code error.
Maximum output voltage is VREF – 1 LSB (ideal), where 1 LSB
(ideal) is VREF/256. The LSB size will vary over the VREF range.
Hence the zero code error will, relative to the LSB size, increase
as VREF decreases. Accordingly, the total unadjusted error,
which includes the zero code error, will also vary in terms of
LSBs over the VREF range. As a result, total unadjusted error is
specified for a fixed reference voltage of +10 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 VREF = 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.
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 allowing for zero code error and full-scale error and is normally
expressed in LSBs or as a percentage of full-scale reading.
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 appears 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 outputs and is expressed as a ratio in dBs.
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.
FULL-SCALE ERROR
Full-Scale Error is defined as:
Measured Value – Zero Code Error – Ideal Value
–4–
REV. B
Typical Performance Characteristics–AD7225
TA = 258C, VDD = +15 V, VSS = –5 V unless otherwise noted.
REV. B
Figure 1. Channel-to-Channel Matching
Figure 2. Relative Accuracy vs. VREF
Figure 3. Differential Nonlinearity vs. VREF
Figure 4. Power Supply Current vs. Temperature
Figure 5. Zero Code Error vs. Temperature
Figure 6. Broadband Noise
–5–
AD7225
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 supply 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 8. Variation of ISINK with VOUT
Additionally, the negative VSS gives more headroom to the output amplifiers which results in better zero code performance and
improved slew rate at the output, than can be obtained in the
single supply mode.
Figure 7. D/A Simplified Circuit Diagram
The input impedance at any of the reference inputs is code dependent 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.
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 input protection is achieved by an on-chip distributed diode between 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 (VDD and DGND) as practically possible.
Each VOUT pin can be considered as a digitally programmable
voltage source with an output voltage of:
VOUTX = DX • VREFX
where DX is fractional representation of the digital input code
and can vary from 0 to 255/256.
INTERFACE LOGIC INFORMATION
The output impedance is that of the output buffer amplifier.
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 signal is LOW, the input latches of the selected DAC are transparent. 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.
OP-AMP SECTION
Each voltage mode D/A converter output is buffered by a unity
gain noninverting CMOS amplifier. This buffer amplifier is capable of developing +10 V across a 2 kΩ load and can drive capacitive loads of 3300 pF.
The AD7225 can be operated single or dual supply; operating
with dual supplies results in enhanced performance in some parameters which cannot be achieved with single supply operation.
In single supply operation (VSS = 0 V = AGND) the sink capability 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
VSS to –5 V. This is indicated in Figure 8.
Table I. AD7225 Addressing
A1
A0
Selected Input Register
L
L
H
H
L
H
L
H
DAC A Input Register
DAC B Input Register
DAC C Input Register
DAC D Input Register
Settling-time for negative-going output signals approaching
AGND is similarly affected by VSS. Negative-going settling-time
for single supply operation is longer than for dual supply operation. Positive-going settling-time is not affected by VSS.
–6–
REV. B
AD7225
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 t6
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.
Figure 9. Input Control Logic
Table II. AD7225 Truth Table
WR LDAC
Function
H
L
No Operation. Device not selected
Input Register of Selected DAC Transparent
Input Register of Selected DAC Latched
All Four DAC Registers Transparent
(i.e. Outputs respond to data held in respective
input registers)
Input Registers are Latched
All Four DAC Registers Latched
DAC Registers and Selected Input Register
Transparent Output follows Input Data for
Selected Channel.
H
H
H
H
L
H
L
L
g
g
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 section) careful layout and grounding is important to minimize
analog crosstalk between the four channels. The dynamic performance of the four DACs depends upon the optimum choice
of board layout. Figure 11 shows the relationship between input
Figure 12. Suggested PCB Layout for AD7225.
Layout Shows Component Side (Top View)
Figure 11. Channel-to-Channel Isolation
REV. B
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. VREF was limited to between 2 V and 3.24 V to
avoid slew rate limiting effects from the output amplifier during
measurements.
–7–
AD7225
SPECIFICATION RANGES
Table III. Unipolar Code Table
For the AD7225 to operate to rated specifications, its input reference voltage must be at least 4 V below the VDD power supply
voltage. This voltage differential is the overhead voltage required by the output amplifiers.
DAC Latch Contents
MSB
LSB
The AD7225 is specified to operate over a VDD range from
+12 V ± 5% to +15 V ± 10% (i.e., from +11.4 V to +16.5 V)
with a VSS of –5 V ± 10%. Operation is also specified for a single
+15 V ± 5% VDD supply. Applying a VSS of –5 V results in improved 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 (VDD – 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 requires 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 VREF. The AD7225 can be operated single supply
(VSS = AGND) or with positive/negative supplies (see op-amp
section which outlines the advantages of having negative VSS).
Connections for the unipolar output operation are shown in Figure 13. The voltage at any of the reference inputs must never be
negative with respect to DGND. Failure to observe this precaution may cause parasitic transistor action and possible device destruction. The code table for unipolar output operation is shown
in Table III.
Analog Output
1111
1111
 255 
+V REF 

 256 
1000
0001
 129 
+V REF 

 256 
1000
0000
 128 
V REF
+V REF 
 =+ 2
 256 
0111
1111
 127 
+V REF 

 256 
0000
0001
 1 
+V REF 

 256 
0000
0000
0V
( )
 1 
Note: 1 LSB = (V REF ) 2−8 = V REF 

 256 
BIPOLAR OUTPUT OPERATION
Each of the DACs of the AD7225 can be individually configured 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

 R2 
R2 
V OUT = 1 +
 ⋅ ( DAV REF ) – 
 ⋅ (V REF )

 R1 
R1 
With R1 = R2
VOUT = (2 DA – 1) • VREF
where DA is a fractional representation of the digital word in
latch A. (0 ≤ DA ≤ 255/256)
Mismatch between R1 and R2 causes gain and offset errors and,
therefore, these resistors must match and track over temperature. 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.
Figure 13. Unipolar Output Circuit
–8–
REV. B
AD7225
For a given VIN, increasing AGND above system GND will reduce the effective VDD–VREF which must be at least 4 V to ensure 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 VDD and VSS
of the AD7225 should be referenced to DGND.
AC REFERENCE SIGNAL
Figure 14. AD7225 Bipolar Output Circuit
Table IV. Bipolar (Offset Binary) Code Table
DAC Latch Contents
MSB
LSB
Analog Output
1111
1111
 127 
+V REF 

 128 
1000
0001
 1 
+V REF 

 128 
1000
0000
0V
0111
1111
 1 
–V REF 

 128 
0000
0001
 127 
–V REF 

 128 
0000
0000
 128 
–V REF 
 = –V REF
 128 
In some applications it may be desirable to have ac reference
signals. The AD7225 has multiplying capability within the upper (VDD – 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 inputs. Figure 16 shows a sine wave signal applied to VREF A. For
input signal frequencies up to 50 kHz the output distortion typically 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
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, VOUT A,
can be expressed as:
VOUT A = VBIAS + DA (VIN)
where DA is a fractional representation of the digital word in
DAC latch A. (0 ≤ DA ≤ 255/256).
A discrete-time filter may be described by either multiplication
in the frequency domain or convolution in the time domain i.e.
N
Y ( ω ) = H ( ω ) X ( ω ) or yn = ∑ hkXn –k+1
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 coefficients, the resulting products being accumulated to form the
output. The tap weights or coefficients hk are actually the nonzero 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 15. AGND Bias Circuit
Figure 17. Transversal Filter
REV. B
–9–
AD7225
FILTER
I/P
ACCUMULATOR
O/P
VOUT A
DELAYED
I/P
I/P
VOUT B
AD7820
Am29520
AD7225
ADC
TLD
QUAD DAC
VOUT C
FILTER
O/P
AD585
+
SHA
VOUT D
SAMPLES
SAMPLES
VREF A
h1
h2
VOUT A
AD584
Am7224
+10V
REF
VREF A
h3
VREF A
VOUT A
VOUT A
Xn–1
Xn
h4
FILTER
I/P
T
VOUT A
1
Xn–2
Xn–3
T
2
h1
AD7226
VOUT
DAC
VREF
VREF A
h2
T
3
4
h3
h4
QUAD DAC
VREF
+
FILTER
O/P
GAIN SET
Yn
TAP WEIGHTS
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 provided 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 products are accumulated to yield the convolution sum output
sample which is held by the AD585.
0
–10
–20
filter with the coefficients indicated. Although the theoretical
prediction does not take into account the quantization of the input 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 VREF B then the output
from DACB, VOUT B, can be expressed as:
VOUT B = DA • DB • VREF A
GAIN – dB
–30
–40
h1
h2
h3
h4
–50
–60
= 0.117
= 0.417
= 0.417
= 0.417
where DA and DB are the fractional representations of the
digital words in DAC latches A and B respectively.
If DA = DB = D then the result is D2 • VREF A
–70
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:
–80
–90
–100
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
NORMALIZED FREQUENCY – f/fs
Y = –(x4 + 2x3 + 3x2 + 2x + 4) • VIN
Figure 19. Predicted (Theoretical) Response
where x is the digital code which is applied to all four DAC
latches.
+15V
25kΩ
VDD
VIN
VREF A
100kΩ
50kΩ
VOUT A
AD7225*
33kΩ
VREF B
VOUT B
VREF C
VOUT C
VREF D
VOUT D
Y
50kΩ
100kΩ
Figure 20. Actual Response
Low pass, bandpass and high pass filters may be synthesized using 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
–10–
AGND
DGND
VSS
*DIGITAL INPUTS OMITTED
FOR CLARITY
Figure 21. Complex Waveform Generation
REV. B
AD7225
MICROPROCESSOR INTERFACE
A23
A15
ADDRESS BUS
A8
8085A/
8088
ADDRESS
DECODE
WR
ALE
ADDRESS BUS
A1
A0
A1
LDAC
AD7225*
AS
WR
LATCH
EN
A0
A1
ADDRESS
DECODE
68008
AD7225*
EN
WR
LDAC
R/W
DB7
DTACK
DB0
AD0
DB7
DB0
AD7
ADDRESS DATA BUS
D7
*LINEAR CIRCUITRY OMITTED FOR CLARITY
D0
Figure 22. AD7225 to 8085A/8088 Interface,
Double-Buffered Mode
ADDRESS BUS
A0
R/W
A0
A1
ADDRESS
DECODE
LDAC
AD7225*
EN
WR
E OR φ2
DB7
DB0
D7
D0
DATA BUS
*LINEAR CIRCUITRY OMITTED FOR CLARITY
Figure 23. AD7225 to 6809/6502 Interface,
Single-Buffered Mode
A15
Operating the AD7225 from dual supplies results in enhanced
performance over single supply operation on a number of parameters 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 VDD 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 filtered. 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 ISS requirement over the
commercial operating temperature range.
ADDRESS BUS
A8
1/6
CD4049AE
A0
A1
Z-80
ADDRESS
DECODE
MREQ
Figure 25. AD7225 to 68008 Interface,
Single-Buffered Mode
VSS GENERATION
A15
6809/
6502
1/6
CD4049AE
LDAC
1/6
CD4049AE
AD7225*
EN
510k
DB7
5.1k
DB0
D0
1/6
CD4049AE
1/6
CD4049AE
WR
WR
D7
DATA BUS
*LINEAR CIRCUITRY OMITTED FOR CLARITY
0.02µF
DATA BUS
1/6
CD4049AE
+
510Ω
47µF
–VOUT
*LINEAR CIRCUITRY OMITTED FOR CLARITY
1N4001
1N4001
Figure 24. AD7225 to Z-80 Interface,
Double-Buffered Mode
+
47µF
Figure 26. VSS Generation Circuit
REV. B
–11–
5V1
AD7225
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Lead SOIC (R-24)
24-Pin Cerdip (Q-24)
28-Terminal Leadless
Ceramic Chip Carrier (E-28A)
C927a–5–5/86
24-Pin Plastic (N-24)
PRINTED IN U.S.A.
28-Lead PLCC (P-28A)
–12–
REV. B