a
Quad 8-Bit Multiplying CMOS
D/A Converter with Memory
DAC8408
FEATURES
Four DACs in a 28 Pin, 0.6 Inch Wide DIP or 28-Pin JEDEC
Plastic Chip Carrier
61/4 LSB Endpoint Linearity
Guaranteed Monotonic
DACs Matched to Within 1%
Microprocessor Compatible
Read/Write Capability (with Memory)
TTL/CMOS Compatible
Four-Quadrant Multiplication
Single-Supply Operation (+5 V)
Low Power Consumption
Latch-Up Resistant
Available In Die Form
APPLICATIONS
Voltage Set Points in Automatic Test Equipment
Systems Requiring Data Access for Self-Diagnostics
Industrial Automation
Multichannel Microprocessor-Controlled Systems
Digitally Controlled Op Amp Offset Adjustment
Process Control
Digital Attenuators
GENERAL DESCRIPTION
The DAC8408 is a monolithic quad 8-bit multiplying digital-toanalog CMOS converter. Each DAC has its own reference input,
feedback resistor, and onboard data latches that feature
read/write capability. The readback function serves as memory
for those systems requiring self-diagnostics.
A common 8-bit TTL/CMOS compatible input port is used to
load data into any of the four DAC data-latches. Control lines
DS1, DS2, and A/B determine which DAC will accept data.
Data loading is similar to that of a RAMs write cycle. Data can
be read back onto the same data bus with control line R/W. The
DAC8408 is bus compatible with most 8-bit microprocessors,
including the 6800, 8080, 8085, and Z80. The DAC8408 operates on a single +5 volt supply and dissipates less than 20 mW.
The DAC8408 is manufactured using PMI’s highly stable,
thin-film resistors on an advanced oxide-isolated, silicon-gate,
CMOS process. PMI’s improved latch-up resistant design eliminates the need for external protective Schottky diodes.
ORDERING INFORMATION1
Model
INL
DNL
Temperature
Range
Package
Description
DAC8408GP
DAC8408ET
DAC8408AT 2
DAC8408FT
DAC8408BT 2
DAC8408FPC 3
DAC8408FS
DAC8408FP
± 1/4 LSB
± 1/4 LSB
± 1/4 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1/2 LSB
± 1 LSB
± 1 LSB
± 1 LSB
± 1 LSB
± 1 LSB
0°C to +70°C
–40°C to +85°C
–55°C to +125°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
28-Pin Plastic DIP
28-Pin Cerdip
28-Pin Cerdip
28-Pin Cerdip
28-Pin Cerdip
28-Contact PLCC
28-Pin SOL
28-Pin Plastic DIP
NOTES
1
Burn-in is available on commercial and industrial temperature range parts
in cerdip, plastic DIP, and TO-can packages. For outline information see Package Information section.
2
For devices processed in total compliance to MIL-STD-883, add /883 after
part number. Consult factory for 883 data sheet.
3
For availability and burn-in information on SO and PLCC packages, contact
your local sales office.
FUNCTIONAL BLOCK DIAGRAM
DAC8408
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
DAC8408
ELECTRICAL CHARACTERISTICS
(@ VDD = +5 V; VREF = 610 V; VOUTA, B, C, D = 0 V; TA = –558C to +1258C apply for
DAC8408AT/BT, TA = –408C to +858C apply for DAC8408ET/FT/FP/FPC/FS; TA = 08C to +708C apply for DAC8408GP, unless otherwise noted.
Specifications apply for DAC A, B, C, & D.)
Parameter
Symbol
STATIC ACCURACY
Resolution
Nonlinearity1, 2
N
INL
Differential
Nonlinearity
Gain Error
Gain Tempco3, 6
Power Supply Rejection
(∆VDD = ± 10%)
IOUT 1A, B, C, D
Leakage Current13
DNL
GFSE
TCGFS
Conditions
Min
DAC8408
Typ
Max
8
± 1/4
± 1/2
± 1/2
±1
±1
± 40
Bits
LSB
LSB
LSB
LSB
LSB
ppm/°C
0.001
%FSR/%
TA =+25°C
TA = Full Temperature Range
± 30
± 100
nA
nA
RA, B, C, D
± 20
±1
14
V
%
kΩ
0.8
V
V
µA
µA
pF
DAC8408A/E/G
DAC8408B/F/H
DAC8408A/E/G
DAC8408B/F/H
(Using Internal RFB)
±2
PSR
ILKG
REFERENCE INPUT
Input Voltage Range
Input Resistance Match4
Input Resistance
RIN
6
DIGITAL INPUTS
Digital Input Low
Digital Input High
Input Current5
VIL
VIH
2.4
Input Capacitance6
IIN
CIN
DATA BUS OUTPUTS
Digital Output Low
Digital Output High
Output Leakage Current
VOL
VOH
ILKG
DAC OUTPUTS6
Propagation Delay7
Settling Time11,12
Output Capacitance
tPD
tS
COUT
AC Feedthrough
FT
Units
TA = +25°C
TA = Full Temperature Range
16 mA Sink
400 µA Source
TA = +25°C
TA = Full Temperature Range
10
± 0.01
0.4
4
± 0.005 ± 1.0
± 0.075 ± 10.0
150
190
DAC Latches All “0s”
DAC Latches All “1s”
(20 Vp-p @ F = 100 kHz)
± 1.0
± 10.0
8
180
250
30
50
V
V
µA
µA
54
ns
ns
pF
pF
dB
90
145
150
175
10
0
0
0
0
220
350
320
430
200
270
0
0
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
6, 10
SWITCHING CHARACTERISTICS
Write to Data Strobe Time
Data Valid to Strobe Set-Up Time
tDS1 or
tDS2
tDSU
Data Valid to Strobe Hold Time
DAC Select to Strobe Set-Up Time
DAC Select to Strobe Hold Time
Write Select to Strobe Set-Up Time
Write Select to Strobe Hold Time
Read to Data Strobe Width
tDH
tAS
tAH
tWSU
tWH
tRDS
Data Strobe to Output Valid Time
tCO
Output Data to Deselect Time
tOTD
Read Select to Strobe Set-Up Time
Read Select to Strobe Hold Time
tRSU
tRH
TA = +25°C
TA = Full Temperature Range
TA = +25°C
TA = Full Temperature Range
TA = +25°C
TA = Full Temperature Range
TA = +25°C
TA = Full Temperature Range
TA = +25°C
TA = Full Temperature Range
Specifications subject to change without notice.
–2–
REV. A
DAC8408
ELECTRICAL CHARACTERISTICS
@ VDD = +5 V; VREF = 610 V; VOUTA, B, C, D = 0 V; TA = –558C to +1258C apply for
DAC8408AT/BT, TA = –408C to +858C apply for DAC8408ET/FT/FP/FPC/FS; TA = 08C to +708C apply for DAC8408GP, unless otherwise noted.
Specifications apply for DAC A, B, C, & D. Continued
Parameter
Symbol
POWER SUPPLY
Voltage Range
Supply Current8
Supply Current9
VDD
IDD
IDD
Conditions
Min
DAC8408
Typ
Max
4.5
5.5
50
1.0
1.5
TA = +25°C
TA = Full Temperature Range
NOTES
1
This is an end-point linearity specification.
2
Guaranteed to be monotonic over the full operating temperature range.
3
ppm/°C of FSR (FSR = Full Scale Range = VREF-1 LSB.)
4
Input Resistance Temperature Coefficient = +300ppm/°C.
5
Logic Inputs are MOS gates. Typical input current at +25°C Is less than 10 nA.
6
Guaranteed by design.
Units
V
µA
mA
mA
7
From Digital Input to 90% of final analog output current.
All Digital Inputs “0” or V DD.
All Digital Inputs V IH or VIL.
10
See Timing Diagram.
11
Digital Inputs = 0 V to V DD or VDD to 0 V.
12
Extrapolated: t S (1/2 LSB) = tPD + 6.2τ where τ = the measured first time constant of the final RC decay.
13
All Digital Inputs = 0 V; V REF = +10 V.
Specifications subject to change without notice.
8
9
PIN CONNECTIONS
DAC8408
TOP VIEW
(Not to Scale)
ABSOLUTE MAXIMUM RATINGS
(TA = +25°C, unless otherwise noted.)
VDD to IOUT 2A, IOUT 2B, IOUT 2C, IOUT 2D . . . . . . . . . . 0 V, +7 V
VDD to DGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 V, +7 V
IOUT 1A, IOUT 1B,
IOUT 1C, IOUT 1D to DGND . . . . . . . . . –0.3 V to VDD +0.3 V
RFBA, RFBB, RFBC, RFBD to IOUT . . . . . . . . . . . . . . . . . ± 25 V
IOUT 2A, IOUT 2B,
IOUT 2C, IOUT 2D to DGND . . . . . . . . . –0.3 V to VDD + 0.3 V
DB0 through DB7 to DGND . . . . . . . . –0.3 V to VDD + 0.3 V
Control Logic
Input Voltage to DGND . . . . . . . . . . –0.3 V + VDD + 0.3 V
VREFA, VREFB, VREFC, VREFD to
IOUT 2A, IOUT 2B, IOUT 2C, IOUT 2D . . . . . . . . . . . . . . . . ± 25 V
Operating Temperature Range
Commercial Grade (GP) . . . . . . . . . . . . . . . . 0°C to +70°C
Industrial Grade (ET, FT, FP, FPC, FS) . –40°C to +85°C
Military Grade (AT, BT) . . . . . . . . . . . . . . –55°C to +125°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . . +300°C
REV. A
Package Type
uJA*
uJC
Units
28-Pin Hermetic DIP (T)
28-Pin Plastic DIP (P)
28-Pin SOL (S)
28-Contact PLCC (PC)
55
53
68
66
10
27
23
29
°C/W
°C/W
°C/W
°C/W
*θJA is specified for worst case mounting conditions, i.e., θJA is specified for
device in socket for cerdip and P-DIP packages; θJA is specified for device
soldered to printed circuit board for SOL and PLCC packages.
CAUTION
1. Do not apply voltages higher than VDD +0.3 V or less than
–0.3 V potential on any terminal except VREF and RFB.
2. The digital control inputs are diode-protected; however,
permanent damage may occur on unconnected inputs from
high energy electrostatic fields. Keep in conductive foam at
all times until ready to use.
3. Use proper antistatic handling procedures.
4. Absolute Maximum Ratings apply to both packaged devices
and DICE. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device.
–3–
DAC8408
Burn-in Circuit
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 DAC8408 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.
WARNING!
ESD SENSITIVE DEVICE
DICE CHARACTERISTICS
1. VDD
2. VREFA
3. RFBA
4. IOUT 1A
5. IOUT 2A/IOUT 2B
6. IOUT 1B
7. RFBB
8. VREFB
9. DB0 (LSB)
10. DB1
11. DB2
12. DB3
13. DB4
14. DB5
15. DB6
16. DB7 (MSB)
17. A/B
18. R/W
19. DS1
20. DS2
21. VREFD
22. RFBD
23. IOUT 1D
24. IOUT 2C/IOUT 2D
25. IOUT 1C
26. RFBC
27. VREFC
28. DGND
DIE SIZE 0.130 × 0.124 inch, 16,120 sq. mils
(3.30 × 3.15 mm, 10.4 sq. mm)
–4–
REV. A
DAC8408
WAFER TEST LIMITS at V
DD
= +5 V; VREF = 610 V; VOUTA, B, C, D = 0 V; TA = +258C, unless otherwise noted. Specifications apply for
DAC A, B, C, & D.
Conditions
DAC8408G
Limits
Units
Using Internal RFB
Using Internal RFB
8
± 1/2
±1
±1
0.001
Bits min
LSB max
LSB max
LSB max
%FSR/% max
All Digital Inputs = 0 V
± 30
nA max
RIN
6/14
kΩ min/max
RIN
±1
% max
DIGITAL INPUTS
Digital Input Low
Digital Input High
Input Current4
VIL
VIH
IIN
0.8
2.4
± 1.0
V max
V min
µA max
DATA BUS OUTPUTS
Digital Output Low
Digital Output High
Output Leakage Current
VOL
VOH
ILKG
0.4
4
± 1.0
V max
V min
µA max
POWER SUPPLY
Supply Current5
Supply Current6
IDD
IDD
50
1.0
µA max
mA max
Parameter
STATIC ACCURACY
Resolution
Nonlinearity1
Differential Nonlinearity
Gain Error
Power Supply Rejection
(∆VDD = ± 10%)2
IOUT 1A, B, C, D Leakage Current
REFERENCE INPUT
Reference Input
Resistance3
Input Resistance Match
Symbol
N
INL
DNL
GFSE
PSR
ILKG
VREF = +10 V
1.6 mA Sink
400 µA Source
NOTES
1
This is an endpoint linearity specification.
2
FSR is Full Scale Range = V REF –1 LSB.
3
Input Resistance Temperature Coefficient approximately equals +300 ppm/ °C.
4
Logic inputs are MOS gates.Typical input current at +25°C is less than 10 nA.
5
All Digital Inputs are either “0” or V DD.
6
All Digital Inputs are either V IH or VIL.
Electrical tests are performed at wafer probe to the limits shown. Due to variations in assembly methods and normal yield loss, yield after packaging is not guaranteed
for standard product dice. Consult factory to negotiate specifications based on dice lot qualification through sample lot assembly and testing.
REV. A
–5–
DAC8408
TYPICAL PERFORMANCE CHARACTERISTICS
Analog Crosstalk vs. Frequency
Supply Current vs. Logic Level
–6–
REV. A
DAC8408
Timing Diagram
PARAMETER DEFINITIONS
RESOLUTION
AC FEEDTHROUGH ERROR
Resolution is the number of states (2n) that the full-scale range
(FSR) of a DAC is divided (or resolved) into.
This is the error caused by capacitance coupling from VREF to
the DAC output with all switches off.
NONLINEARITY
SETTLING TIME
Nonlinearity (Relative Accuracy) is a measure of the maximum
deviation from a straight line passing through the end-points of
the DAC transfer function. It is measured after adjusting for
ideal zero and full-scale and is expressed in LSB, %, or ppm of
full-scale range.
Settling Time is the time required for the output function of the
DAC to settle to within 1/2 LSB for a given digital input signal.
DIFFERENTIAL NONLINEARITY
PROPAGATION DELAY
This is a measure of the internal delays of the DAC. It is defined
as the time from a digital input change to the analog output current reaching 90% of its final value.
Differential Nonlinearity is the worst case deviation of any adjacent analog outputs from the ideal 1 LSB step size. A specified
differential nonlinearity of ± 1 LSB maximum over the operating
temperature range ensures monotonicity.
CHANNEL-TO-CHANNEL ISOLATION
GAIN ERROR
DIGITAL CROSSTALK
Gain Error (full-scale error) is a measure of the output error between the ideal and actual DAC output. The ideal full-scale
output is VREF –1 LSB.
This is the portion of input signal that appears at the output of a
DAC from another DAC’s reference input. It is expressed as a
ratio in dB.
Digital Crosstalk is the glitch energy transferred to the output of
one DAC due to a change in digital input code from other
DACs. It is specified in nVs.
OUTPUT CAPACITANCE
Output Capacitance is that capacitance between IOUT 1A, IOUT 1B,
IOUT 1C, or IOUT 1D and AGND.
REV. A
–7–
DAC8408
CIRCUIT INFORMATION
The DAC8408 combines four identical 8-bit CMOS DACs
onto a single monolithic chip. Each DAC has its own reference
input, feedback resistor, and on-board data latches. It also features a read/write function that serves as an accessible memory
location for digital-input data words. The DAC’s three-state
readback drivers place the data word back onto the data bus.
D/A CONVERTER SECTION
Each DAC contains a highly stable, silicon-chromium, thin-film,
R-2R resistor ladder network and eight pairs of current steering
switches. These switches are in series with each ladder resistor
and are single-pole, double-throw NMOS transistors; the gates
of these transistors are controlled by CMOS inverters. Figure 1
shows a simplified circuit of the R-2R resistor ladder section,
and Figure 2 shows an approximate equivalent switch circuit.
The current through each resistor leg is switched between IOUT 1
and IOUT 2. This maintains a constant current in each leg, regardless of the digital input logic states.
Figure 1. Simplified D/A Circuit of DAC8408
Each transistor switch has a finite “ON” resistance that can introduce errors to the DAC’s specified performance. These resistances must be accounted for by making the voltage drop across
each transistor equal to each other. This is done by binarilyscaling the transistor’s “ON” resistance from the most significant bit (MSB) to the least significant bit (LSB). With 10 volts
applied at the reference input, the current through the MSB
switch is 0.5 mA, the next bit is 0.25 mA, etc.; this maintains a
constant 10 mV drop across each switch and the converter’s accuracy is maintained. It also results in a constant resistance appearing at the DAC’s reference input terminal; this allows the
DAC to be driven by a voltage or current source, ac or dc of
positive or negative polarity.
Figure 2. N-Channel Current Steering Switch
Shown in Figure 3 is an equivalent output circuit for DAC A.
The circuit is shown with all digital inputs high. The leakage
current source is the combination of surface and junction leakages to the substrate. The 1/256 current source represents the
constant 1-bit current drain through the ladder terminating resistor. The situation is reversed with all digital inputs low, as
shown in Figure 4. The output capacitance is code dependent,
and therefore, is modulated between the low and high values.
Figure 3. Equivalent DAC Circuit (AII Digital Inputs HIGH)
–8–
REV. A
DAC8408
INTERFACE LOGIC SECTION
DAC Operating Modes
• All DACs in HOLD MODE.
• DAC A, B, C, or D individually selected (WRITE MODE).
• DAC A, B, C, or D individually selected (READ MODE).
• DACs A and C simultaneously selected (WRITE MODE).
• DACs B and D simultaneously selected (WRITE MODE).
DAC Selection: Control inputs, DS1, DS2, and A/B select
which DAC can accept data from the input port (see Mode Selection Table).
Figure 4. Equivalent DAC Circuit (AII Digital Inputs LOW)
DIGITAL SECTION
Figure 5 shows the digital input/output structure for one bit.
The digital WR, WR, and RD controls shown in the figure are
internally generated from the external A/B, R/W, DS1, and DS2
signals. The combination of these signals decide which DAC is
selected. The digital inputs are CMOS inverters, designed such
that TTL input levels (2.4 V and 0.8 V) are converted into
CMOS logic levels. When the digital input is in the region of 1.2 V
to 1.8 V, the input stages operate in their linear region and draw
current from the +5 V supply (see Typical Supply Current vs.
Logic Level curve on page 6). It is recommended that the digital
input voltages be as close to VDD and DGND as is practical in
order to minimize supply currents. This allows maximum savings in power dissipation inherent with CMOS devices. The
three-state readback digital output drivers (in the active mode)
provide TTL-compatible digital outputs with a fan-out of one
TTL load. The three state digital readback leakage-current is
typically 5 nA.
Figure 5. Digital Input/Output Structure
Mode Selection: Control inputs DS and R/W control the operating mode of the selected DAC.
Write Mode: When the control inputs DS and R/W are both
low, the selected DAC is in the write mode. The input data
latches of the selected DAC are transparent, and its analog output responds to activity on the data inputs DB0–DB7.
Hold Mode: The selected DAC latch retains the data that was
present on the bus line just prior to DS or R/W going to a high
state. All analog outputs remain at the values corresponding to
the data in their respective latches.
Read Mode: When DS is low and R/W is high, the selected
DAC is in the read mode, and the data held in the appropriate
latch is put back onto the data bus.
MODE SELECTION TABLE
DS1
Control Logic
DS2 A/B R/W
Mode
DAC
L
L
H
H
H
H
L
L
H
L
H
L
L
L
L
L
WRITE
WRITE
WRITE
WRITE
A
B
C
D
L
L
H
H
H
H
L
L
H
L
H
L
H
H
H
H
READ
READ
READ
READ
A
B
C
D
L
L
L
L
H
L
L
L
WRITE
WRITE
A&C
B&D
H
L
L
H
L
L
X
H
L
X
H
H
HOLD
HOLD
HOLD
A/B/C/D
A/B/C/D
A/B/C/D
L = Low State, H = High State, X = Irrelevant
REV. A
–9–
DAC8408
BASIC APPLICATIONS
Some basic circuit configurations are shown in Figures 6 and 7.
Figure 6 shows the DAC8408 connected in a unipolar configuration (2-Quadrant Multiplication), and Table I shows the Code
Table. Resistors R1, R2, R3, and R4 are used to trim full scale
output. Full-scale output voltage = VREF –1 LSB = VREF (1–2–8)
or VREF × (255/256) with all digital inputs high. Low temperature coefficient (approximately 50 ppm/°C) resistors or trimmers should be selected if used. Full scale can also be adjusted
using VREF voltage. This will eliminate resistors R1, R2, R3, and
R4. In many applications, R1 through R4 are not required, and
the maximum gain error will then be that of the DAC.
Each DAC exhibits a variable output resistance that is codedependent. This produces a code-dependent, differential nonlinearity term at the amplifier’s output which can have a maximum value of 0.67 × the amplifier’s offset voltage. This differential nonlinearity term adds to the R-2R resistor ladder differential-nonlinearity; the output may no longer be monotonic. To
maintain monotonicity and minimize gain and linearity errors, it
is recommended that the op amp offset voltage be adjusted to
less than 10% of 1 LSB (1 LSB = 2–8 × VREF or 1/256 × VREF),
or less than 3.9 mV over the operating temperature range. Zeroscale output voltage (with all digital inputs low) may be adjusted
using the op amp offset adjustment. Capacitors C1, C2, C3,
and C4 provide phase compensation and help prevent overshoot
and ringing when using high speed op amps.
Figure 7 shows the recommended circuit configuration for the
bipolar operation (4-quadrant multiplication), and Table II shows
the Code Table. Trimmer resistors R17, R18, R19, and R20
are used only if gain error adjustments are required and range
between 50 Ω and 1000 Ω. Resistors R21, R22, R23, and R24
will range betwen 50 Ω and 500 Ω. If these resistors are used, it
is essential that resistor pairs R9–R13, R10–R14, R11–R15,
R12–R16 are matched both in value and tempco. They should
be within 0.01%; wire wound or metal foil types are preferred
for best temperature coefficient matching. The circuits of Figure
6 and 7 can either be used as a fixed reference D/A converter, or
as an attenuator with an ac input voltage.
Table I. Unipolar Binary Code Table (Refer to Figure 6)
DAC Data Input
MSB
LSB
Analog Output
255
1 1 1 1 1 1 1 1
–VREF
256
1 0 0 0 0 0 0 1
–VREF
256
1 0 0 0 0 0 0 0
–VREF
=
256
2
0 1 1 1 1 1 1 1
–VREF
256
0 0 0 0 0 0 0 1
–VREF
256
0 0 0 0 0 0 0 0
–VREF
=0
256
129
128
–VIN
127
1
0
NOTE
1 LSB = (2–8) (VREF) =
1
256
(VREF)
Figure 6. Quad DAC Unipolar Operation (2-Quadrant Multiplication)
–10–
REV. A
DAC8408
Figure 7. Quad DAC Bipolar Operation (4-Quadrant Multiplication)
Table II. Bipolar (Offset Binary) Code Table
(Refer to Figure 7)
DAC Data Input
MSB
LSB
Analog Output
(DAC A OR DAC B)
127
1 1 1 1 1 1 1 1
+VREF
128
1 0 0 0 0 0 0 1
+VREF
128
1 0 0 0 0 0 0 0
0
0 1 1 1 1 1 1 1
–VREF
128
0 0 0 0 0 0 0 1
–VREF
128
0 0 0 0 0 0 0 0
–VREF
128
1
1
127
128
APPLICATION HINTS
General Ground Management: AC or transient voltages between AGND and DGND can appear as noise at the DAC8408’s
analog output. Note that in Figures 5 and 6, IOUT2A/IOUT2B and
IOUT 2C/IOUT 2D are connected to AGND. Therefore, it is recommended that AGND and DGND be tied together at the
DAC8408 socket. In systems where AGND and DGND are tied
together on the backplane, two diodes (1N914 or equivalent)
should be connected in inverse parallel between AGND and
DGND.
Write Enable Timing: During the period when both DS and
R/W are held low, the DAC latches are transparent and the analog output responds directly to the digital data input. To prevent unwanted variations of the analog output, the R/W should
not go low until the data bus is fully settled (DATA VALID).
NOTE
1 LSB = (2–7) (VREF) =
REV. A
1
128
(VREF)
–11–
DAC8408
SINGLE SUPPLY, VOLTAGE OUTPUT OPERATION
Table III. Single Supply Binary Code Table (Refer to Figure 8)
The DAC8408 can be connected with a single +5 V supply to
produce DAC output voltages from 0 V to +1.5 V. In Figure 8,
the DAC8408 R-2R ladder is inverted from its normal connection. A +1.500 V reference is connected to the current output pin
4 (IOUT 1A), and the normal VREF input pin becomes the DAC
output. Instead of a normal current output, the R-2R ladder outputs a voltage. The OP-490, consisting of four precision low
power op amps that can operate its inputs and outputs to zero
volts, buffers the DAC to produce a low impedance output voltage from 0 V to +1.5 V full-scale. Table III shows the code table.
With the supply and reference voltages as shown, better than 1/2
LSB differential and integral nonlinearity can be expected. To
maintain this performance level, the +5 V supply must not drop
below 4.75 V. Similarly, the reference voltage must be no higher
than 1.5 V. This is because the CMOS switches require a minimum level of bias in order to maintain the linearity performance.
DAC Data Input
MSB
LSB
Analog Output
255
1 1 1 1 1 1 1 1
VREF
, +1.4941 V
256
1 0 0 0 0 0 0 1
, +0.7559 V
VREF
256
1 0 0 0 0 00 0
, +0.7500 V
VREF
256
0 1 1 1 1 1 1 1
, +0.7441 V
VREF
256
0 0 0 0 0 0 0 1
, +0.0059 V
VREF
256
0 0 0 0 0 0 0 0
VREF
, 0.0000 V
256
129
128
127
1
0
Figure 8. Unipolar Supply, Voltage Output DAC Operation
–12–
REV. A
DAC8408
Figure 9. A Digitally Programmable Universal Active Filter
A DIGITALLY PROGRAMMABLE ACTIVE FILTER
A powerful D/A converter application is a programmable active
filter design as shown in Figure 9. The design is based on the
state-variable filter topology which offers stable and repeatable
filter characteristics. DAC B and DAC D can be programmed in
tandem with a single digital byte load which sets the center frequency of the filter. DAC A sets the Q of the filter. DAC C sets
the gain of the filter transfer function. The unique feature of this
design is that varying the gain of filter does not affect the Q of
the filter. Similarly, the reverse is also true. This makes the programmability of the filter extremely reliable and predictable.
Note that low-pass, high-pass, and bandpass outputs are available. This sophisticated function is achieved in only two IC
packages.
The network analyzer photo shown in Figure 10 superimposes
five actual bandpass responses ranging from the lowest frequency of 75 Hz (1 LSB ON) to a full-scale frequency of 19.132
kHz (all bits ON), which is equivalent to a 256 to 1 dynamic
range. The frequency is determined by fC = 1/2πRC where R is
the ladder resistance (RIN) of the DAC8408, and C is 1000 pF.
Note that from device to device, the resistance RIN varies. Thus
some tuning may be necessary.
REV. A
Figure 10. Programmable Active Filter Band-Pass
Frequency Response
All components used are available off-the-shelf. Using low drift
thin-film resistors, the DAC8408 exhibits very stable performance over temperature. The wide bandwidth of the OP-470
produces excellent high frequency and high Q response. In addition, the OP470’s low input offset voltage assures an unusually
low dc offset at the filter output.
–13–
DAC8408
Figure 11. A Digitally Programmable, Low-Distortion Sinewave Oscillator
A LOW-DISTORTION, PROGRAMMABLE
SINEWAVE OSCILLATOR
By varying the previous state-variable filter topology slightly,
one can obtain a very low distortion sinewave oscillator with
programmable frequency feature as shown in Figure 11. Again,
DAC B and DAC D in tandem control the oscillating frequency
based on the relationship fC = 1/2πRC. Positive feedback is
accomplished via the 82.5 kΩ and the 20 kΩ potentiometer.
The Q of the oscillator is determined by the ratio of 10 kΩ and
475Ω in series with the FET transistor, which acts as an automatic gain control variable resistor. The AGC action maintains
a very stable sinewave amplitude at any frequency. Again, only
two ICs accomplish a very useful function.
At the highest frequency setting, the harmonic distortion level
measures 0.016%. As the frequencies drop, distortion also drops
to a low of 0.006%. At the lowest frequency setting, distortion
came back up to a worst case of 0.035%.
–14–
REV. A
–15–
–16–
PRINTED IN U.S.A.
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