Page 1
LC2MOS Dual, Complete,
a
FEATURES
Two 12-Bit/14-Bit DACs with Output Amplifiers
AD7242: 12-Bit Resolution
AD7244: 14-Bit Resolution
On-Chip Voltage Reference
Fast Settling Time
AD7242: 3 ms to 61/2 LSB
AD7244: 4 ms to 61/2 LSB
High Speed Serial Interface
Operates from 65 V Supplies
Specified Over –408C to +858C in Plastic Packages
Low Power – 130 mW typ
GENERAL DESCRIPTION
The AD7242/AD7244 is a fast, complete, dual 12-bit/14-bit
voltage output D/A converter. It consists of a 12-bit/14-bit
DAC, 3 V buried Zener reference, DAC output amplifiers and
high speed serial interface logic.
Interfacing to both DACs is serial, minimizing pin count and
allowing a small package size. Standard control signals allow
interfacing to most DSP processors and microcontrollers.
Asynchronous control of DAC updating for both DACs is made
possible with a separate
The AD7242/AD7244 operates from ± 5 V power supplies,
providing an analog output range of ±3 V. A REF OUT/REF
IN function allows the DACs to be driven from the on-chip 3 V
reference or from an external reference source.
The AD7242/AD7244 is fabricated in Linear Compatible
CMOS (LC
combines precision bipolar circuits with low power CMOS
logic. Both parts are available in a 24-pin, 0.3-inch wide, plastic
or hermetic dual-in-line package (DIP) and in a 28-pin, plastic
small outline (SOIC) package. The AD7242 and AD7244 are
available in the same pinout to allow easy upgrade from 12-bit
to 14-bit performance.
2
MOS), an advanced mixed technology process that
LDAC input for each DAC.
12-Bit/14-Bit Serial DACs
AD7242/AD7244
FUNCTIONAL BLOCK DIAGRAM
PRODUCT HIGHLIGHTS
1. Complete, Dual 12-Bit/14-Bit DACs
The AD7242/AD7244 provides the complete function for
generating voltages to 12-bit/14-bit resolution. The part
features an on-chip reference, output buffer amplifiers and
two 12-bit/14-bit D/A converters.
2. High Speed Serial Interface
The AD7242/AD7244 provides a high speed, easy-to-use,
serial interface allowing direct interfacing to DSP processors
and microcontrollers. A separate serial port is provided for
each DAC.
3. Small Package Size
The AD7242/AD7244 is available in a 24-pin DIP and a 28pin SOIC package offering considerable space saving over
comparable solutions.
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 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703 © Analog Devices, Inc., 1996
Page 2
(VDD = +5 V 6 5% VSS = –5 V 6 5%, AGND = DGND = 0 V, REF INA =
AD7242/AD7244–SPECIFICATIONS
All Specifications T
Parameter J, A Versions1K, B Versions1Units Test Conditions/Comments
DC ACCURACY
Resolution 12 12 Bits
Integral Nonlinearity ±1 ±1/2 LSB max
Differential Nonlinearity ±1 ±1 LSB max Guaranteed Monotonic
Bipolar Zero Error ±5 ±5 LSB max
Positive Full-Scale Error
Negative Full-Scale Error
REFERENCE OUTPUT
REF OUT @ +25°C 2.99/3.01 2.99/3.01 V min/V max
to T
T
MIN
, V
MAX
OUTB
REF OUT Tempco 35 35 ppm/°C typ
Reference Load Change
(∆REF OUT vs. ∆I) –1 –1 mV max Reference Load Current Change (0 µA–500 µA)
REFERENCE INPUTS
REF INA, REF INB Input Range 2.85/3.15 2.85/3.15 V min/V max 3 V ± 5%
Input Current 1 1 µA max
LOGIC INPUTS
LDACA, LDACB, TFSA, TFSB,
(
TCLKA, TCLKB, DTA, DTB)
Input High Voltage, V
Input Low Voltage, V
Input Current, I
Input Capacitance, C
ANALOG OUTPUTS
(V
OUTA
Output Voltage Range ±3 ±3 V nom
DC Output Impedance 0.1 0.1 Ω typ
Short Circuit Current 20 20 mA typ
AC CHARACTERISTICS
Voltage Output Settling Time Settling Time to Within ±1/2 LSB of Final Value
Positive Full-Scale Change 3 3 µs max Typically 2 µs
Negative Full-Scale Change 3 3 µs max Typically 2 µs
Digital-to-Analog Glitch Impulse 10 10 nV secs typ DAC Code Change All 1s to All 0s
Digital Feedthrough 2 2 nV secs typ
Channel-to-Channel Isolation 110 110 dB typ V
POWER REQUIREMENTS
V
DD
V
SS
I
DD
I
SS
Total Power Dissipation 195 195 mW max Typically 130 mW
NOTES
1
Temperature ranges are as follows: J, K Versions: –40°C to +85°C; A, B Versions: –40°C to +85°C.
2
Measured with respect to REF IN and includes bipolar offset error.
3
For capacitive loads greater than 50 pF, a series resistor is required (see Internal Reference section).
4
Sample tested @ +25°C to ensure compliance.
Specifications subject to change without notice.
to T
MIN
unless otherwise noted.)
MAX
AD7242
2
2
3
±5 ±5 LSB max
±5 ±5 LSB max
2.98/3.02 2.98/3.02 V min/V max
INH
INL
IN
4
IN
2.4 2.4 V min VDD = 5 V ± 5%
0.8 0.8 V max VDD = 5 V ± 5%
±10 ±10 µA max VIN = 0 V to V
10 10 pF max
)
4
+5 +5 V nom ±5% for Specified Performance
–5 –5 V nom ±5% for Specified Performance
27 27 mA max Cumulative Current from the Two VDD Pins
15 15 mA max Cumulative Current from the Two VSS Pins
AD7242 ORDERING GUIDE
REF INB = +3 V. V
, V
OUTA
load to AGND: RL = 2 kV, CL = 100 pF.
OUTB
DD
= 10 kHz Sine Wave
OUT
Temperature Integral Package
Model Range Nonlinearity Option*
AD7242JN –40°C to +85°C ±1 LSB max N-24
AD7242KN –40°C to +85°C ±1/2 LSB max N-24
AD7242JR –40°C to +85°C ±1 LSB max R-28
AD7242KR –40°C to +85°C ±1/2 LSB max R-28
AD7242AQ –40°C to +85°C ±1 LSB max Q-24
AD7242BQ –40°C to +85°C ±1/2 LSB max Q-24
*N = Plastic DIP; Q = Cerdip; R = Small Outline IC (SOIC).
–2–
REV. A
Page 3
AD7242/AD7244
AD7244
Parameter J, A Versions1S Version
1
Units Test Conditions/Comments
DC ACCURACY
Resolution 14 14 Bits
Integral Nonlinearity ±2 ±2 LSB max
Differential Nonlinearity ±1 ±1 LSB max Guaranteed Monotonic
Bipolar Zero Error ±10 ±10 LSB max
Positive Full-Scale Error
Negative Full-Scale Error
REFERENCE OUTPUT
2
2
3
±10 ±10 LSB max
±10 ±10 LSB max
REF OUT @ +25°C 2.99/3.01 2.99/3.01 V min/V max
to T
T
MIN
MAX
2.98/3.02 2.98/3.02 V min/V max
REF OUT Tempco 35 35 ppm/°C typ
Reference Load Change
(∆REF OUT vs. ∆I) –1 –1 mV max Reference Load Current Change (0 µA–500 µA)
REFERENCE INPUTS
REF INA, REF INB Input Range 2.85/3.15 2.85/3.15 V min/V max 3 V ± 5%
Input Current 1 1 µA max
LOGIC INPUTS
LDACA, LDACB, TFSA, TFSB,
(
TCLKA, TCLKB, DTA, DTB)
Input High Voltage, V
Input Low Voltage, V
Input Current, I
Input Capacitance, C
IN
IN
INH
INL
4
2.4 2.4 V min VDD = 5 V ± 5%
0.8 0.8 V max VDD = 5 V ± 5%
±10 ±10 µA max VIN = 0 V to V
10 10 pF max
DD
ANALOG OUTPUTS
, V
(V
OUTA
OUTB
)
Output Voltage Range ±3 ±3 V nom
DC Output Impedance 0.1 0.1 Ω typ
Short Circuit Current 20 20 mA typ
AC CHARACTERISTICS
4
Voltage Output Settling Time Settling Time to Within ±1/2 LSB of Final Value
Positive Full-Scale Change 4 4 µs max Typically 2.5 µs
Negative Full-Scale Change 4 4 µs max Typically 2.5 µs
Digital-to-Analog Glitch Impulse 10 10 nV secs typ DAC Code Change All 1s to All 0s
Digital Feedthrough 2 2 nV secs typ
Channel-to-Channel Isolation 110 110 dB typ V
= 10 kHz Sine Wave
OUT
POWER REQUIREMENTS
V
DD
V
SS
I
DD
I
SS
+5 +5 V nom ±5% for Specified Performance
–5 –5 V nom ±5% for Specified Performance
27 28 mA max Cumulative Current from the Two VDD Pins
15 15 mA max Cumulative Current from the Two VSS Pins
Total Power Dissipation 195 205 mW max Typically 130 mW
NOTES
1
Temperature ranges are as follows: J Version: 0°C to +70°C; A Version: –40°C to +85°C; S Version: –55°C to +125°C.
2
Measured with respect to REF IN and includes bipolar offset error.
3
For capacitive loads greater than 50 pF, a series resistor is required (see Internal Reference section).
4
Sample tested @ +25°C to ensure compliance.
Specifications subject to change without notice.
AD7244 ORDERING GUIDE
Temperature Integral Package
Range Nonlinearity Option
2
Model
1
AD7244JN –40°C to +85°C ±2 LSB max N-24
AD7244JR –40°C to +85°C ±2 LSB max R-28
AD7244AQ –40°C to +85°C ±2 LSB max Q-24
AD7244SQ
NOTES
1
To order MIL-STD-883, Class B, processed parts, add /883B to part number.
Contact local sales office for military data sheet and availability.
2
N = Plastic DIP; Q = Cerdip; R = Small Outline IC (SOIC).
3
This grade will be available to /883B processing only.
3
–55°C to +125°C ±2 LSB max Q-24
REV. A
–3–
Page 4
AD7242/AD7244
WARNING!
ESD SENSITIVE DEVICE
TIMING CHARACTERISTICS
1, 2
(VDD = +5 V 6 5%, VSS = –5 V 6 5%, AGND = DGND = 0 V)
Limit at T
MIN
, T
MAX
Limit at T
MIN
, T
MAX
Parameter (J, K, A, B Versions) (S Version) Units Conditions/Comments
t
1
t
2
3
t
3
t
4
t
5
t
6
NOTES
1
Timing specifications are sample tested at +25°C to ensure compliance. All input signals are specified with tr = tf = 5 ns (10% to 90% of 5 V) and timed from a volt-
age level of 1.6 V.
2
See Figure 6.
3
TCLK Mark/Space ratio is 40/60 to 60/40.
ABSOLUTE MAXIMUM RATINGS*
(TA = +25°C unless otherwise noted)
VDD to AGND . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V
V
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . +0.3 V to –7 V
SS
AGND to DGND . . . . . . . . . . . . . . . . –0.3 V to V
V
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . VSS to V
OUT
REF OUT to AGND . . . . . . . . . . . . . . –0.3 V to VDD + 0.3 V
REF INA, REF INB to AGND . . . . . . . –0.3 V to V
Digital Inputs to DGND . . . . . . . . . . . . –0.3 V to V
50 50 ns min TFS to TCLK Falling Edge
75 100 ns min TCLK Falling Edge to TFS
150 200 ns min TCLK Cycle Time
30 40 ns min Data Valid to TCLK Setup Time
75 100 ns min Data Valid to TCLK Hold Time
40 40 ns min LDAC Pulse Width
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . +300°C
Power Dissipation (Any Package) to +75°C . . . . . . . 550 mW
Derates above +75°C by . . . . . . . . . . . . . . . . . . . . . 6 mW/°C
+ 0.3 V
DD
+ 0.3 V
DD
+ 0.3 V
DD
*Stresses above those listed under “Absolute Maximum Ratings” may cause
DD
permanent damage to the device. This is a stress rating only, 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.
Operating Temperature Range
J, K Versions
AD7244 . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
AD7242 . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C
A, B Versions . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C
S Version . . . . . . . . . . . . . . . . . . . . . . . . . –55°C to +125°C
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 AD7242/AD7244 feature 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
SOIC
–4–
REV. A
Page 5
AD7242/AD7244 PIN FUNCTION DESCRIPTION
DIP
Pin No. Mnemonic Description
AD7242/AD7244
1
2
LDACA Load DAC, Logic Input. A new word is transferred into DAC Latch A from input Latch A on the fall-
ing edge of this signal. If
Latch A on the sixteenth falling edge of TCLKA after
LDACA is hard-wired low, data is transferred from input Latch A to DAC
TFSA goes low.
TFSA Transmit Frame Synchronization, Logic Input. This is a frame or synchronization signal for DACA
data with serial data expected after the falling edge of this signal.
3 DTA Transmit Data, Logic Input. This is the data input which is used in conjunction with
TFSA and
TCLKA to transfer serial data to input Latch A.
4 TCLKA Transmit Clock, Logic Input. Serial data bits for DACA are latched on the falling edge of TCLKA
when
TFSA is low.
5 DGND Digital Ground. Both DGND pins for the device must be tied together at the device.
6 TP1 Test Pin 1. Used when testing the device. Do not connect anything to this pin.
7V
DD
Positive Power Supply, 5 V ± 5%. Both V
pins for the device must be tied together at the device.
DD
8 AGND Analog Ground. Both AGND pins for the device must be tied together at the device.
9V
OUTB
Analog Output Voltage from DACB. This output comes from a buffer amplifier. The range is bipolar,
±3 V with REF INB = +3 V.
10 V
SS
Negative Power Supply, –5 V ± 5%. Both V
pins for the device must be tied together at the device.
SS
11 TP2 Test Pin 2. Used when testing the device. Do not connect anything to this pin.
12 REF INB DACB Voltage Reference Input. The voltage reference for DACB is applied to this pin. It is internally
buffered before being applied to DACB. The nominal reference voltage for correct operation of the
AD7242/AD7244 is 3 V.
13
14
LDACB Load DAC, Logic Input. A new word is transferred into DAC Latch B from input Latch B on the fall-
ing edge of this signal. If
Latch B on the sixteenth falling edge of TCLKB after
LDACB is hard-wired low, data is transferred from input Latch B to DAC
TFSB goes low.
TFSB Transmit Frame Synchronization, Logic Input. This is a frame or synchronization signal for DACB
data with serial data expected after the falling edge of this signal.
15 DTB Transmit Data, Logic Input. This is the data input used in conjunction with
TFSB and TCLKB to
transfer serial data to input Latch B.
16 TCLKB Transmit Clock, Logic Input. Serial data bits for DACB are latched on the falling edge of TCLKB
when
TFSB is low.
17 DGND Digital Ground. Both DGND pins for the device must be tied together at the device.
18 TP3 Test Pin 3. Used when testing the device. Do not connect anything to this pin.
19 V
DD
Positive Power Supply, 5 V ± 5%. Both V
pins for the device must be tied together at the device.
DD
20 AGND Analog Ground. Both AGND pins for the device must be tied together at the device.
21 V
OUTA
Analog Output Voltage from DACA. This output comes from a buffer amplifier. The range is bipolar,
±3 V with REF INA = +3 V.
22 V
SS
Negative Power Supply, –5 V ± 5%. Both V
pins for the device must be tied together at the device.
SS
23 REF OUT Voltage Reference Output. To operate the DACs with this internal reference, REF OUT should be
connected to both REF INA and REF INB. The external load capability of the reference is 500 µA.
24 REF INA DACA Voltage Reference Input. The voltage reference for DACA is applied to this pin. It is internally
buffered before being applied to DACA. The nominal reference voltage for correct operation of the
AD7242/AD7244 is 3 V.
REV. A
–5–
Page 6
AD7242/AD7244
CIRCUIT DESCRIPTION
The AD7242/AD7244 contains two 12-bit/14-bit D/A converters, each with an output buffer amplifier. The part also contains
a reference input buffer amplifier for each DAC, and an on-chip
3 V reference.
D/A Section
The AD7242/AD7244 contains two 12-bit/14-bit voltage mode
D/A converters, each consisting of highly stable thin-film resistors
and high speed single-pole, double-throw switches. The simplified
circuit diagram for the DAC section is shown in Figure 1. The
three MSBs of the data word are decoded to drive the seven
switches A-G. On the AD7242, the 9 LSBs switch a
9-bit R-2R ladder structure while on the AD7244, the 11 LSBs
switch an 11-bit R-2R ladder structure. The output voltage
from this converter has the same polarity as the reference
voltage, REF IN.
The REF IN voltage is internally buffered by a unity gain
amplifier before being applied to the D/A converters and the
bipolar bias circuitry. The D/A converter is configured and
scaled for a 3 V reference, and the device is tested with 3 V
applied to REF IN. Operating the AD7242/AD7244 at reference voltages outside the ± 5% tolerance range may result in
degraded performance from the part.
Figure 1. DAC Ladder Structure
Internal Reference
The on-chip reference is a temperature-compensated buried
Zener reference that is factory trimmed for 3 V ± 10 mV. The
reference can be used to provide both the reference voltage for
the two D/A converters and the bipolar biasing circuitry. This is
achieved by connecting REF OUT to REF INA and REF INB.
The reference voltage can also be used for other components
and is capable of providing up to 500 µA to an external load.
The maximum recommended capacitance on the reference
output pin for normal operation is 50 pF. If the reference
output is required to drive a capacitive load greater than 50 pF,
a 200 Ω resistor should be placed in series with the capacitive
load. Decoupling the REF OUT pin with a series 200 Ω resistor
and a parallel combination of a 10 µF tantalum capacitor and a
0.1 µF ceramic capacitor as in Figure 2 reduces the noise
spectral density of the reference (see Figure 4). Using this
decoupling scheme to generate the reference voltage for REF
INA and REF INB gives a channel-to-channel isolation number
of 110 dB (connecting REF OUT directly to REF INA and
REF INB gives 80 dB). The channel-to-channel isolation is 110
dB using an external reference.
External Reference
In some applications, the user may require a system reference or
some other external reference to drive the AD7242/AD7244
reference inputs. Figure 3 shows how the AD586 reference can
be conditioned to provide the 3 V reference required by the
AD7242/AD7244 reference inputs.
Figure 2. Circuit Connection for REF OUT with an External
Capacitive Load of Greater Than 50 pF
Figure 3. AD586 Driving AD7242/AD7244 Reference Inputs
–6–
REV. A
Page 7
AD7242/AD7244
Output Amplifier
The outputs from each of the voltage-mode DACs are buffered
by a noninverting amplifier. The buffer amplifier is capable of
developing ±3 V across a 2 kΩ and 100 pF load to ground, and
can produce 6 V peak-to-peak sine wave signals to a frequency
of 20 kHz. The output is updated on the falling edge of the
respective
LDAC input. The output voltage settling time, to
within 1/2 LSB of its final value, is typically less than 2 µs for
the AD7242 and 2.5 µs for the AD7244.
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 30 nV/√
Hz at a frequency of
1 kHz. The broadband noise from the amplifier exhibits a
typical peak-to-peak figure of 150 µV for a 1 MHz output
bandwidth. Figure 4 shows a typical plot of noise spectral
density versus frequency for the output buffer amplifier and for
the on-chip reference (including and excluding the decoupling
components).
For the AD7242, the output voltage can be expressed in terms
of the input code, N, using the following relationship:
OUT
=
2 • N • REF IN
4096
V
where –2048 ≤ N ≤ +2047
For the AD7244, the output voltage can be expressed in terms
of the input code, N, using the following relationship:
OUT
=
2 • N • REF IN
16384
V
where –8192 ≤ N ≤ +8191
Table I. AD7242 Ideal Input/Output Code Table Code
DAC Latch Contents
MSB LSB Analog Output, V
OUT
*
01 11 1111 1111 +2.998535 V
01 11 1111 1110 +2.99707 V
00 00 0000 0001 +0.001465 V
00 00 0000 0000 0 V
11 11 1111 1111 –0.001465 V
10 00 0000 0001 –2.998535 V
10 00 0000 0000 –3 V
*Assuming REF IN = +3 V.
Figure 4. Noise Spectral Density vs. Frequency
TRANSFER FUNCTION
The basic circuit configuration for the AD7242/AD7244 is
shown in Figure 5. Table I and Table II show the ideal input
code to output voltage relationship for the AD7242 and
AD7244 respectively. Input coding for the AD7242/AD7244 is
2s complement.
Table II. AD7244 Ideal Input/Output Code Table Code
DAC Latch Contents
MSB LSB Analog Output, V
OUT
*
01 1111 1111 1111 +2.999634 V
01 1111 1111 1110 +2.99268 V
00 0000 0000 0001 +0.000366 V
00 0000 0000 0000 0 V
11 1111 1111 1111 –0.000366 V
10 0000 0000 0001 –2.999634 V
10 0000 0000 0000 –3 V
*Assuming REF IN = +3 V.
REV. A
Figure 5. Basic Connection Diagram
–7–
Page 8
AD7242/AD7244
TIMING AND CONTROL
Communication with the AD7242/AD7244 is via six serial logic
inputs. These consist of separate serial clocks, word framing and
data lines for each DAC. DAC updating is controlled by two
digital inputs:
updating V
the microprocessor by an external timer when precise updating
intervals are required. Alternatively, the
inputs can be driven from a decoded address bus allowing the
microprocessor control over DAC updating as well as data
communication to the AD7242/AD7244 input latches.
The AD7242/AD7244 contains two latches per DAC, an input
latch and a DAC latch. Data must be loaded to the input latch
under the control of TCLKA,
A and TCLKB,
transferred from input Latch A to DAC Latch A under the control
of the
LDACA signal, while LDACB controls the loading of DAC
Latch B from input Latch B. Only the data held in the DAC
latches determines the analog outputs of the AD7242/AD7244.
Data is loaded to the input latches under control of the respective TCLK,
expects a 16-bit stream of serial data on its DT inputs. Data
must be valid on the falling edge of TCLK. The
provides the frame synchronization signal that tells the AD7242/
AD7244 that valid serial data will be available on the DT input
for the next 16 falling edges of TCLK. Figure 6 shows the
LDACA for updating V
. These inputs can be asserted independently of
OUTB
TFSA and DTA for input Latch
TFSB and DTB for input Latch B. Data is then
TFS and DT signals. The AD7242/AD7244
and LDACB for
OUTA
LDACA and LDACB
TFS input
timing diagram for operation of either of the two serial input
ports on the part.
Although 16 bits of data are clocked into the input latch, only
12 bits are transferred into the DAC latch for the AD7242 and
14 bits are transferred for the AD7244. Therefore, 4 bits in the
AD7242 data stream and 2 bits in the AD7244 data stream are
don’t cares since their value does not affect the DAC latch data.
The bit positions are the don’t cares followed by the DAC data
starting with the MSB (see Figure 6).
The respective
respective DAC latches. Normally, data is loaded to the DAC
latch on the falling edge of
low, serial data is loaded to the DAC latch on the sixteenth
falling edge of TCLK. If
serial data to the input latch, no DAC latch update takes place
on the falling edge of
transfer is completed, then the update takes place on the sixteenth
falling edge of TCLK. If
data transfer is completed, no DAC latch update takes place.
If seventeen or more TCLK edges occur while
seventeenth (and beyond) clock edges are ignored, i.e., no
further data is clocked into the input latch after the sixteenth
TCLK edge following a falling edge on
LDAC signals control the transfer of data to the
LDAC. However, if LDAC is held
LDAC goes low during the loading of
LDAC. If LDAC stays low until the serial
LDAC returns high before the serial
TFS is low, the
TFS.
Figure 6. AD7242/AD7244 Timing Diagram
–8–
REV. A
Page 9
AD7242/AD7244
MICROPROCESSOR INTERFACING
Microprocessor interfacing to the AD7242/AD7244 is via a
serial bus that uses standard protocol compatible with DSP
processors and microcontrollers. The communication interface
consists of a separate transmit section for each of the DACs.
Each section has a clock signal, a data signal and a frame or
strobe pulse.
Figures 7 through 11 show the AD7242/AD7244 configured
for interfacing to a number of popular DSP processors and
microcontrollers.
AD7242/AD7244 to ADSP-2101/ADSP-2102 Interface
Figure 7 shows a serial interface between the AD7242/AD7244
and the ADSP-2101/ADSP-2102 DSP processor. The ADSP2101/ADSP-2102 has two serial ports and, in the interface
shown, both serial ports are used, one for each DAC. Both serial
ports do not have to be used; in the case where only one serial
port is used, an extra line (DACA/
serial interfaces) would have to decode the one
provide
TFSA and TFSB lines for the AD7242/AD7244.
DACB as shown in the other
TFS line to
control or address line of the ADSP-2101/ADSP-2102 could be
used to drive these inputs. Alternatively, the
LDACA and
LDACB inputs of the AD7242/AD7244 could be hardwired
low; in this case the update of the DAC latches and analog
outputs takes place on the 16th falling edge of SCLK (after the
respective
AD7242/AD7244 to DSP56000 Interface
TFS signal goes low).
A serial interface between the AD7242/AD7244 and the
DSP56000 is shown in Figure 8. The DSP56000 is configured
for normal mode, asynchronous operation with gated clock. It is
also set up for a 16-bit word with SCK and SC2 as outputs and
the FSL control bit set to a 0. SCK is internally generated on
the DSP56000 and applied to both the TCLKA and TCLKB
inputs of the AD7242/AD7244. Data from the DSP56000 is
valid on the falling edge of SCK. The serial data line, STD
drives the DTA and DTB serial input data lines of the
AD7242/AD7244.
The SC2 output provides the framing pulse for valid data. This
is an active high output and is gated with a DACA/
control line before being applied to the
of the AD7242/AD7244. The DACA/
TFSA and TFSB inputs
DACB line determines
which DAC serial data is to be transferred to, i.e., which
DACB
TFS
line is active when SC2 is active.
As in the previous interface, a common
driving the
Once again, these
which case V
falling edge of SCK after the
LDACA and LDACB inputs of the AD7242/AD7244.
LDAC inputs could be hardwired low, in
OUTA
or V
will be updated on the sixteenth
OUTB
TFSA or TFSB input goes low.
LDAC input is shown
Figure 7. AD7242/AD7244 to ADSP-2101/ADSP-2102
Interface
The three serial lines of the first serial port, SPORT1, of the
ADSP-2101/ADSP-2102 connect directly to the three serial
input lines of DACA of the AD7242/AD7244. The three serial
lines of SPORT2 connect directly to the three serial lines on the
DACB serial input port. Data from the ADSP-2101/ADSP-2102 is
valid on the falling edge of SCLK. A common LDAC signal is
used to drive the
LDACA and LDACB inputs. This is shown to
be generated from a timer or clock recovery circuit but another
REV. A
–9–
Figure 8. AD7242/AD7244 to DSP56000 Interface
Page 10
AD7242/AD7244
AD7242/AD7244 to TMS320C25 Interface
Figure 9 shows a serial interface between the AD7242/AD7244
and the TMS320C25 DSP processor. In this interface, the
CLKX and FSX signals of the TMS320C25 are generated from
the clock/timer circuitry. The FSX pin of the TMS320C25
must be configured as an input. CLKX is used to provide both
the TCLKA and TCLKB inputs of the AD7242/AD7244. DX
of the TMS320C25 is also routed to the serial data line of each
input port of the AD7242/AD7244.
Data from the TMS32020 is valid on the falling edge of CLKX
after FSX goes low. This FSX signal is gated with the DACA/
DACB control line to determine whether TFSA or TFSB goes
low when FSX goes low.
The clock/timer circuitry also generates the
LDAC signal for the
AD7242/AD7244 to synchronize the update of the outputs with
the serial transmission. As in the previous interface diagrams, a
common
LDAC input is shown driving the LDACA and
LDACB inputs of the AD7242/AD7244. Once again, these
LDAC inputs could be hardwired low, in which case V
V
will be updated on the sixteenth falling edge of CLKX
OUTB
after the
AD7242/AD7244 to 87C51 Interface
TFSA or TFSB input goes low.
Figure 9. AD7242/AD7244 to TMS320C25 Interface
OUTA
or
A serial interface between the AD7242/AD7244 and the 87C51
microcontroller is shown in Figure 10. TXD of the 87C51
drives TCLKA and TCLKB of the AD7242/AD7244 while
RXD drives the two serial data lines of the part. The
TFSA and
TFSB signals are derived from P3.2 and P3.3, respectively.
The 87C51 provides the LSB of its SBUF register as the first bit
in the serial data stream. Therefore, the user will have to ensure
that the data in the SBUF register is correctly arranged so the
don’t care bits are the first to be transmitted to the AD7242/
AD7244; the last bit to be sent is the LSB of the word to be
loaded to the AD7242/AD7244. When data is to be transmitted
to the part, P3.2 (for DACA) or P3.3 (for DACB) is taken low.
Data on RXD is valid on the falling edge of TXD. The 87C51
transmits its serial data in 8-bit bytes with only eight falling
clock edges occurring in the transmit cycle. To load data to the
AD7242/AD7244, P3.2 (for DACA) or P3.3 (for DACB) is left
low after the first eight bits are transferred and a second byte of
data is then serially transferred to the AD7242/AD7244. When
the second serial transfer is complete, the P3.2 line (for DACA)
or the P3.3 line (for DACB) is taken high.
Figure 10 shows both
LDAC inputs of the AD7242/AD7244
hardwired low. As a result, the DAC latch and the analog
–10–
output of one of the DACs will be updated on the sixteenth
falling edge of TXD after the respective
TFS signal for that
DAC has gone low. Alternatively, the scheme used in previous
interfaces, whereby the
LDAC inputs are driven from a timer,
can be used.
Figure 10. AD7242/AD7244 to 87C51 Interface
AD7242/AD7244 to 68HC11 Interface
Figure 11 shows a serial interface between the AD7242/AD7244
and the 68HC11 microcontroller. SCK of the 68HC11 drives
TCLKA and TCLKB of the AD7242/AD7244 while the MOSI
output drives the two serial data lines of the AD7242/AD7244.
The
TFSA and TFSB signals are derived from PC6 and PC7,
respectively.
For correct operation of this interface, the 68HC11 should be
configured such that its CPOL bit is a 0 and its CPHA bit is a 1.
When data is to be transmitted to the part, PC6 (for DACA) or
PC7 (for DACB) is taken low. When the 68HC11 is configured
like this, data on MOSI is valid on the falling edge of SCK. The
68HCll transmits its serial data in 8-bit bytes with only eight
falling clock edges occurring in the transmit cycle. To load data
to the AD7242/AD7244, PC6 (for DACA) or PC7 (for DACB)
is left low after the first eight bits are transferred and a second
byte of data is then serially transferred to the AD7242/AD7244.
When the second serial transfer is complete, the PC6 line (for
DACA) or the PC7 line (for DACB) is taken high.
Figure 11. AD7242/AD7244 to 68HC11 Interface
Figure 11 shows both LDAC inputs of the AD7242/AD7244
hardwired low. As a result, the DAC latch and the analog
output of one of the DACs will be updated on the sixteenth
falling edge of SCK after the respective
TFS signal for that
DAC has gone low. Alternatively, the scheme used in previous
interfaces, whereby the
LDAC inputs are driven from a timer,
can be used.
REV. A
Page 11
AD7242/AD7244
APPLYING THE AD7242/AD7244
Good printed circuit board layout is as important as the overall
circuit design itself in achieving high speed converter performance. The AD7242 works on an LSB size of 1.465 mV, while
the AD7244 works on an LSB size of 366 µV. Therefore, the
designer must be conscious of minimizing noise in both the
converter itself and in the surrounding circuitry. Switching
mode power supplies are not recommended as the switching
spikes can feed through to the on-chip amplifier. Other causes
of concern are ground loops and digital feedthrough from
microprocessors. These are factors that influence any high
performance converter, and a proper PCB layout that minimizes
these effects is essential for best performance.
LAYOUT HINTS
Ensure that the layout for the printed circuit board has separated
digital and analog lines as much as possible. Take care not to
run any digital track alongside an analog signal track. Establish a
single point analog ground (star ground) separate from the logic
system ground. Place this star ground as close as possible to the
AD7242/AD7244. Connect all analog grounds to this star
ground and also connect the AD7242/AD7244 DGND pins to
this ground. Do not connect any other digital grounds to this
analog ground point.
Low impedance analog and digital power supply common
returns are essential to low noise operation of high performance
converters. Therefore, the foil width for these tracks should be
kept as wide as possible. The use of ground planes minimizes
impedance paths and also guards the analog circuitry from
digital noise.
NOISE
Keep the signal leads on the V
signal return leads to AGND as short as possible to minimize
noise coupling. In applications where this is not possible, use a
shielded cable between the DAC outputs and their destination.
Reduce the ground circuit impedance as much as possible since
any potential difference in grounds between the DAC and its
destination device appears as an error voltage in series with the
DAC output.
OUTA
and V
signals and the
OUTB
REV. A
–11–
Page 12
AD7242/AD7244
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
Plastic DIP (N-24)
C1421–10–10/90
Cerdip (Q-24)
SOIC (R-28)
–12–
PRINTED IN U.S.A.
REV. A
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