AD AD7847

a
LC2MOS
Complete, Dual 12-Bit MDACs
AD7837/AD7847
FEATURES
Two 12-Bit MDACs with Output Amplifiers
4-Quadrant Multiplication
Space-Saving 0.3", 24-Lead DIP and 24-Terminal
SOIC Package
Parallel Loading Structure: AD7847
(8 + 4) Loading Structure: AD7837
FUNCTIONAL BLOCK DIAGRAMS
VDD
MS INPUT
LATCH
LS INPUT
LATCH
4
8
AD7837
DAC LATCH A
APPLICATIONS
Automatic Test Equipment
Function Generation
Waveform Reconstruction
Programmable Power Supplies
Synchro Applications
VREFA
AGNDA
RFBB
DB7
DAC B
LDAC
12
The AD7837/AD7847 is fabricated in Linear Compatible CMOS
(LC2MOS), an advanced, mixed technology process that combines precision bipolar circuits with low power CMOS logic.
A novel low leakage configuration (U.S. Patent No. 4,590,456)
ensures low offset errors over the specified temperature range.
AGNDB
CONTROL
LOGIC
4
8
MS INPUT
LATCH
LS INPUT
LATCH
A1
The AD7837/AD7847 is a complete, dual, 12-bit multiplying
digital-to-analog converter with output amplifiers on a monolithic CMOS chip. No external user trims are required to
achieve full specified performance.
The amplifier feedback resistors are internally connected to
VOUT on the AD7847.
VOUTB
DAC LATCH B
A0
The output amplifiers are capable of developing ± 10 V across a
2 kΩ load. They are internally compensated with low input offset voltage due to laser trimming at wafer level.
VOUTA
DB0
WR
Both parts are microprocessor compatible, with high speed data
latches and interface logic. The AD7847 accepts 12-bit parallel
data which is loaded into the respective DAC latch using the
WR input and a separate Chip Select input for each DAC. The
AD7837 has a double-buffered 8-bit bus interface structure
with data loaded to the respective input latch in two write operations. An asynchronous LDAC signal on the AD7837 updates
the DAC latches and analog outputs.
RFBA
VREFB
CS
GENERAL DESCRIPTION
12
DAC A
VSS
DGND
VDD
AD7847
DAC LATCH A
VOUTA
DAC A
VREFA
VREFB
AGNDA
DB0
DB11
DAC B
VOUTB
WR
CSA
AGNDB
CONTROL
LOGIC
DAC LATCH B
CSB
DGND
VSS
PRODUCT HIGHLIGHTS
1. The AD7837/AD7847 is a dual, 12-bit, voltage-out MDAC
on a single chip. This single chip design offers considerable
space saving and increased reliability over multichip designs.
2. The AD7837 and the AD7847 provide a fast versatile interface to 8-bit or 16-bit data bus structures.
REV. C
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: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2000
1
(V = +15 V 5%, V = –15 V 5%, AGNDA = AGNDB = DGND
AD7837/AD7847–SPECIFICATIONS
= O V. V = V = +10 V, R = 2 k, C = 100 pF [V connected to R AD7837]. All specifications T to T unless otherwise noted.)
DD
REFA
REFB
L
Parameter
L
OUT
SS
FB
MIN
MAX
A Version
B Version
S Version
Units
Test Conditions/Comments
12
±1
±1
12
± 1/2
±1
12
±1
±1
Bits
LSB max
LSB max
Guaranteed Monotonic
±2
±4
±2
±3
±2
±4
mV max
mV max
DAC Latch Loaded with All 0s
Temperature Coefficient = ± 5 µV/°C typ
±4
±5
±2
±3
±4
±5
LSB max
LSB max
DAC Latch Loaded with All 1s
Temperature Coefficient = ± 2 ppm of
FSR/°C typ
REFERENCE INPUTS
VREF Input Resistance
VREFA, VREFB Resistance Matching
8/13
±2
8/13
±2
8/13
±2
kΩ min/max
% max
Typical Input Resistance = 10 kΩ
Typically ± 0.25%
DIGITAL INPUTS
Input High Voltage, VINH
Input Low Voltage, VINL
Input Current
Input Capacitance3
2.4
0.8
±1
8
2.4
0.8
±1
8
2.4
0.8
±1
8
V min
V max
µA max
pF max
ANALOG OUTPUTS
DC Output Impedance
Short Circuit Current
0.2
11
0.2
11
0.2
11
Ω typ
mA typ
14.25/15.75
–14.25/–15.75
14.25/15.75
14.25/15.75
–14.25/–15.75 –14.25/–15.75
V min/max
V min/max
± 0.01
± 0.01
8
± 0.01
± 0.01
8
± 0.01
± 0.01
8
% per % max
% per % max
mA max
6
6
6
mA max
3
5
3
5
3
5
µs typ
µs max
11
10
11
10
11
10
V/µs typ
nV secs typ
–95
–95
–95
dB typ
–95
–95
–95
dB typ
Multiplying Feedthrough Error
–90
–90
–90
dB typ
Unity Gain Small Signal BW
750
750
750
kHz typ
Full Power BW
175
175
175
kHz typ
Total Harmonic Distortion
–88
–88
–88
dB typ
Digital Crosstalk
1
1
1
nV secs typ
Output Noise Voltage @ +25°C
(0.1 Hz to 10 Hz)
Digital Feedthrough
2
1
2
1
2
1
µV rms typ
nV secs typ
STATIC PERFORMANCE
Resolution
Relative Accuracy2
Differential Nonlinearity2
Zero Code Offset Error2
@ +25°C
TMIN to TMAX
Gain Error2
@ +25°C
TMIN to TMAX
POWER REQUIREMENTS
VDD Range
VSS Range
Power Supply Rejection
∆Gain/∆VDD
∆Gain/∆VSS
IDD
Digital Inputs at 0 V and VDD
VOUT Connected to AGND
4
ISS
AC CHARACTERISTICS2, 3
Voltage Output Settling Time
Slew Rate
Digital-to-Analog Glitch Impulse
Channel-to-Channel Isolation
VREFA to VOUTB
VREFB to VOUTA
VDD = 15 V ± 5%, VREF = –10 V
VSS = –15 V ± 5%, VREF = +10 V
Outputs Unloaded. Inputs at Thresholds.
Typically 5 mA
Outputs Unloaded. Inputs at Thresholds.
Typically 3 mA
Settling Time to Within ± 1/2 LSB of Final
Value. DAC Latch Alternately Loaded
with All 0s and All 1s
1 LSB Change Around Major Carry
VREFA = 20 V p-p, 10 kHz Sine Wave.
DAC Latches Loaded with All 0s
VREFB = 20 V p-p, 10 kHz Sine Wave.
DAC Latches Loaded with All 0s
VREF = 20 V p-p, 10 kHz Sine Wave.
DAC Latch Loaded with All 0s
VREF = 100 mV p-p Sine Wave. DAC
Latch Loaded with All 1s
VREF = 20 V p-p Sine Wave. DAC
Latch Loaded with All 1s
VREF = 6 V rms, 1 kHz. DAC Latch
Loaded with All 1s
Code Transition from All 0s to All 1s and
Vice Versa
See Typical Performance Graphs
Amplifier Noise and Johnson Noise of RFB
NOTES
1
Temperature ranges are as follows: A, B Versions, –40°C to +85°C; S Version, –55°C to +125°C.
2
See Terminology.
3
Guaranteed by design and characterization, not production tested.
4
The Devices are functional with V DD/VSS = ± 12 V (See typical performance graphs.).
Specifications subject to change without notice.
–2–
REV. C
AD7837/AD7847
TIMING CHARACTERISTICS1, 2, 3
(VDD = +15 V 5%, VSS = –15 V 5%, AGNDA = AGNDB = DGND = O V)
Parameter
Limit at TMIN, TMAX
(All Versions)
Unit
Conditions/Comments
t1
t2
t3
t4
t5
t6 4
t7 4
t8 4
0
0
30
80
0
0
0
50
ns min
ns min
ns min
ns min
ns min
ns min
ns min
ns min
CS to WR Setup Time
CS to WR Hold Time
WR Pulsewidth
Data Valid to WR Setup Time
Data Valid to WR Hold Time
Address to WR Setup Time
Address to WR Hold Time
LDAC Pulsewidth
NOTES
1
All input signals are specified with tr = tf = 5 ns (10% to 90% of 5 V) and timed from a voltage level of 1.6 V.
2
See Figures 3 and 5.
3
Guaranteed by design and characterization, not production tested.
4
AD7837 only.
ABSOLUTE MAXIMUM RATINGS*
ORDERING GUIDE
(TA = +25°C unless otherwise noted)
VDD to DGND, AGNDA, AGNDB . . . . . . . –0.3 V to +17 V
VSS1 to DGND, AGNDA, AGNDB . . . . . . . +0.3 V to –17 V
VREFA, VREFB to AGNDA, AGNDB
. . . . . . . . . . . . . . . . . . . . . . . . . . VSS – 0.3 V to VDD + 0.3 V
AGNDA, AGNDB to DGND . . . . . . . –0.3 V to VDD + 0.3 V
VOUTA2, VOUTB2 to AGNDA, AGNDB
. . . . . . . . . . . . . . . . . . . . . . . . . . VSS – 0.3 V to VDD + 0.3 V
RFBA3, RFBB3 to AGNDA, AGNDB
. . . . . . . . . . . . . . . . . . . . . . . . . . VSS – 0.3 V to VDD + 0.3 V
Digital Inputs to DGND . . . . . . . . . . . –0.3 V to VDD + 0.3 V
Operating Temperature Range
Commercial/Industrial (A, B Versions) . . . –40°C to +85°C
Extended (S Version) . . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 secs) . . . . . . . . . . . . . 300°C
Power Dissipation (Any Package) to +75°C . . . . . . 1000 mW
Derates above +75°C by . . . . . . . . . . . . . . . . . . . . 10 mW/°C
NOTES
1
If VSS is open circuited with V DD and either AGND applied, the V SS pin will float
positive, exceeding the Absolute Maximum Ratings. If this possibility exists, a
Schottky diode connected between V SS and AGND (cathode to AGND) ensures
the Maximum Ratings will be observed.
2
The outputs may be shorted to voltages in this range provided the power dissipation
of the package is not exceeded.
3
AD7837 only.
Model1
Temperature
Range
Relative
Accuracy
Package
Option2
AD7837AN
AD7837BN
AD7837AR
AD7837BR
AD7837AQ
AD7837BQ
AD7837SQ
–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
–55°C to +125°C
± 1 LSB
± 1/2 LSB
± 1 LSB
± 1/2 LSB
± 1 LSB
± 1/2 LSB
± 1 LSB
N-24
N-24
R-24
R-24
Q-24
Q-24
Q-24
AD7847AN
AD7847BN
AD7847AR
AD7847BR
AD7847AQ
AD7847BQ
AD7847SQ
–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
–55°C to +125°C
± 1 LSB
± 1/2 LSB
± 1 LSB
± 1/2 LSB
± 1 LSB
± 1/2 LSB
± 1 LSB
N-24
N-24
R-24
R-24
Q-24
Q-24
Q-24
NOTES
1
To order MIL-STD-883, Class B processed parts, add /883B to part number.
2
N = Plastic DIP; Q = Cerdip; R = SOIC.
*Stresses above those listed under Absolute Maximum Ratings may cause 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. Only one Absolute
Maximum Rating may be applied at any one time.
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 these devices 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.
REV. C
–3–
WARNING!
ESD SENSITIVE DEVICE
AD7837/AD7847
Channel-to-Channel Isolation
TERMINOLOGY
Relative Accuracy (Linearity)
This is an ac error due to capacitive feedthrough from the VREF
input on one DAC to VOUT on the other DAC. It is measured
with the DAC latches loaded with all 0s.
Relative accuracy, or endpoint linearity, is a measure of the
maximum deviation of the DAC transfer function from a
straight line passing through the endpoints. It is measured after
allowing for zero and full-scale errors and is expressed in LSBs
or as a percentage of full-scale reading.
Digital Feedthrough
Digital feedthrough is the glitch impulse injected from the digital inputs to the analog output when the data inputs change state,
but the data in the DAC latches is not changed.
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 or less
over the operating temperature range ensures monotonicity.
For the AD7837, it is measured with LDAC held high. For the
AD7847, it is measured with CSA and CSB held high.
Digital Crosstalk
Digital crosstalk is the glitch impulse transferred to the output
of one converter due to a change in digital code on the DAC
latch of the other converter. It is specified in nV secs.
Zero Code Offset Error
Zero code offset error is the error in output voltage from VOUTA
or VOUTB with all 0s loaded into the DAC latches. It is due to a
combination of the DAC leakage current and offset errors in the
output amplifier.
Digital-to-Analog Glitch Impulse
This is the voltage spike that appears at the output of the DAC
when the digital code changes, before the output settles to its
final value. The energy in the glitch is specified in nV secs and is
measured for a 1 LSB change around the major carry transition
(0111 1111 1111 to 1000 0000 0000 and vice versa).
Gain Error
Gain error is a measure of the output error between an ideal
DAC and the actual device output with all 1s loaded. It does
not include offset error.
Unity Gain Small Signal Bandwidth
Total Harmonic Distortion
This is the ratio of the root-mean-square (rms) sum of the harmonics to the fundamental, expressed in dBs.
This is the frequency at which the small signal voltage output
from the output amplifier is 3 dB below its dc level. It is measured with the DAC latch loaded with all 1s.
Multiplying Feedthrough Error
Full Power Bandwidth
This is an ac error due to capacitive feedthrough from the VREF
input to VOUT of the same DAC when the DAC latch is loaded
with all 0s.
This is the maximum frequency for which a sinusoidal input
signal will produce full output at rated load with a distortion
less than 3%. It is measured with the DAC latch loaded with
all 1s.
AD7837 PIN FUNCTION DESCRIPTION (DIP AND SOIC PIN NUMBERS)
Pin
Mnemonic
Description
1
2
3
4
5
6
7
8
9
10
11
12
13
CS
RFBA
VREFA
VOUTA
AGNDA
VDD
VSS
AGNDB
VOUTB
VREFB
DGND
RFBB
WR
14
LDAC
15
16
17–20
21–24
A1
A0
DB7–DB4
DB3–DB0
Chip Select. Active low logic input. The device is selected when this input is active.
Amplifier Feedback Resistor for DAC A.
Reference Input Voltage for DAC A. This may be an ac or dc signal.
Analog Output Voltage from DAC A.
Analog Ground for DAC A.
Positive Power Supply.
Negative Power Supply.
Analog Ground for DAC B.
Analog Output Voltage from DAC B.
Reference Input Voltage for DAC B. This may be an ac or dc signal.
Digital Ground. Ground reference for digital circuitry.
Amplifier Feedback Resistor for DAC B.
Write Input. WR is an active low logic input which is used in conjunction with CS, A0 and A1 to
write data to the input latches.
DAC Update Logic Input. Data is transferred from the input latches to the DAC latches when LDAC
is taken low.
Address Input. Most significant address input for input latches (see Table II).
Address Input. Least significant address input for input latches (see Table II).
Data Bit 7 to Data Bit 4.
Data Bit 3 to Data Bit 0 (LSB) or Data Bit 11 (MSB) to Data Bit 8.
–4–
REV. C
AD7837/AD7847
AD7847 PIN FUNCTION DESCRIPTION (DIP AND SOIC PIN NUMBERS)
Pin
Mnemonic
Description
11
12
13
14
15
16
17
18
19
10
11
12
13
CSA
CSB
VREFA
VOUTA
AGNDA
VDD
VSS
AGNDB
VOUTB
VREFB
DGND
DB11
WR
14–24
DB10–DB0
Chip Select Input for DAC A. Active low logic input. DAC A is selected when this input is low.
Chip Select Input for DAC B. Active low logic input. DAC B is selected when this input is low.
Reference Input Voltage for DAC A. This may be an ac or dc signal.
Analog Output Voltage from DAC A.
Analog Ground for DAC A.
Positive Power Supply.
Negative Power Supply.
Analog Ground for DAC B.
Analog Output Voltage from DAC B.
Reference Input Voltage for DAC B. This may be an ac or dc signal.
Digital Ground.
Data Bit 11 (MSB).
Write Input. WR is a positive edge triggered input which is used in conjunction with CSA and CSB
to write data to the DAC latches.
Data Bit 10 to Data Bit 0 (LSB).
AD7837 PIN CONFIGURATION
AD7847 PIN CONFIGURATION
DIP AND SOIC
DIP AND SOIC
CS
1
24 DB0
CSA
1
24 DB0
RFBA
2
23 DB1
CSB
2
23 DB1
VREFA
3
22 DB2
VREFA
3
22 DB2
VOUTA
4
21 DB3
VOUTA
4
21 DB3
AGNDA
5
20 DB4
AGNDA
5
VDD
6
TOP VIEW 19 DB5
7 (Not to Scale) 18 DB6
VDD
6
AGNDB
8
17 DB7
9
16 DB8
VSS
VSS
AD7847
20 DB4
TOP VIEW 19 DB5
7 (Not to Scale) 18 DB6
AGNDB
8
17 DB7
VOUTB
9
16 A0
VOUTB
VREFB 10
15 A1
VREFB 10
15 DB9
DGND 11
14 LDAC
DGND 11
14 DB10
RFBB 12
REV. C
AD7837
13 WR
DB11 12
–5–
13 WR
AD7837/AD7847–Typical Performance Graphs
0.6
25
10
VDD = +15V
VSS = –15V
0.4
0.2
20
0
VDD = +15V
VSS = –15V
VREF = +20Vp–p
–20
DAC CODE = 111...111
–30
104
105
15
10
5
106
0
10
107
FREQUENCY – Hz
0.5
0.3
INL
0.2
0.1
DNL
0
11
13
15
VDD /VSS – Volts
17
Figure 4. Linearity vs. Power Supply
–0.6
0.6
0.4
–0.2
DAC CODE = 111...111
–0.4
100
1k
LOAD RESISTANCE – DAC B
0
–0.6
10k
2048
CODE
0
4095
Figure 3. DAC-to-DAC Linearity
Matching
–40
–50
300
VDD = +15V
VSS = –15V
VREF = 0V
DAC CODE = 111...111
200
–60
THD – dB
NOISE SPECTRAL DENSITY – nV/ Hz
ERROR – LSB
VREF = 7.5V
–0.4
0.2
400
0.4
–0.2
VDD = +15V
VSS = –15V
VREF = +20Vp–p @ 1kHz
Figure 2. Output Voltage Swing vs.
Resistive Load
Figure 1. Frequency Response
0.0
ERROR – LSB
VOUT – Volts p–p
GAIN – dB
0
–10
DAC A
–70
VDD = +15V
VSS = –15V
VREF = 6V rms
DAC CODE = 111...111
–80
100
–90
0
0.01
0.1
10
1
FREQUENCY – Hz
100
Figure 5. Noise Spectral Density vs.
Frequency
–100
0.1
100
1
10
FREQUENCY – kHz
Figure 6. THD vs. Frequency
–50
A1 –0.01V
FEEDTHROUGH – dB
–60
–70
VDD = +15V
VSS = –15V
VREF = 20V p-p
DAC CODE = 000...000
FULL SCALE
–80
VOUT
–90
ZERO SCALE
–100
0.1
1
100
10
FREQUENCY – kHz
1000
Figure 7. Multiplying Feedthrough
Error vs. Frequency
200mV
50mV
B
Lw
2s
VERT 2V/DIV
HORIZ 2s/DIV
Figure 8. Large Signal Pulse
Response
–6–
Figure 9. Small Signal Pulse
Response
REV. C
AD7837/AD7847
CIRCUIT INFORMATION
D/A SECTION
Table I. AD7847 Truth Table
A simplified circuit diagram for one of the D/A converters and
output amplifier is shown in Figure 10.
A segmented scheme is used whereby the 2 MSBs of the 12-bit
data word are decoded to drive the three switches A-C. The
remaining 10 bits drive the switches (S0–S9) in a standard R-2R
ladder configuration.
Each of the switches A–C steers 1/4 of the total reference current with the remaining 1/4 passing through the R-2R section.
The output amplifier and feedback resistor perform the current
to voltage conversion giving
CSA
CSB
WR
Function
X
1
0
1
0
g
1
g
X
1
1
0
0
1
g
g
1
X
g
g
g
0
0
0
No Data Transfer
No Data Transfer
Data Latched to DAC A
Data Latched to DAC B
Data Latched to Both DACs
Data Latched to DAC A
Data Latched to DAC B
Data Latched to Both DACs
X = Don’t Care. g = Rising Edge Triggered.
VOUT = – D × VREF
CSA, CSB
where D is the fractional representation of the digital word. (D
can be set from 0 to 4095/4096.)
t1
The output amplifier can maintain ± 10 V across a 2 kΩ load. It
is internally compensated and settles to 0.01% FSR (1/2 LSB)
in less than 5 µs. Note that on the AD7837, VOUT must be connected externally to RFB.
R
R
t2
t3
WR
t5
t4
VALID
DATA
DATA
Figure 12. AD7847 Write Cycle Timing Diagram
R
VREF
2R
2R
2R
2R
2R
2R
C
B
A
S9
S8
S0
2R
INTERFACE LOGIC INFORMATION—AD7837
R /2
VOUT
SHOWN FOR ALL 1s ON DAC
AGND
Figure 10. D/A Simplified Circuit Diagram
INTERFACE LOGIC INFORMATION—AD7847
The input control logic for the AD7847 is shown in Figure 11.
The part contains a 12-bit latch for each DAC. It can be treated
as two independent DACs, each with its own CS input and a common WR input. CSA and WR control the loading of data to the
DAC A latch, while CSB and WR control the loading of the
DAC B latch. The latches are edge triggered so that input data
is latched to the respective latch on the rising edge of WR. If CSA
and CSB are both low and WR is taken high, the same data will
be latched to both DAC latches. The control logic truth table is
shown in Table I, while the write cycle timing diagram for the
part is shown in Figure 12.
CSA
WR
CSB
The input loading structure on the AD7837 is configured for
interfacing to microprocessors with an 8-bit-wide data bus. The
part contains two 12-bit latches per DAC—an input latch and
a DAC latch. Each input latch is further subdivided into a leastsignificant 8-bit latch and a most-significant 4-bit latch. Only the
data held in the DAC latches determines the outputs from the part.
The input control logic for the AD7837 is shown in Figure 13,
while the write cycle timing diagram is shown in Figure 14.
LDAC
CS
WR
DAC A
LATCH
DAC B
LATCH
12
12
4
A0
A1
DAC A MS
INPUT
LATCH
8
DAC A LS
INPUT
LATCH
4
DAC B LS
INPUT
LATCH
DAC A LATCH
DAC B LATCH
8
DB7 DB0
Figure 11. AD7847 Input Control Logic
Figure 13. AD7837 Input Control Logic
REV. C
8
DAC B LS
INPUT
LATCH
–7–
AD7837/AD7847
UNIPOLAR BINARY OPERATION
A0/A1
ADDRESS DATA
t6
Figure 15 shows DAC A on the AD7837/AD7847 connected
for unipolar binary operation. Similar connections apply for
DAC B. When VIN is an ac signal, the circuit performs 2-quadrant multiplication. The code table for this circuit is shown in
Table III. Note that on the AD7847 the feedback resistor RFB is
internally connected to VOUT.
t7
CS
t1
t2
t3
WR
t4
VDD
t5
VDD
AD7837
AD7847
VALID
DATA
DATA
VREFA
t8
DGND
VSS
AGNDA
Figure 14. AD7837 Write Cycle Timing Diagram
Table III. Unipolar Code Table
DAC Latch Contents
MSB
LSB
CS WR A1 A0 LDAC Function
X
X
0
0
1
1
X
X
X
0
1
0
1
X
1
1
1
1
1
1
0
*INTERNALLY
CONNECTED
ON AD7847
Figure 15. Unipolar Binary Operation
No Data Transfer
No Data Transfer
DAC A LS Input Latch Transparent
DAC A MS Input Latch Transparent
DAC B LS Input Latch Transparent
DAC B MS Input Latch Transparent
DAC A and DAC B DAC Latches
Updated Simultaneously from the
Respective Input Latches
Analog Output, VOUT
1111 1111 1111
 4095 
–VIN × 

 4096 
1000 0000 0000
 2048 
–V IN × 
 = –1/ 2 VIN
 4096 
0000 0000 0001
 1 
–V IN × 

 4096 
0000 0000 0000
0V
Table II. AD7837 Truth Table
X
1
0
0
0
0
1
VOUT
VSS
CS, WR, A0 and A1 control the loading of data to the input
latches. The eight data inputs accept right-justified data. Data
can be loaded to the input latches in any sequence. Provided that
LDAC is held high, there is no analog output change as a result
of loading data to the input latches. Address lines A0 and A1
determine which latch data is loaded to when CS and WR are low.
The control logic truth table for the part is shown in Table II.
1
X
0
0
0
0
1
*
VOUTA
DAC A
VIN
LDAC
RFBA
Note 1 LSB =
V IN
.
4096
X = Don’t Care.
The LDAC input controls the transfer of 12-bit data from the
input latches to the DAC latches. When LDAC is taken low, both
DAC latches, and hence both analog outputs, are updated at
the same time. The data in the DAC latches is held on the rising
edge of LDAC. The LDAC input is asynchronous and independent of WR. This is useful in many applications especially in the
simultaneous updating of multiple AD7837s. However, care must
be taken while exercising LDAC during a write cycle. If an LDAC
operation overlaps a CS and WR operation, there is a possibility
of invalid data being latched to the output. To avoid this, LDAC
must remain low after CS or WR return high for a period equal
to or greater than t8, the minimum LDAC pulsewidth.
–8–
REV. C
AD7837/AD7847
BIPOLAR OPERATION
(4-QUADRANT MULTIPLICATION)
APPLICATIONS
Figure 16 shows the AD7837/AD7847 connected for bipolar
operation. The coding is offset binary as shown in Table IV.
When VIN is an ac signal, the circuit performs 4-quadrant multiplication. To maintain the gain error specifications, resistors R1,
R2 and R3 should be ratio matched to 0.01%. Note that on the
AD7847 the feedback resistor RFB is internally connected to
VOUT.
R2
20k
R1
20k
VDD
VDD
AD7837
AD7847
VREFA
VIN
DAC A
DGND
AGNDA
AD711
RFBA
*
VOUTA
PROGRAMMABLE GAIN AMPLIFIER (PGA)
The dual DAC/amplifier combination along with access to RFB
make the AD7837 ideal as a programmable gain amplifier. In this
application, the DAC functions as a programmable resistor in the
amplifier feedback loop. This type of configuration is shown
in Figure 17 and is suitable for ac gain control. The circuit consists of two PGAs in series. Use of a dual configuration provides
greater accuracy over a wider dynamic range than a single PGA
solution. The overall system gain is the product of the individual
gain stages. The effective gains for each stage are controlled by
the DAC codes. As the code decreases, the effective DAC
resistance increases, and so the gain also increases.
VOUT
R3
10k
RFBA
VIN
VOUTA
*INTERNALLY
CONNECTED
ON AD7847
VSS
VREFA
DAC A
AGNDA
VREFB
VSS
Figure 16. Bipolar Offset Binary Operation
AD7837
DAC B
RFBB
VOUTB
VOUT
AGNDB
Table IV. Bipolar Code Table
DAC Latch Contents
MSB
LSB
Figure 17. Dual PGA Circuit
Analog Output, VOUT
1111 1111 1111
 2047 
+V IN × 

 2048 
1000 0000 0001
 1 
+V IN × 

 2048 
1000 0000 0000
0V
0111 1111 1111
 1 
–V IN × 

 2048 
0000 0000 0000
 2048 
–V IN × 
 = –V IN
 2048 
Note 1 LSB =
V IN
2048
The transfer function is given by
VOUT REQA REQB
=
×
V IN
RFBA RFBB
(1)
where REQA, REQB are the effective DAC resistances controlled
by the digital input code:
REQ =
212 RIN
N
(2)
where RIN is the DAC input resistance and is equal to RFB and
N = DAC input code in decimal.
The transfer function in (1) thus simplifies to
212 212
VOUT
=
×
V IN
N A NB
.
(3)
where NA = DAC A input code in decimal and NB = DAC B
input code in decimal.
NA, NB may be programmed between 1 and (212–1). The zero
code is not allowed as it results in an open loop amplifier
response. To minimize errors, the digital codes NA and NB
should be chosen to be equal to or as close as possible to each
other to achieve the required gain.
REV. C
–9–
AD7837/AD7847
ANALOG PANNING CIRCUIT
0.6
TOTAL POWER VARIATION – dB
In audio applications it is often necessary to digitally “pan” or
split a single signal source into a two-channel signal while maintaining the total power delivered to both channels constant. This
may be done very simply by feeding the signal into the VREF
input of both DACs. The digital codes are chosen such that the
code applied to DAC B is the two's complement of that applied
to DAC A. In this way the signal may be panned between both
channels as the digital code is changed. The total power variation with this arrangement is 3 dB.
For applications which require more precise power control the
circuit shown in Figure 18 may be used. This circuit requires
the AD7837/AD7847, an AD712 dual op amp and eight equal
value resistors.
R
R
R
R
0.2
0.1
1
512
1024
1536
2048
2560
3072
DIGITAL INPUT CODE NA
3584
4095
Figure 19. Power Variation for Circuit in Figure 9
AC or transient voltages between the analog and digital grounds
i.e., between AGNDA/AGNDB and DGND can cause noise
injection into the analog output. The best method of ensuring
that both AGNDs and DGND are equal is to connect them
together at the AD7837/AD7847 on the circuit board. In more
complex systems where the AGND and DGND intertie is on the
backplane, it is recommended that two diodes be connected in
inverse parallel between the AGND and DGND pins (1N914 or
equivalent).
VREFA
AD7837/
AD7847
VOUTA
VIN
VOUTB
1/2
AD712
R
0.3
APPLYING THE AD7837/AD7847
General Ground Management
1/2
AD712
R
0.4
0.0
Again both channels are driven with two's complementary data.
The maximum power variation using this circuit is only 0.5 dBs.
R
0.5
VREFB
R
Power Supply Decoupling
VOUTA
VOUTB
RLA
In order to minimize noise it is recommended that the VDD and
the VSS lines on the AD7837/AD7847 be decoupled to DGND
using a 10 µF in parallel with a 0.1 µF ceramic capacitor.
RLB
Figure 18. Analog Panning Circuit
Operation with Reduced Power Supply Voltages
The AD7837/AD7847 is specified for operation with VDD/VSS =
± 15 V ± 5%. The part may be operated down to VDD/VSS =
± 10 V without significant linearity degradation. See typical
performance graphs. The output amplifier however requires
approximately 3 V of headroom so the VREF input should not
approach within 3 V of either power supply voltages in order to
maintain accuracy.
The voltage output expressions for the two channels are as
follows:
 N

VOUTA = –V IN  12 A 
2
+
N

A
 N

VOUT B = –V IN  12 B 
 2 + NB 
MICROPROCESSOR INTERFACING–AD7847
where NA = DAC A input code in decimal (1 ≤ NA ≤ 4095)
Figures 20 to 22 show interfaces between the AD7847 and three
popular 16-bit microprocessor systems, the 8086, MC68000 and
the TMS320C10. In all interfaces, the AD7847 is memorymapped with a separate memory address for each DAC latch.
and NB = DAC B input code in decimal (1 ≤ NB ≤ 4095)
with NB = 2s complement of NA.
The two's complement relationship between NA and NB causes
NB to increase as NA decreases and vice versa.
AD7847–8086 Interface
Hence NA + NB = 4096.
With NA = 2048, then NB = 2048 also; this gives the balanced
condition where the power is split equally between both channels. The total power variation as the signal is fully panned from
Channel B to Channel A is shown in Figure 19.
Figure 20 shows an interface between the AD7847 and the 8086
microprocessor. A single MOV instruction loads the 12-bit word
into the selected DAC latch and the output responds on the rising edge of WR.
–10–
REV. C
AD7837/AD7847
MICROPROCESSOR INTERFACING–AD7837
ADDRESS BUS
8086
ALE
ADDRESS
DECODE
16 BIT
LATCH
CSA
CSB
AD7847*
WR
WR
DB11
DB0
AD15
AD0
ADDRESS/DATA BUS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 20. AD7847 to 8086 Interface
AD7847–MC68000 Interface
Figure 21 shows an interface between the AD7847 and the
MC68000. Once again a single MOVE instruction loads the
12-bit word into the selected DAC latch. CSA and CSB are
AND-gated to provide a DTACK signal when either DAC
latch is selected.
A23
A1
MC68000
AS
Figures 23 to 25 show the AD7837 configured for interfacing to
microprocessors with 8-bit data bus systems. In all cases, data is
right-justified and the AD7837 is memory-mapped with the two
lowest address lines of the microprocessor address bus driving
the A0 and A1 inputs of the AD7837. Five separate memory
addresses are required, one for the each MS latch and one for
each LS latch and one for the common LDAC input. Data is
written to the respective input latch in two write operations.
Either high byte or low byte data can be written first to the
input latch. A write to the AD7837 LDAC address transfers the
data from the input latches to the respective DAC latches and
updates both analog outputs. Alternatively, the LDAC input
can be asynchronous and can be common to several AD7837s
for simultaneous updating of a number of voltage channels.
AD7837–8051/8088 Interface
Figure 23 shows the connection diagram for interfacing the
AD7837 to both the 8051 and the 8088. On the 8051, the
signal PSEN is used to enable the address decoder while DEN
is used on the 8088.
ADDRESS BUS
ADDRESS
DECODE
EN
A15
CSA
CSB
8051/8088
AD7847*
DTACK
ADDRESS BUS
A8
PSEN OR DEN
LDS
R/W
A0 A1
ADDRESS
DECODE
CS
LDAC
EN
WR
DB11
ALE
AD7837*
OCTAL
LATCH
DB0
WR
D15
D0
WR
DB7
DATA BUS
DB0
*ADDITIONAL PINS OMITTED FOR CLARITY
AD7
AD0
Figure 21. AD7847 to MC68000 Interface
AD7847–TMS320C10 Interface
Figure 22 shows an interface between the AD7847 and the
TMS320C10 DSP processor. A single OUT instruction loads
the 12-bit word into the selected DAC latch.
A11
A0
TMS320C10
MEN
ADDRESS BUS
ADDRESS
DECODE
EN
Figure 23. AD7837 to 8051/8088 Interface
AD7837–MC68008 Interface
An interface between the AD7837 and the MC68008 is shown
in Figure 24. In the diagram shown, the LDAC signal is derived
from an asynchronous timer but this can be derived from the
address decoder as in the previous interface diagram.
CSA
CSB
TIMER
A19
AD7847*
A0
WR
WE
DB11
MC68008
D0
AS
DATA BUS
ADDRESS BUS
ADDRESS
DECODE
DB0
D15
ADDRESS/DATA BUS
*ADDITIONAL PINS OMITTED FOR CLARITY
EN
A0 A1
CS
LDAC
DTACK
*ADDITIONAL PINS OMITTED FOR CLARITY
AD7837*
DS
WR
DB7
R/W
Figure 22. AD7847 to TMS320C10 Interface
DB0
D7
D0
DATA BUS
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 24. AD7837 to 68008 Interface
REV. C
–11–
AD7837/AD7847
AD7837–6502/6809 Interface
A15
C01007a–0–8/00 (rev. C)
Figure 25 shows an interface between the AD7837 and the 6502
or 6809 microprocessor. For the 6502 microprocessor, the φ2
clock is used to generate the WR, while for the 6809 the E signal is used.
ADDRESS BUS
A0
ADDRESS
DECODE
6502/6809
LDAC
EN
R/W
A0 A1
CS
AD7837*
2 OR E
WR
DB7
DB0
D7
DATA BUS
D0
*ADDITIONAL PINS OMITTED FOR CLARITY
Figure 25. AD7837 to 6502/6809 Interface
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
24-Lead Plastic DIP (N-24)
24-Lead Cerdip (Q-24)
1.228 (31.19)
1.226 (31.14)
1
24
13
12
0.261 0.001
(6.61 0.03)
PIN 1
13
PIN 1
1
0.32 (8.128)
0.30 (7.62)
12
1.290 (32.77) MAX
0.225 (5.715)
MAX
0.130 (3.30)
0.128 (3.25)
0.295
(7.493)
MAX
0.070 (1.778)
0.020 (0.508)
0.320 (8.128)
0.290 (7.366)
0.180
(4.572)
MAX
0.125 (3.175)
MIN
SEATING
0.012 (0.305)
0.021 (0.533) 0.110 (2.794) 0.065 (1.651) PLANE
15°
0.008 (0.203)
0°
0.015 (0.381) 0.090 (2.286) 0.055 (1.397)
TYP
TYP
TYP
1. LEAD NO. 1 IDENTIFIED BY A DOT OR NOTCH.
2. CERDIP LEADS WILL EITHER BE TIN PLATED OR SOLDER DIPPED.
IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS
SEATING
PLANE
0.011 (0.28)
0.11 (2.79)
0.07 (1.78)
15° 0.009 (0.23)
0.02 (0.5)
0°
0.09 (2.28)
0.05 (1.27)
0.016 (0.41)
1. LEAD NO. 1 IDENTIFIED BY A DOT OR NOTCH.
2. PLASTIC LEADS WILL EITHER BE SOLDER DIPPED OR TIN LEAD PLATED.
IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS.
24-Lead SOIC (R-24)
0.608 (15.45)
0.596 (15.13)
24
PRINTED IN U.S.A.
24
13
0.299 (7.6)
0.291 (7.39)
1
12
PIN 1
0.096 (2.44)
0.089 (2.26)
0.414 (10.52)
0.398 (10.10)
0.03 (0.76)
0.02 (0.51)
6
0
SEATING
0.042 (1.067)
0.013 (0.32)
PLANE
0.018 (0.457)
0.009 (0.23)
1. LEAD NO. 1 IDENTIFIED BY A DOT.
2. SOIC LEADS WILL EITHER BE TIN PLATED OR SOLDER DIPPED
IN ACCORDANCE WITH MIL-M-38510 REQUIREMENTS.
0.01 (0.254) 0.05
0.006 (0.15) (1.27)
0.019 (0.49)
0.014 (0.35)
–12–
REV. C