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AD7937 Datasheet(PDF) 4 Page - Analog Devices

Part No. AD7937
Description  LC2MOS 84 Loading Dual 12-Bit DAC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7937 Datasheet(HTML) 4 Page - Analog Devices

   
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AD7937
–4–
REV. 0
PIN FUNCTION DESCRIPTIONS
Pin
Mnemonic
Description
1
AGNDA
Analog Ground for DAC A.
2IOUTA
Current output terminal of DAC A.
3RFBA
Feedback resistor for DAC A.
4VREFA
Reference input to DAC A.
5
CS
Chip Select Input Active low.
6–11
DB0–DB7
Eight data inputs, DB0–DB7.
13, 14
12
DGND
Digital Ground.
15
A0
Address Line 0.
16
A1
Address Line 1.
17
CLR
Clear Input. Active low. Clears all
registers.
18
WR
Write Input. Active low.
19
UPD
Updates DAC Registers from inputs
registers.
20
VDD
Power supply input. Nominally 5 V to
15 V, with
±10% tolerance.
21
VREFB
Reference input to DAC B.
22
RFBB
Feedback resistor for DAC B.
23
IOUTB
Current output terminal of DAC B.
24
AGNDB
Analog Ground for DAC B.
PIN CONFIGURATION
SOIC
TOP VIEW
(Not to Scale)
24
23
22
21
20
19
18
17
16
15
14
13
1
2
3
4
5
6
7
8
9
10
11
12
AD7937
DGND
DB5
DB4
DB3
DB2
AGNDA
IOUTA
RFBA
VREFA
DB1
DB0
CS
DB6
DB7
A0
A1
CLR
AGNDB
IOUTB
RFBB
VREFB
WR
UPD
VDD
CIRCUIT INFORMATION – D/A SECTION
The AD7937 contains two identical 12-bit multiplying D/A
converters. Each DAC consists of a highly stable R-2R ladder
and 12 N-channel current steering switches. Figure 2 shows a
simplified D/A circuit for DAC A. In the R-2R ladder, binary
weighted currents are steered between IOUTA and AGNDA. The
current flowing in each ladder leg is constant, irrespective of
switch state. The feedback resistor RFBA is used with an op amp
(see Figures 4 and 5) to convert the current flowing in IOUTA to
a voltage output.
IOUTA
AGNDA
RFBA
VREFA
R
R
2R
S11
2R
S10
2R
S0
2R
R
Figure 2. Simplified Circuit Diagram for DAC A
EQUIVALENT CIRCUIT ANALYSIS
Figure 3 shows the equivalent circuit for one of the D/A con-
verters (DAC A) in the AD7937. A similar equivalent circuit
can be drawn for DAC B.
IOUTA
AGNDA
RO
VREFA
R
D.VREF
R
ILKG
COUT
R
RFBA
Figure 3. Equivalent Analog Circuit for DAC A
COUT is the output capacitance due to the N-channel switches
and varies from about 50 pF to 100 pF with digital input code.
The current source ILKG is composed of surface and junction
leakages and approximately doubles every 10
°C. R
O is the equiva-
lent output resistance of the device which varies with input code.
DIGITAL CIRCUIT INFORMATION
The digital inputs are designed to be both TTL and 5 V CMOS
compatible. All logic inputs are static protected MOS gates with
typical input currents of less than 1 nA.
Table I. AD7937 Truth Table
CLR UPD CS WR A1 A0 Function
1
1
1
X
X
X
No Data Transfer
1
1
X
1
X
X
No Data Transfer
0
X
X
X
X
X
All Registers Cleared
1
1
0
0
0
0
DAC A LS Input Register
Loaded with DB7–DB0 (LSB)
1
1
0
0
0
1
DAC A MS Input Register
Loaded with DB3 (MSB)–DB0
1
1
0
0
1
0
DAC B LS Input Register
Loaded with DB7–DB0 (LSB)
1
1
0
0
1
1
DAC B MS Input Register
Loaded with DB3 (MSB)–DB0
1
0
1
0
X
X
DAC A, DAC B Registers
Updated Simultaneously from
Input Registers
1
0
0
0
X
X
DAC A, DAC B Registers are
Transparent
NOTE: X = Don’t care


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