AD7537
PIN FUNCTION DESCRIPTION (DIP)
PIN
1
2
3
4
5
6–14
12
15
16
17
18
19
20
21
22
23
24
MNEMONIC
AGNDA
I
OUTA
R
FBA
V
REFA
CS
DB0–DB7
DGND
A0
A1
CLR
WR
UPD
V
DD
V
REFB
R
FBB
I
OUTB
AGNDB
DESCRIPTION
Analog Ground for DAC A.
Current output terminal of DAC A.
Feedback resistor for DAC A.
Reference input to DAC A.
Chip Select Input Active low.
Eight data inputs, DB0–DB7.
Digital Ground.
Address Line 0.
Address Line 1.
Clear Input. Active low. Clears all
registers.
Write Input. Active low.
Updates DAC Registers from inputs
registers.
Power supply input. Nominally +12 V
to +15 V, with
±
10% tolerance.
Reference input to DAC B.
Feedback resistor for DAC B.
Current output terminal of DAC B.
Analog Ground for DAC B.
current flowing in each ladder leg is constant, irrespective of
switch state. The feedback resistor R
FBA
is used with an op amp
(see Figures 4 and 5) to convert the current flowing in I
OUTA
to
a voltage output.
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 AD7537. A similar equivalent circuit
can be drawn for DAC B.
C
OUT
is the output capacitance due to the N-channel switches
and varies from about 50 pF to 150 pF with digital input code.
The current source I
LKG
is composed of surface and junction
leakages and approximately doubles every 10°C. R
0
is the
equivalent output resistance of the device which varies with
input code.
DIGITAL CIRCUIT INFORMATION
PIN CONFIGURATIONS
DIP
LCCC
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. AD7537 Truth Table
CLR UPD CS WR
A1 A0 FUNCTION
1
1
0
1
PLCC
1
1
X
1
1
1
1
0
1
X
X
0
0
0
0
1
X
1
X
0
0
0
0
0
X
X
X
0
0
1
1
X
X
X
X
0
1
0
1
X
1
1
1
1
1
0
0
0
X
X
No Data Transfer
No Data Transfer
All Registers Cleared
DAC A LS Input Register
Loaded with DB7–DB0 (LSB)
DAC A MS Input Register
Loaded with DB3 (MSB)–DB0
DAC B LS Input Register
Loaded with DB7–DB0 (LSB)
DAC B MS Input Register
Loaded with DB3 (MSB)–DB0
DAC A, DAC B Registers
Updated Simultaneously from
Input Registers
DAC A, DAC B Registers are
Transparent
NOTES: X = Don’t care
CIRCUIT INFORMATION – D/A SECTION
The AD7537 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 I
OUTA
and AGNDA. The
–4–
Figure 3. Equivalent Analog Circuit for DAC A
REV. 0
AD [ ANALOG DEVICES ]
