AD7538 Analog Devices, AD7538 Datasheet - Page 11

• Manufacturer Part Number: AD7538
• Description: LC2MOS Microprocessor-Compatible 14-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7538

• Resolution (bits): 14bit
• Dac Update Rate: 667kSPS
• Dac Settling Time: 1.5µs
• Max Pos Supply (v): +15.75V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

BIPOLAR OPERATION (4-QUADRANT
MULTIPLICATION)
The recommended circuit diagram for bipolar operation is
shown in Figure 8. Offset binary coding is used. The code table
for Figure 8 is given in Table 7.
With the DAC loaded to 10 0000 0000 0000, adjust R1 for V
0 V. Alternatively, one can omit R1 and R2 and adjust the ratio
of R5 and R6 for V
plished by adjusting the amplitude of V
value of R7.
The values given for R1, R2 are the minimum necessary to
calibrate the system for Resistors R5, R6, R7 ratio matched to
0.1%. System linearity error is independent of resistor ratio
matching and is affected by DAC linearity error only.
When operating over a wide temperature range, it is important
that the resistors be of the same type so that their temperature
coefficients match.
LOW LEAKAGE CONFIGURATION
For CMOS multiplying DAC, as the device is operated at higher
temperatures, the output leakage current increases. For a 14-bit
resolution system, this can be a significant source of error. The
AD7538 features a leakage reduction configuration to keep the
leakage current low over an extended temperature range. One
may operate the device with or without this configuration. If V
(Pin 24) is tied to AGND then the DAC exhibits normal output
leakage currents at high temperatures. To use the low leakage
facility, V
as in Figure 6 and Figure 8. A simple resistor divider (R3, R4)
produces approximately −300 mV from −15 V. The C2
capacitor in parallel with R3 is an integral part of the low
leakage configuration and must be 4.7 μF or greater. Figure 7
is a plot of leakage current vs. temperature for both conditions.
It clearly shows the improvement gained by using the low
leakage configuration.
SS
should be tied to a voltage of approximately −0.3 V
O
= 0 V. Full-scale trimming can be accom-
LDAC
V
IN
WR
CS
20
21
22
50Ω
DB13 TO DB0 DGND
IN
INPUT DATA
R1
LDAC
WR
CS
V
or by varying the
6
REF
1
19
AD7538
DIGITAL
GND
V
V
23
DD
DD
5
R
V
Figure 8. Bipolar Operation
24
2
FB
SS
O
Rev. B | Page 11 of 16
=
SS
AGND
1kΩ
I
R3
OUT
22Ω
R2
4.7µF
47kΩ
C2
3
4
R4
+
Table 7. Bipolar Code Table for the Offset Binary Circuit
of Figure 8
Binary Number In
DAC Register
MSB
11 1111 1111 1111
10 0000 0000 0001
10 0000 0000 0000
01 1111 1111 1111
00 0000 0000 0000
C1
33pF
ANALOG
GND
Figure 7. Graph of Typical Leakage Current vs. Temperature for AD7538
A1
–15V
AD711
60
50
40
30
20
10
5kΩ, 10%
0
R8
10kΩ
R5
30
R6
20kΩ
LSB
40
V
V
DD
REF
20kΩ
A2
R7
50
= 15V
AD711
= 10V
TEMPERATURE (°C)
60
70
V
0 V
Analog Output V
+V
+V
−V
−V
O
80
IN
IN
IN
IN
(8191/8192)
(1/8192)
(1/8192)
(8191/8192)
90
V
100 110 120
SS
= 0V
V
SS
OUT
= –0.3V
AD7538