Datasheet

AD7538
Rev. B | Page 11 of 16
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
O
=
0 V. Alternatively, one can omit R1 and R2 and adjust the ratio
of R5 and R6 for V
O
= 0 V. Full-scale trimming can be accom-
plished by adjusting the amplitude of V
IN
or by varying the
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
SS
(Pin 24) is tied to AGND then the DAC exhibits normal output
leakage currents at high temperatures. To use the low leakage
facility, V
SS
should be tied to a voltage of approximately −0.3 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.
Table 7. Bipolar Code Table for the Offset Binary Circuit
of Figure 8
Binary Number In
DAC Register
Analog Output V
OUT
MSB LSB
11 1111 1111 1111 +V
IN
(8191/8192)
10 0000 0000 0001 +V
IN
(1/8192)
10 0000 0000 0000 0 V
01 1111 1111 1111 −V
IN
(1/8192)
00 0000 0000 0000 −V
IN
(8191/8192)
30 40 50 60 70 80 90 100 110 120
TEMPERATURE (°C)
LEAKAGE CURRENT (nA)
60
50
40
30
20
10
0
V
DD
= 15V
V
REF
= 10V
V
SS
= 0V
V
SS
= –0.3V
01139-008
Figure 7. Graph of Typical Leakage Current vs. Temperature for AD7538
619
5 24
231
20
21
22
2
3
4
DB13 TO DB0 DGND
V
DD
V
REF
R
FB
I
OUT
V
DD
–15V
AGND
V
SS
LDAC
CS
WR
LDAC
CS
WR
AD7538
A1
R3
1kΩ
R2
22Ω
R5
10kΩ
R7
20kΩ
R6
20kΩ
R8
5kΩ, 10%
R1
50Ω
R4
47kΩ
INPUT DATA
DIGITAL
GND
C2
4.7µF
C1
33pF
ANALOG
GND
AD711
A2
AD711
01139-007
V
IN
V
O
+
Figure 8. Bipolar Operation