AD7528
APPLICATIONS INFORMATION
Application Hints
To ensure system performance consistent with AD7528 specifi-
cations, careful attention must be given to the following points:
ship between input frequency and channel to channel isolation.
Figure 7 shows a printed circuit layout for the AD7528 and the
AD644 dual op amp which minimizes feedthrough and crosstalk.
SINGLE SUPPLY APPLICATIONS
1. GENERAL GROUND MANAGEMENT: AC or transient
voltages between the AD7528 AGND and DGND can cause
noise injection into the analog output. The simplest method
of ensuring that voltages at AGND and DGND are equal is
to tie AGND and DGND together at the AD7528. In more
complex systems where the AGND–DGND intertie is on the
backplane, it is recommended that diodes be connected in
inverse parallel between the AD7528 AGND and DGND
pins (1N914 or equivalent).
2. OUTPUT AMPLIFIER OFFSET: CMOS DACs exhibit a
code-dependent output resistance which in turn causes a
code-dependent amplifier noise gain. The effect is a code-
dependent differential nonlinearity term at the amplifier
output which depends on VOS (VOS is amplifier input offset
voltage). This differential nonlinearity term adds to the R/2R
differential nonlinearity. To maintain monotonic operation, it
is recommended that amplifier VOS be no greater than 10% of
1 LSB over the temperature range of interest.
The AD7528 DAC R-2R ladder termination resistors are con-
nected to AGND within the device. This arrangement is par-
ticularly convenient for single supply operation because AGND
may be biased at any voltage between DGND and VDD. Figure
8 shows a circuit which provides two +5 V to +8 V analog out-
puts by biasing AGND +5 V up from DGND. The two DAC
reference inputs are tied together and a reference input voltage
is obtained without a buffer amplifier by making use of the
constant and matched impedances of the DAC A and DAC B
reference inputs. Current flows through the two DAC R-2R
ladders into R1 and R1 is adjusted until the VREF A and VREF
B
inputs are at +2 V. The two analog output voltages range from
+5 V to +8 V for DAC codes 00000000 to 11111111.
V
= +15V
DD
DAC A
V
A = +5V TO +8V
B = +5V TO +8V
OUT
DB0
DB7
DATA
INPUTS
SUGGESTED
OP AMP:
AD644
CS
3. HIGH FREQUENCY CONSIDERATIONS: The output
capacitance of a CMOS DAC works in conjunction with the
amplifier feedback resistance to add a pole to the open loop
response. This can cause ringing or oscillation. Stability can
be restored by adding a phase compensation capacitor in
parallel with the feedback resistor.
AD7528
DAC B
WR
DAC A/DAC B
V
OUT
2 VOLTS
R1
10k⍀
AD584J
V
R2
1k⍀
DD
GND
DYNAMIC PERFORMANCE
The dynamic performance of the two DACs in the AD7528 will
depend upon the gain and phase characteristics of the output
amplifiers together with the optimum choice of the PC board
layout and decoupling components. Figure 6 shows the relation
Figure 8. AD7528 Single Supply Operation
Figure 9 shows DAC A of the AD7528 connected in a positive
reference, voltage switching mode. This configuration is useful
in that VOUT is the same polarity as VIN allowing single supply
operation. However, to retain specified linearity, VIN must be in
the range 0 V to +2.5 V and the output buffered or loaded with
a high impedance, see Figure 10. Note that the input voltage is
connected to the DAC OUT A and the output voltage is taken
from the DAC VREF A pin.
–100
T
= +25؇C
A
V
V
= +15V
–90
–80
–70
–60
–50
DD
= 20V PEAK TO PEAK
IN
V
OUT
V
(0V TO +2.5V)
IN
V
A
REF
V
DD
DAC A
OUT A
+15V
AD7528
20k
50k
100k
200k
500k
1M
INPUT FREQUENCY – Hz
Figure 9. AD7528 in Single Supply, Voltage Switching Mode
Figure 6. Channel-to-Channel Isolation
3
PIN 8 OF TO-5 CAN (AD644)
AD644
V+
T
= +25؇C
A
V
= +15V
DD
2
1
V–
NONLINEARITY
AGND
AD7528 PIN 1
C1 LOCATION
V
*NOTE
INPUT SCREENS
TO REDUCE
FEEDTHROUGH.
LAYOUT SHOWS
COPPER SIDE
C2 LOCATION
A*
B*
REF
V
DIFFERENTIAL
NONLINEARITY
V
DD
REF
WR
CS
DGND
DAC A/DAC B
MSB
AD7528
(i.e., BOTTOM VIEW).
LSB
2.5
3
3.5
4
4.5
V
5
5.5
6
6.5
7
7.5
A – Volts
IN
Figure 7. Suggested PC Board Layout for AD7528 with
AD644 Dual Op Amp
Figure 10. Typical AD7528 Performance in Single Supply
Voltage Switching Mode (K/B/T, L/C/U Grades)
–6–
REV. B
ROCHESTER [ Rochester Electronics ]
