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參數(shù)資料
| 型號： | AD7628TE |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | CMOS Dual 8-Bit Buffered Multiplying DAC |
| 中文描述： | DUAL, PARALLEL, 8 BITS INPUT LOADING, 8-BIT DAC, CQCC20 |
| 封裝： | CERAMIC, LCC-20 |
| 文件頁數(shù)： | 6/8頁 |
| 文件大小： | 193K |
| 代理商： | AD7628TE |
AD7628
–6–
REV. A
APPLICAT IONS INFORMAT ION
Application Hints
T o ensure system performance consistent with AD7628 specifi-
cations, careful attention must be given to the following points:
1. GENERAL GROUND MANAGEMENT : AC or transient
voltages between the AD7628 AGND and DGND can cause
noise injection into the analog output. T he simplest method
of ensuring that voltages at AGND and DGND are equal is
to tie AGND and DGND together at the AD7628. In more
omplex systems where the AGND–DGND intertie is on the
backplane, it is recommended that diodes be connected in
inverse parallel between the AD7628 AGND and DGND
pins (1N914 or equivalent).
2. OUT PUT AMPLIFIER OFFSET : CMOS DACs exhibit a
code-dependent output resistance which, in turn, causes a
code-dependent amplifier noise gain. T he effect is a code-
dependent differential nonlinearity term at the amplifier
output that depends on V
OS
(V
OS
is amplifier input offset
voltage). T his differential nonlinearity term adds to the R/2R
differential nonlinearity. T o maintain monotonic operation, it
is recommended that amplifier V
OS
be no greater than 10% of
1 LSB over the temperature range of interest.
3. HIGH FREQUENCY CONSIDERAT IONS: T he output
capacitance of a CMOS DAC works in conjunction with the
amplifier feedback resistance to add a pole to the open loop
response. T his can cause ringing or oscillation. Stability can
be restored by adding a phase compensation capacitor in
parallel with the feedback resistor.
DY NAMIC PE RFORMANCE
T he dynamic performance of the two DACs in the AD7628 will
depend on the gain and phase characteristics of the output am-
plifiers, together with the optimum choice of the PC board lay-
out and decoupling components. Figure 6 shows the relationship
between input frequency and channel-to-channel isolation.
Figure 6. Channel-to-Channel Isolation
Figure 7. Suggested PC Board Layout for AD7628 with
AD644 Dual Op Amp
Figure 7 shows a printed circuit layout for the AD7628 and the
AD644 dual op amp, which minimizes feedthrough and crosstalk.
SINGLE SUPPLY APPLICAT IONS
T he AD7628 DAC R-2R ladder termination resistors are con-
nected to AGND within the device. T his arrangement is par-
ticularly convenient for single supply operation because AGND
may be biased at any voltage between DGND and V
DD
. Figure
8 shows a circuit that provides two +5 V to +8 V analog outputs
by biasing AGND +5 V up from DGND. T he two DAC refer-
ence inputs are tied together and a reference input voltage is ob-
tained 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 V
REF
A and V
REF
B inputs are
at +2 V. T he two analog output voltages range from +5 V to
+8 V for DAC codes 00000000 to l l l l l l l l .
Figure 8. AD7628 Single Supply Operation
Figure 9 shows DAC A of the AD7628 connected in a positive
reference, voltage switching mode. T his configuration is useful
because V
OUT
is the same polarity as V
IN
, allowing single supply
operation. However, to retain specified linearity, V
IN
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 V
REF
A pin.
Figure 9. AD7628 Single Supply, Voltage Switching Mode
Figure 10. Typical AD7628 Performance in Single Supply
Voltage Switching Mode
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