Data Sheet DAC08
Rev. D | Page 15 of 21
If one of the outputs is not required, it must be connected to
ground or to a point capable of sourcing I
FS
; do not leave an
unused output pin open.
Both outputs have an extremely wide voltage compliance
enabling fast direct current to voltage conversion through a
resistor tied to ground or other voltage source. Positive compli-
ance is 36 V above V− and is independent of the positive supply.
Negative compliance is given by
V− + (I
REF
× 1 kΩ) + 2.5 V
The dual outputs enable double the usual peak-to-peak load
swing when driving loads in quasi-differential fashion. This
feature is especially useful in cable driving, CRT deflection and
in other balanced applications such as driving center-tapped
coils and transformers.
POWER SUPPLIES
The DAC08 operates over a wide range of power supply voltages
from a total supply of 9 V to 36 V. When operating at supplies
of ±5 V or lower, I
REF
≤ 1 mA is recommended. Low reference
current operation decreases power consumption and increases
negative compliance (Figure 11), reference amplifier negative
common-mode range (Figure 14), negative logic input range
(Figure 15), and negative logic threshold range (Figure 16). For
example, operation at −4.5 V with I
REF
= 2 mA is not recommended
because negative output compliance reduces to near zero.
Operation from lower supplies is possible; however, at least
8 V total must be applied to ensure turn on of the internal bias
network.
Symmetrical supplies are not required, as the DAC08 is quite
insensitive to variations in supply voltage. Battery operation is
feasible because no ground connection is required; however, an
artificial ground can ensure logic swings, etc., remain between
acceptable limits. Power consumption is calculated as follows:
(
) ( )
( ) (
)
−−
+++=
VIV
IP
D
A useful feature of the DAC08 design is that supply current is
constant and independent of input logic states. This is useful in
cryptographic applications and further reduces the size of the
power supply bypass capacitors.
TEMPERATURE PERFORMANCE
The nonlinearity and monotonicity specifications of the DAC08
are guaranteed to apply over the entire rated operating temperature
range. Full-scale output current drift is low, typically ±10 ppm/°C,
with zero-scale output current and drift essentially negligible
compared to 1/2 LSB.
The temperature coefficient of the reference resistor R14 must
match and track that of the output resistor for minimum overall
full-scale drift. Settling times of the DAC08 decrease approximately
10% at –55°C. At +125°C, an increase of about 15% is typical.
The reference amplifier must be compensated by using a capacitor
from Pin 16 to V−. For fixed reference operation, a 0.01 µF
capacitor is recommended. For variable reference applications,
refer to the Reference Amplifier Compensation for Multiplying
Applications section.
MULTIPLYING OPERATION
The DAC08 provides excellent multiplying performance with an
extremely linear relationship between I
FS
and I
REF
over a range of
4 µA to 4 mA. Monotonic operation is maintained over a typical
range of I
REF
from 100 µA to 4.0 mA.
SETTLING TIME
The DAC08 is capable of extremely fast settling times, typically
85 ns at I
REF
= 2.0 mA. Judicious circuit design and careful board
layout must obtain full performance potential during testing
and application. The logic switch design enables propagation
delays of only 35 ns for each of the 8 bits. Settling time to within
1/2 LSB of the LSB is therefore 35 ns, with each progressively
larger bit taking successively longer. The MSB settles in 85 ns, thus
determining the overall settling time of 85 ns. Settling to 6-bit
accuracy requires about 65 ns to 70 ns. The output capacitance
of the DAC08, including the package, is approximately 15 pF;
therefore the output RC time constant dominates settling time if
R
L
> 500 Ω.
Settling time and propagation delay are relatively insensitive to
logic input amplitude and rise and fall times, due to the high
gain of the logic switches. Settling time also remains essentially
constant for I
REF
values. The principal advantage of higher I
REF
values lies in the ability to attain a given output level with lower
load resistors, thus reducing the output RC time constant.
Measuring the settling time requires the ability to accurately
resolve ±4 µA; therefore a 1 kΩ load is needed to provide adequate
drive for most oscilloscopes. The settling time fixture shown in
Figure 33 uses a cascade design to permit driving a 1 kΩ load
with less than 5 pF of parasitic capacitance at the measurement
node. At I
REF
values of less than 1.0 mA, excessive RC damping
of the output is difficult to prevent while maintaining adequate
sensitivity. However, the major carry from 01111111 to 10000000
provides an accurate indicator of settling time. This code change
does not require the normal 6.2 time constants to settle to within
±0.2% of the final value, and thus settling time is observed at
lower values of I
REF
.
DAC08 switching transients or “glitches” are very low and can
be further reduced by small capacitive loads at the output at a
minor sacrifice in settling time. Fastest operation can be obtained
by using short leads, minimizing output capacitance and load
resistor values, and by adequate bypassing at the supply, reference,
and V
LC
terminals. Supplies do not require large electrolytic bypass
capacitors because the supply current drain is independent of
input logic states; 0.1 µF capacitors at the supply pins provide
full transient protection.
