Fastest settling times are obtained when Pin 1 sees a low
impedance. If Pin 1 is connected to a 1 kΩ divider, for example,
it should be bypassed to ground by a 0.01 µF capacitor.
ANALOG OUTPUT CURRENTS
Both true and complemented output sink currents are provided
where IO + IO = IFS. Current appears at the true (IO) output when
a 1 (logic high) is applied to each logic input. As the binary
count increases, the sink current at Pin 4 increases proportionally,
in the fashion of a positive logic DAC. When a 0 is applied to
any input bit, that current is turned off at Pin 4 and turned on at
Pin 2. A decreasing logic count increases IO as in a negative or
inverted logic DAC. Both outputs may be used simultaneously.
If one of the outputs is not required, it must be connected to
ground or to a point capable of sourcing IFS; 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− + (IREF × 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.
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, IREF ≤ 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 IREF = 2 mA is not
recommended because negative output compliance would be
reduced 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 may be used to ensure logic swings, etc., remain
between acceptable limits. Power consumption is calculated as
PD = (I +) (V +) + (I −) (V −)
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.
The nonlinearity and monotonicity specifications of the DAC08
are guaranteed to apply over the entire rated operating tempera-
ture 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 should
match and track that of the output resistor for minimum overall
full-scale drift. Settling times of the DAC08 decrease approxi-
mately 10% at –55°C. At +125°C, an increase of about 15% is
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.
The DAC08 provides excellent multiplying performance with
an extremely linear relationship between IFS and IREF over a
range of 4 µA to 4 mA. Monotonic operation is maintained over
a typical range of IREF from 100 µA to 4.0 mA.
The DAC08 is capable of extremely fast settling times, typically
85 ns at IREF = 2.0 mA. Judicious circuit design and careful
board layout must be used to 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 RL > 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 IREF values. The principal advantage of higher IREF
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 IREF values of less than 1.0 mA, excessive
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