Philips Semiconductors Linear Products
Product specification
8-Bit high-speed multiplying D/A converter
DAC08 Series
is 2mA or less, and at least 8V more positive than the negative
supply when the reference current is between 2mA and 4mA. This is
necessary to avoid saturation of the output transistors, which would
cause serious accuracy degradation.
TYPICAL APPLICATION
+V
REF
OPTIONAL RESISTOR
FOR OFFSET
INPUTS
R
Output Current Range
IN
R
R
REF
14
15
4
2
Any time the full-scale current exceeds 2mA, the negative supply
must be at least 8V more negative than the output voltage. This is
due to the increased internal voltage drops between the negative
supply and the outputs with higher reference currents.
0V
REQ
=200Ω
16
P
NO CAP
Accuracy
NOTES:
REQ = R || R
IN
Typical Values
Absolute accuracy is the measure of each output current level with
respect to its intended value, and is dependent upon relative
accuracy, full-scale accuracy and full-scale current drift. Relative
accuracy is the measure of each output current level as a fraction of
the full-scale current after zero-scale current has been nulled out.
The relative accuracy of the DAC08 series is essentially constant
over the operating temperature range due to the excellent
temperature tracking of the monolithic resistor ladder. The reference
current may drift with temperature, causing a change in the absolute
accuracy of output current. However, the DAC08 series has a very
low full-scale current drift over the operating temperature range.
P
R
= 5kΩ
IN
+V = 10V
IN
Pulsed Referenced Operation
FUNCTIONAL DESCRIPTION
Reference Amplifier Drive and Compensation
The reference amplifier input current must always flow into Pin 14
regardless of the setup method or reference supply voltage polarity.
The DAC08 series is guaranteed accurate to within ± LSB at +25°C
at a full-scale output current of 1.992mA. The relative accuracy test
circuit is shown in Figure 1. The 12-bit converter is calibrated to a
full-scale output current of 1.99219mA, then the DAC08 full-scale
Connections for a positive reference voltage are shown in Figure 1.
The reference voltage source supplies the full reference current. For
bipolar reference signals, as in the multiplying mode, R can be
15
tied to a negative voltage corresponding to the minimum input level.
current is trimmed to the same value with R so that a zero value
14
R
may be eliminated with only a small sacrifice in accuracy and
appears at the error amplifier output. The counter is activated and
the error band may be displayed on the oscilloscope, detected by
comparators, or stored in a peak detector.
15
temperature drift.
The compensation capacitor value must be increased as R value
14
is increased. This is in order to maintain proper phase margin. For
Two 8-bit D-to-A converters may not be used to construct a 16-bit
accurate D-to-A converter. 16-bit accuracy implies a total of ± part in
65,536, or ±0.00076%, which is much more accurate than the
±0.19% specification of the DAC08 series.
R
values of 1.0, 2.5, and 5.0kΩ, minimum capacitor values are 15,
14
37, and 75pF, respectively. The capacitor may be tied to either V
EE
or ground, but using V increases negative supply rejection.
EE
(Fluctuations in the negative supply have more effect on accuracy
than do any changes in the positive supply.)
Monotonicity
A monotonic converter is one which always provides analog output
greater than or equal to the preceding value for a corresponding
increment in the digital input code. The DAC08 series is monotonic
for all values of reference current above 0.5mA. The recommended
range for operation is a DC reference current between 0.5mA and
4.0mA.
A negative reference voltage may be used if R is grounded and
14
the reference voltage is applied to R as shown. A high input
15
impedance is the main advantage of this method. The negative
reference voltage must be at least 3.0V above the V supply.
EE
Bipolar input signals may be handled by connecting R to a positive
14
reference voltage equal to the peak positive input level at Pin 15.
Settling Time
When using a DC reference voltage, capacitive bypass to ground is
recommended. The 5.0V logic supply is not recommended as a
reference voltage, but if a well regulated 5.0V supply which drives
The worst-case switching condition occurs when all bits are
switched on, which corresponds to a low-to-high transition for all
input bits. This time is typically 70ns for settling to within LSB for
logic is to be used as the reference, R should be formed of two
14
8-bit accuracy. This time applies when R <500Ω and C <25pF. The
series resistors with the junction of the two resistors bypassed with
0.1µF to ground. For reference voltages greater than 5.0V, a clamp
diode is recommended between Pin 14 and ground.
L
O
slowest single switch is the least significant bit, which typically turns
on and settles in 65ns. In applications where the DAC functions in a
positive-going ramp mode, the worst-case condition does not occur
and settling times less than 70ns may be realized.
If Pin 14 is driven by a high impedance such as a transistor current
source, none of the above compensation methods applies and the
amplifier must be heavily compensated, decreasing the overall
bandwidth.
Extra care must be taken in board layout since this usually is the
dominant factor in satisfactory test results when measuring settling
time. Short leads, 100µF supply bypassing for low frequencies,
minimum scope lead length, and avoidance of ground loops are all
mandatory.
Output Voltage Range
The voltage at Pin 4 must always be at least 4.5V more positive than
the voltage of the negative supply (Pin 3) when the reference current
724
August 31, 1994
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