Philips Semiconductors Linear Products
Product specification
8-bit multiplying D/A converter
MC1508-8/1408-8
4.5V (or 8V — see above) above the negative supply voltage
without significant degradation of accuracy. Philips Semiconductors
MC1508/MC1408 can be used in sockets designed for other
manufacturers’ MC1508/MC1408 without circuit modification.
TYPICAL PERFORMANCE CHARACTERISTICS
D-to-A TRANSFER CHARACTERISTICS
0
Output Current Range
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.
1.0
2.0
(00000000) INPUT DIGITAL WORD (11111111)
Accuracy
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 MC1508/MC1408 is essentially
constant over the operating temperature range because of 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
MC1508/MC1408 has a very low full-scale current drift over the
operating temperature range.
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.
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.
R
may be eliminated and Pin 15 grounded, with only a small
15
The MC1508/MC1408 series is guaranteed accurate to within
±1/2LSB at +25°C at a full-scale output current of 1.99mA. The
relative accuracy test circuit is shown in Figure 3. The 12-bit
converter is calibrated to a full-scale output current of 1.99219mA;
then the MC1508/MC1408’s full-scale current is trimmed to the
sacrifice in accuracy and temperature drift.
The compensation capacitor value must be increased with
increasing values of R to maintain proper phase margin. For R
14
14
values of 1.0, 2.5, and 5.0kΩ, minimum capacitor values are 15, 37,
and 75pF. The capacitor may be tied to either V or ground, but
EE
same value with R so that a zero value appears at the error
14
using V increases negative supply rejection. (Fluctuations in the
EE
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.
negative supply have more effect on accuracy than do any changes
in the positive supply.)
A negative reference voltage may be used if R is grounded and
14
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 ±1/2
part in 65,536, or ±0.00076%, which is much more accurate than the
±0.19% specification of the MC1508/MC1408.
the reference voltage is applied to R , as shown in Figure 2. A
15
high input impedance is the main advantage of this method. The
negative reference voltage must be at least 3.0V above the V
EE
supply. Bipolar input signals may be handled by connecting R to a
14
positive reference voltage equal to the peak positive input level at
Pin 15.
Monotonicity
A monotonic converter is one which always provides an analog
output greater than or equal to the preceding value for a
corresponding increment in the digital input code. The
MC1508/MC1408 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.
Capacitive bypass to ground is recommended when a DC reference
voltage is used. The 5.0V logic supply is not recommended as a
reference voltage, but if a well regulated 5.0V supply which drives
logic is to be used as the reference, R should be formed of two
14
series resistors and 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.
Settling Time
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 1/2LSB for
If Pin 14 is driven by a high impedance such as a transistor current
source, none of the above compensation methods apply and the
amplifier must be heavily compensated, decreasing the overall
bandwidth.
8-bit accuracy. This time applies when R < 500Ω and C < 25pF.
L
O
The slowest single switch is the least significant bit, which typically
turns on and settles in 65ns. In applications where the D-to-A
converter functions in a positive going ramp mode, the worst-case
condition does not occur and settling times less than 70ns may be
realized.
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
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 degradation of accuracy.
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, good ground planes, and avoidance of
ground loops are all mandatory.
Philips Semiconductors MC1508/MC1408 does not need a range
control because the design extends the compliance range down to
740
August 31, 1994
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