Philips Semiconductors Linear Products

Product specification

MC1508-8/1408-8

8-bit multiplying D/A converter

August 31, 1994

740

TYPICAL PERFORMANCE CHARACTERISTICS

D-to-A TRANSFER CHARACTERISTICS

0

1.0

2.0

(00000000) INPUT DIGITAL WORD

(11111111)

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.

tied to a negative voltage corresponding to the minimum input level.

sacrifice in accuracy and temperature drift.

The compensation capacitor value must be increased with

values of 1.0, 2.5, and 5.0k

Ω, minimum capacitor values are 15, 37,

negative supply have more effect on accuracy than do any changes

in the positive supply.)

high input impedance is the main advantage of this method. The

positive reference voltage equal to the peak positive input level at

Pin 15.

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

series resistors and the junction of the two resistors bypassed with

0.1

µF to ground. For reference voltages greater than 5.0V, a clamp

diode is recommended between Pin 14 and ground.

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.

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.

Philips Semiconductors MC1508/MC1408 does not need a range

control because the design extends the compliance range down to

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.

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.

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.

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

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.

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.

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.

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

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.

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, 100

µF supply bypassing for low frequencies,

minimum scope lead length, good ground planes, and avoidance of

ground loops are all mandatory.