Interfacing Digital To Analog Converters - Computer Science Engineering (CSE) PDF Download

Interfacing ADC0808 with 8086

Interfacing Digital To Analog Converters:

The digital to analog converters convert binary numbers into their analog equivalent voltages. The DAC find applications in areas like digitally controlled gains, motor speed controls, programmable gain amplifiers, etc.

DAC0800 8-bit Digital to Analog Converter

• The DAC 0800 is a monolithic 8-bit DAC manufactured by National Semiconductor.
• It has settling time around 100ms and can operate on a range of power supply voltages i.e. from 4.5V to +18V.
• Usually the supply V+ is 5V or +12V.
• The V-pin can be kept at a minimum of -12V.

Pin Diagram of DAC 0800

Interfacing DAC0800 with 8086 Ad 7523 8-Bit Multiplying DAC:

• Intersil‟s AD 7523 is a 16 pin DIP, multiplying digital to analog converter, containing R-2R ladder(R=10KΩ) for digital to analog conversion along with single pole double through NMOS switches to connect the digital inputs to the ladder.

Pin Diagram of AD7523

• The supply range extends from +5V to +15V , while Vref may be anywhere between -10V to +10V. The maximum analog output voltage will be +10V, when all the digital inputs are at logic high state. Usually a Zener is connected between OUT1 and OUT2 to save the DAC from negative transients.
• An operational amplifier is used as a current to voltage converter at the output of AD 7523 to convert the current output of AD7523 to a proportional output voltage.
• It also offers additional drive capability to the DAC output. An external feedback resistor acts to control the gain. One may not connect any external feedback resistor, if no gain control is required.

Interfacing AD7523 with 8086 Stepper Motor Interfacing:

• A stepper motor is a device used to obtain an accurate position control of rotating shafts. It employs rotation of its shaft in terms of steps, rather than continuous rotation as in case of AC or DC motors. To rotate the shaft of the stepper motor, a sequence of pulses is needed to be applied to the windings of the stepper motor, in a proper sequence.
• The number of pulses required for one complete rotation of the shaft of the stepper motor is equal to its number of internal teeth on its rotor. The stator teeth and the rotor teeth lock with each other to fix a position of the shaft.
• With a pulse applied to the winding input, the rotor rotates by one teeth position or an angle x. The angle x may be calculated as:

X=360⁰/no. of rotor teeth 

• After the rotation of the shaft through angel x, the rotor locks itself with the next tooth in the sequence on the internal surface of stator.
• The internal schematic of a typical stepper motor with four windings is shown in fig.1.
• The stepper motors have been designed to work with digital circuits. Binary level pulses of 0-5V are required at its winding inputs to obtain the rotation of shafts. The sequence of the pulses can be decided, depending upon the required motion of the shaft.
• Fig.2 shows a typical winding arrangement of the stepper motor.
• Fig.3 shows conceptual positioning of the rotor teeth on the surface of rotor, for a six teeth rotor.

Fig.1 Internal schematic of a four winding stepper motor

Fig.2 Winding arrangement of a stepper motor.

Fig.3 Stepper motor rotor

• The circuit for interfacing a winding Wn with an I/O port is given in fig.4. Each of the windings of a stepper motor needs this circuit for its interfacing with the output port. A typical stepper motor may have parameters like torque 3 Kg-cm, operating voltage 12V, current rating 0.2 A and a step angle 1.80 i.e. 200 steps/revolution (number of rotor teeth).
• A simple schematic for rotating the shaft of a stepper motor is called a wave scheme. In this scheme, the windings Wa, Wb, Wc and Wd are applied with the required voltages pulses, in a cyclic fashion. By reversing the sequence of excitation, the direction of rotation of the stepper motor shaft may be reversed.
• Table.1 shows the excitation sequences for clockwise and anticlockwise rotations.
  Another popular scheme for rotation of a stepper motor shaft applies pulses to two successive windings at a time but these are shifted only by one position at a time. This scheme for rotation of stepper motor shaft is shown in table2.

Fig.4 interfacing stepper motor winding.

Table.1 Excitation sequence of a stepper motor using wave switching scheme.

 Table.2 An alternative scheme for rotating stepper motor shaft