B1-05.03 DATA CONVERSION

• Analogue data is continuous and identifies every nuance of what is being measured.
• Digital data uses sampling to encode measurements.
• Analogue data is unfiltered raw data, whereas digital data is filtered for practical use.
• Analogue-to-Digital Converters (ADC) convert an analog signal to a digital signal.
• Digital-to-Analogue Converters (DAC) convert a digital signal to an analog signal.
• ADCs and DACs allow computers to interface with the analogue world for monitoring and control.
• A typical system interfacing a computer to the analogue world involves a transducer, ADC, computer, DAC, and actuator.
• Op-amps, used as comparators, are fundamental to understanding ADC and DAC operations.

Transducer

• Converts a physical variable into an electrical signal.
• The physical variable is typically a non-electric quantity.

ADC

• Receives the transducer's electrical analogue output as input.
• Converts the analogue input into a digital output, represented by a number of bits corresponding to the analogue value.
• For instance, an analogue voltage range of 800 to 1500 mV from the transducer can be converted to 01010000 (80) to 10010110 (150) by the ADC

Computer

• Processes the digital representation received from the ADC.
• Performs calculations or other operations.
• Produces a digital output to manipulate the physical variable.

DAC

• Converts the computer's digital output into a proportional analogue voltage or current.
• The computer outputting a digital range between 00000000 and 11111111, will result in a DAC converting it to a voltage ranging from 0 to 10 V.

Actuator

• Receives the analogue signal from the DAC.
• Interfaces to some device used to physically control or adjust the physical variable.

Operational Amplifiers

• Compares the amplitude of one voltage with another, using an open-loop configuration with the input voltage on one input and a reference voltage on the other.
• They are high-gain DC-coupled amplifiers with two inputs and one output, exemplified by the 741 op-amp IC.
  • High gain, on the order of a million.
  • High-input impedance, low-output impedance.
  • Used with split supply (usually +/- 15 V).
  • Used with feedback, with gain determined by the feedback network.

Zero Level Detection

• An op-amp application where it acts as a comparator to detect when an input voltage exceeds a certain level.
• By grounding the inverting input, a zero level is created, and the input signal is applied to the non-inverting input.
• Even a small voltage difference between the inputs will drive the op-amp into saturation due to high open-loop voltage gain.
• The op-amp output will be at its maximum negative level when the sine wave is negative, and when the sine wave input crosses zero, the output goes to its maximum positive level.

Non-Zero Level Detection

• Modifies the zero-level detector, by connecting a fixed reference voltage to the inverting input to detect voltages other than zero.
• This can involve using a battery or a voltage divider to set the reference voltage; a Zener diode can also be used.
• As long as the input voltage, Vin, exceeds the reference voltage, VREF, the output goes to its maximum positive voltage.

Non-Inverting Amplifier

• Op-amp connected in a closed-loop configuration.
• Functions as a non-inverting amplifier with a controlled amount of voltage gain.

Inverting Amplifier

• Applies the input signal to the inverting input.
• The output is fed back to the inverting input through a feedback circuit formed by input and feedback resistors (R₁ and Rf).
• It creates negative feedback.
• Has a gain of -1 with equal resistors.
• Acts as an inverting buffer in digital circuits.

Digital-to-Analogue Converters

• Converts digital numbers representing voltage values into actual analogue voltages, essential in electronics for signal conversion.
• Digital input values, like 1, 2, 4, and 8, are input into the op-amp through weighted resistors.
• The resulting voltage from the resistors is then applied to the inverting input of the op-amp.

Binary Weighted Resistor DAC

• Uses a 4-bit binary number to convert the applied number to a matching (inverted) output voltage.
• Assumes a standard with +5V as logic 1 and 0V as logic 0.
• Numbers 1, 2, 4, and 8 represent the relative weights assigned to each input of the binary number.
  • "1" is the Least Significant Bit (LSB).
  • "8" is the Most Significant Bit (MSB).

R/2R Ladder DAC

• Overcomes the limitations of binary weighted resistor DACs in maintaining correct ratios when temperature varies due to large resistance value differences between LSB and MSB.
• The R/2R ladder uses only two resistance values, minimizing the impact of temperature variations on accuracy and voltage levels.
• Consists of resistors with values R and 2R.

R/2R Ladder DAC Operation

• Inputs are selected as either five volts or zero volts, this will determines the configuration of the resistive circuit.
• Includes a 4-bit R/2R Ladder DAC circuit.
• Two parallel resistors of equal value have an overall circuit resistance of one half of the value of an individual resistor.
• Having a binary input of 0001, for example, means the "1" has five volts on S1, which is the most significant bit (MSB), and S4 is the least significant bit (LSB).

Analogue to Digital Conversion Methods

• Converts from analogue to digital signals.
• Commonly interfaces analogue and digital systems.
• Methods: flash/simultaneous and digital-ramp/counter-type.
• ADC design and construction is complex, but techniques provide insight into factors determining ADC performance.

Flash ADC

• To convert a digital code to an analogue voltage.
• Assigns an appropriate voltage to each bit, and then combines them.
• Requires a set of comparators, with the number needed increasing exponentially with the number of binary bits used to store the code.
  • converting a 0 to 9-V range to a binary number would require nine comparators
  • A 4-bit binary number counting from 0 to 15, requires 15 comparators
  • A typical 8-bit circuit requires 255 comparators
• The comparators changes the state from false to true.
• Additional digital circuitry is required to encode these signals into the corresponding digital number.

Flash ADC Encoder

• Due to the nature of the sequential comparator output states (each comparator saturating 'high' in sequence from lowest to highest), the same highest-order-input selection effect may be realised through a set of Exclusive-OR (XOR) gates
• This allows the use of a simpler, non-priority encoder.
• The encoder circuit itself can be made from a matrix of diodes, demonstrating just how simply this converter design may be constructed.
• An advantage of the flash converter is that the diagram shows a float sensor in a fuel tank.