DATA CONVERTERS (Linear Circuits) S.Senthil Kumar, Dept. of Aero, KCT

Data Conversion Basically we have two types of data in nature: Analog and Digital. Data transmission – Electrical wires (for analog) and databus (for digital). Most of the real-world physical quantities are in analog form. Data storage and processing – Analog versus Digital. (That’s the idea for data conversion!!!)

Data Conversion Digital data processors – Microprocessors, DSPs, FPGAs etc. So, when a data conversion needed? – When digital devices interface with analogue devices and vice versa. Can we locate a place in an airplane where digital systems interface with analogue systems? –YES !!! A most critical system – Flight Control System (FCS).

FBW Flight Control System Basic components of a FBW flight control system

Flight Control Computer (FCC) FCC used in LCA Flight Control Computer (Courtesy: DRDO, India) (Courtesy: Curtiss-Wright Controls Defense Solutions, US)

Data Conversion Circuits Digital-to-Analog (D/A) and Analog-to-Digital (A/D) converters (DAC and ADC). Application of A/D and D/A converters Today, both ADC and DAC are available in IC form.

ADC and DAC Applications – digital audio recording and playback, computers, data acquisition, digital multimeter, digital control, digital signal processing, microprocessor based instrumentation. Since most of the ADCs use DACs in their inside, we have to discuss DAC first.

Digital-to-Analog Converter DAC – Current or Voltage outputs (Unipolar and Bipolar), Multiplying type. DAC’s current or voltage analog output is proportional to the weighted sum of digital inputs (characteristic of a linear circuit). Schematic of a DAC

Digital-to-Analog Converter For a voltage output DAC, Binary Weighted Resistor DAC R-2R Ladder DAC Types of DAC Inverted R-2R Ladder DAC In order to understand the above DACs, we have to know the basics of an Operational Amplifier

Operational Amplifier (Op-Amp) (a) Op-amp circuit symbol (b) Ideal op-amp (c) Open loop circuit

Op-Amp – Inverting Amplifier Inverting Summing Amplifier

Binary Weighted Resistor DAC A simple binary weighted resistor DAC

R-2R Ladder DAC A 3-bit R-2R Ladder DAC (Voltage mode) A 3-bit Inverted R-2R (Current mode)

R-2R Ladder DAC – Equation 3-bit R-2R Ladder DAC (Voltage mode)

Analog-to-Digital Converter A-to-D conversion is a quantization process. ADC has two techniques of conversion Comparing a given analog signal with the internally generated equivalent signal. (Types: Successive Approximation, Counter and Flash type) Changing an analog signal into time or frequency and comparing these new parameters with known values. (Types: Integrator and Voltage-to-Frequency (V2F) converters)

Successive Approximation ADC Block diagram of a 4-bit successive approximation ADC

Successive Approximation ADC Flowchart of successive approximation A-to-D conversion

Cycles in Successive Approximation ADC Conversion process in a successive approximation ADC

Reference(s) Tony R. Kuphaldt, Lessons in Electric Circuits, Volume IV – Digital, Design Science License, Fourth Edition, 2002. D. Roy Choudhury and Shail B. Jain, Linear Integrated Circuits, New Age International Publishers, Second Edition, 2003. Anil K. Maini, Digital Electronics Principles, Devices and Applications, John Wiley & Sons, 2007.