Sensors Interfacing

Sensors to ADC Sensors Output span rarely fit input span of ADC Offset (a) – require level shifting Unequal span (b) – require amplification Both (c) –Require both level shift and amplification An OpAmp Level shift and amplify simultaneously

Interfacing components OPAMP Filters Comparators ADC Voltage References

Op-amp Characteristics High Input resistance Low Output resistance Ability to drive capacitive load Low input offset voltage Low input bias current Very high open loop gain High common mode rejection ratio

OPAMP classification criteria Precision opamp Single/dual supply opamp Single ended/differential opamp High Bandwidth opamp Rail to rail IO opamp

Open loop condition

Unity gain – Voltage follower Provide impedance conversion from high level to low level A follower design should have following characteristics For current generating sensors – input bias current of opamp should be at least hundred time smaller than sensors current Input offset voltage should be smaller than required LSB

Instrumentation Amplifier Three opamp IA configuration A IA amplifies the difference between V+ and V-

Instrumentation Amplifiers IA are available as monolithic IC’s Fixed gain range Easy to set desired gain using a single resistor Very high CMRR of the order of 100db and more

Filters To remove unwanted signal components in the input signal Analog Filters Passive filters Designed using passive R,L,C components Simple to design 1st order filters Active filters Based on active component like transistors or opamp Possible to amplify signal of interest Digital Filters

Filter Response Characteristics

Categories of Filters Low Pass Filters: pass all frequencies from dc up to the upper cutoff frequency. High Pass Filters: pass all frequencies that are above its lower cutoff frequency Low-pass response High-pass response

Categories of Filters Band Pass Filters: pass only the frequencies that fall between its values of the lower and upper cutoff frequencies. Band Stop (Notch) Filters: eliminate all signals within the stop band while passing all frequencies outside this band. Band Pass Response Band Stop Response

Single-Pole Low/High-Pass Filter Low Pass Filter High Pass Filter

DAC Is a circuit whose output depend on digital input and associated reference voltage DAC can be implemented using PWM for PWM Vavg=(Ton/T) X Vlh PWM output filtered using RC filter

ADC Essentials Basic I/O Relationship ADC is Rationing System n bits ADC Number of discrete output level : 2n Quantum LSB size Q = LSB = FS / 2n Quantization Error 1/2 LSB Reduced by increasing n Basic I/O Relationship ADC is Rationing System x = Analog input / Reference Fraction: 0 ~ 1

Conversion PARAMETERS Conversion Time Required time (tc) before the converter can provide valid output data Input voltage change during the conversion process introduces an undesirable uncertainty Full conversion accuracy is realized only if this uncertainty is kept low below the converter’s resolution Converter Resolution The smallest change required in the analog input of an ADC to change its output code by one level Converter Accuracy The difference between the actual input voltage and the full-scale weighted equivalent of the binary output code Maximum sum of all converter errors including quantization error

Converting bipolar to unipolar Input signal is scaled and an offset is added Using unipolar converter when input signal is bipolar Scaling down the input Adding an offset Bipolar Converter If polarity information in output is desired Bipolar input range Typically, 0 ~ 5V Bipolar Output 2’s Complement Offset Binary Sign Magnitude … Add offset scaled

Outputs and Analog Reference Signal I/O of typical ADC ADC output Number of bits 8 and 12 bits are typical 10, 14, 16 bits also available Errors in reference signal From Initial Adjustment Drift with time and temperature Cause Gain error in Transfer characteristics To realize full accuracy of ADC Precise and stable reference is crucial Typically, precision IC voltage reference is used 5ppm/C ~ 100ppm/C

Control Signals HBE / LBE Start BUSY / EOC From CPU To read Output word after EOC HBE High Byte Enable LBE Low Byte Enable Start From CPU Initiate the conversion process BUSY / EOC To CPU Conversion is in progress 0=Busy: In progress 1=EOC: End of Conversion

A/D Conversion Techniques Counter or Tracking ADC Successive Approximation ADC Most Commonly Used Dual Slop Integrating ADC Voltage to Frequency ADC Parallel or Flash ADC Fast Conversion

Counter Type ADC Operation Reset and Start Counter Block diagram DAC convert Digital output of Counter to Analog signal Compare Analog input and Output of DAC Vi < VDAC Continue counting Vi = VDAC Stop counting Digital Output = Output of Counter Disadvantage Conversion time is varied 2n Clock Period for Full Scale input Block diagram Suitable for low frequency high resolution conversion

Tracking Type ADC Tracking or Servo Type Using Up/Down Counter to track input signal continuously For slow varying input Advantage There output is continuously available

Successive Approximation ADC Most Commonly used in medium to high speed Converters Based on approximating the input signal with binary code and then successively revising this approximation until best approximation is achieved SAR(Successive Approximation Register) holds the current binary value Block Diagram

Successive Approximation ADC Circuit waveform Logic Flow Conversion Time n clock for n-bit ADC Fixed conversion time Serial Output is easily generated Bit decision are made in serial order

Parallel or Flash ADC Very High speed conversion Up to 100MHz for 8 bit resolution Video, Radar, Digital Oscilloscope Single Step Conversion 2n –1 comparator Precision Resistive Network Encoder Resolution is limited Large number of comparator in IC

Type of ADC’s Type Speed (relative) Cost (relative) Dual Slope Slow Med Flash Very Fast High Successive Appox Medium – Fast Low Sigma-Delta