2 Analog Signal Conditioning 2.1 INTRODUCTION Signal conditioning - refers to operations performed on signals to convert them to a form suitable for interface with other elements in the process control loop. e.g. 1. in circuits, output of the former module should match the input of the latter module 2. the sensor output often doesnt match the input range of the ADC analog conversion in this chapter the conditioned output is still an analog 2.2 PRINCIPLE OF ANALOG SIGNAL CONDITIONING categorize signal conditioning into several general types: 1) Signal-level and bias changes involves adjusting the level (magnitude) and bias (zero value) of some voltage representing a process variable e.g. a sensor varies 0.2 0.6V, connected with equipment with 0 5V first part is called zero shift, or bias adjusting; 12.5 is called a gain amplification and attenuation - is done by a amplifier 5 .122 . 0 outputsensor signal conditioninginitial sensor 2) Linearization Often the dependence that exists between input and output is nonlinear. b - sensor output c - process variable V - signal after conditioned difficult to design such circuits: solution : ?operate only within narrow limits ?computer, software 3) Conversions Often, signal conditioning is used to convert one type of electrical variation into another. e.g. to convert resistance change either to a voltage or a current signal ?Conversion for Signal transmission: an important type of conversion is associated with the process control standard of transmitting signals as 4-20mA current. So, transmitting end - receiving end voltage-to-current converters, current-to-voltage converters ?Conversion for Digital Interface: ADC and DAC, e.g. ADC may need analog input of 0-5V. 4) Filtering and Impedance Matching ?Filtering: industry - spurious signals, 50/60Hz line frequency signal, Motor start transient (pulse or unwanted signals) Using high-pass, low-pass, notch filter to eliminate (passive-resistance, capacitors, inductors; active-gain and feedback, power supply) ?Impedance matching: Transducer internal impedance or line impedance can cause errors in measurement of a dynamic signal, so impedance matching is an important element. (see concept of loading in the next slide for detailed) 5) Concept of loading The loading of one circuit by another introduces uncertainty in the amplitude of a voltage as it is passed through the measurement process. If representing a variable, then we have uncertainty in the value of the variable. Qualitatively, open circuit means that nothing is connected to the output. output Vx.Loading occurs when we do connect something, a load, across the output. output Vy. Vy feedback system - motor - shaft - variable resistor disadvantage: not very fast, is subject to wear, generates electronic noise. solution: use a current source. even a closed-loop system can be constructed providing a self-nulling ability. Principle: splitting one arm into two R4and R5, and R4 R5 if a high impedance is used , then R4 + R2 R5 then a null is reached by adjusting the magnitude and polarity of the current I until IR5equals the voltage difference of the first two terms current source 45 5 245 () b VRR VIR RRR + =+ + 5 542 54 31 3 )()( IR RRR RVR RR VR VVV ba + + + + + = + = EXAMPLE 2.7 A current balance bridge, R1 = R2 = 10k, R4 = 950, R3 = 1k, R5 = 50 and a high-impedance null detector. Find the current required to null the bridge if R3changes by 1. The supply voltage is 10V. solution: for nominal resistance values given, the bridge is at a null with I = 0, because when R3 increase by 1 to 1001, we need an I to ensure V=0 so, we get 345 5 13245 ()() (10 )(1)(10 )(95050 ) (0)(50 ) (101)(1095050 ) 0 ab VRVRR VVVIR RRRRR VkV mA kkk V + = + + = + = 345 5 13245 ()() (10 )(1001 )(10 )(95050 ) (50 ) (101001 )(1095050 ) 0 ab VRVRR VVVIR RRRRR VV I kk V + = + + = + = 16.0IA= AC Bridge the bridge concept can be applied to the matching of impedance. similarly, E=ac offset voltage when Z3Z2 = Z1Z4, E=0 ps: real part and imaginary part of a complex number must be independently equal. 3214 1342 ()() Z ZZ Z EE ZZZZ = + 2.3.3 RC Filters to eliminate unwanted noise signal from measurement. It is necessary to use circuits that block certain frequency or bands of frequency. A simple filter can be constructed from a single resistor and a single capacitor. ?Low pass RC filter: it blocks high frequencies and passes low frequencies. Critical frequency: the frequency for which the ratio of the output to the input voltage is approximately 0.707. )2/(1RCfc= = 2/12 )/(1 1 cin OUT ffV V + = + = Design methods: step1, find the critical frequency. step2, determine the resistor and capacitor to provide the required critical frequency. Note that one equation, two unknown parameters; practical guideline: 1) select a standard capacitor in the F or pF range 2) calculate a required resistanceranging 1k-1m, try a diff