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ELECTRONIC INSTRUMENTATION EKT314/4 4. Signal Conditioning Circuits Contents Introduction Amplifiers Filters 2 Introduction Signal from detector stage has to be modified (conditioned) in order to make it more usable. This signals is then to be use in later stage of system that may consist of indicating, recording and processing elements. Proper selection of signal conditioning circuit can improve the quality and system performance. Signal conditioning functions can be amplification and filtering. 3 Example… Electronic-Aided Measurement Source: [Kalsi2005] pg. 461 4 Signal Conditioning Functions Amplification Filtering Increase the level of input signal to better suit the DAQ. Improve the sensitivity and resolution of the measurement. Reject useless noise within certain frequency range. Prevent signal aliasing and distortion. Attenuation Contrary to amplification. 5 Signal Conditioning Functions …continued Isolation Multiplexing Sequentially transmit a number of signals into single digitiser. Simultaneous Sampling Solve improper grounding problem of the system. Issue of measuring more than one signals at the same time. Digital Signal Conditioning 6 DC Signal Conditioning System Source: [Kalsi2005] pg. 462 7 DC Signal Conditioning System… continued DC bridge can be Wheatstone’s Bridge which can be balanced by a potentiometer or can be calibrated for unbalanced conditions. Amplifier should be thermally good and stable for a long term. Low pass filter for eliminating high frequencies components or noise. Main disadvantages – problem of drift. 8 AC Signal Conditioning System Source: [Kalsi2005] pg. 462 9 AC Signal Conditioning System… continued AC system is to overcome the problem of DC system. Transducer can be variable resistance or variable inductance types. Bridge circuit for modulating the amplitude of the output from the transducer stage. The signal is then amplified and demodulated before pass through the low pass filter. 10 Contents Introduction Amplifiers Filters 11 Amplifiers Required in the system to improve the signal strength which is typically in the low level range of less than a few mV. In some cases, amplifiers is necessary in providing impedance matching and isolation. Basic characteristics involved in designing amplifiers are: Input impedance Output impedance Gain and frequency response Noise 12 Input Impedance Input impedance of an amplifier regularly depends on the output impedance from the transducer stage. Source impedance may vary from few Ω up to hundred MΩ. Considering the loading effect of the input impedance of the amplifier to the transducer, the effective input, ei can formulated as follows. Rin ei es Rin Rs 13 Input Impedance 14 Input Impedance… continued The error between effective input and source voltage reflect the overall sensitivity of the system. A very high input impedance (approaching infinity) amplifier can be used to reduced an error. Practically it is simpler to design an amplifier with input impedance of 10 to 50 times source impedance and calibrate the system sensitivity combining the effect of the amplifier itself and the transducer. In some cases, the amplifier with a very high input impedance is needed to overcome the changes in the sensitivity of the system. 15 Input Impedance… continued High input impedance is not always a good thing though, for example if you want to transfer as much power as possible then the source and load impedance should be equal. For a current input a low input impedance (ideally zero) is desired, for example in a transimpedance (current to voltage) amplifier. 16 Output Impedance Output impedance required for an amplifier depends on the input impedance of the next subsystem. The value of output impedance can be explicit so that the loading effect for next sub-system can be calibrated. Generally the output impedance on an amplifier need to be sufficiently low enough (less than an Ω). Beside output impedance, the factor of output drive capability of an amplifier also need to considered. 17 Gain Gain of an amplifier is the result of an amplification of the input signal. Gain factor of an amplifier can be generally expressed as: aout A ain aout and ain can be output and input power or voltage. Amplifier gain also can be expressed in Decibel (dB). A( dB ) Pout Vout 20 log 10 log V P in in 18 Gain… continued Considering the relativity of the input impedance of an amplifier and the output impedance of the transducer, there are attenuation occur before amplification. The effective amplification is the product of attenuation and the gain factor of the amplifier. Amplifier gain affect the system’s sensitivity and calibration, therefore the high gain stability become and important criteria. 19 Frequency Response The frequency response refers to the variation of the gain of an amplifier with the frequency. •Usually specified in terms of frequency when the -3dB take place in the amplifier’s gain. •Typically the frequency response of an amplifier tends to reduce the amplification at high frequencies. •The response often affected by the output impedance of the amplifier. Need to consider the final load impedance when calculating frequency response of the system. • 20 Noise The performance of the amplifiers that use an active devices are influenced by noise that probably internally generated. Noise added usually frequency dependent and its spectral density can be expressed as noise voltage, current and power. At very low frequencies, the noise spectral density increase due to the flicker noise (pink noise, 1/f) whereas at high frequencies the spectral increase due to the limitations of the frequency response of the amplifier. 21 DC Amplifiers An amplifier that can produce a constant DC output signal. Not necessary produce a zero DC output when input is zero. This error known as offset voltage and it is depends on the input signal not the amplifier gain. This offset voltage also varies with time and temperature. The presence of finite DC current at the input of the amplifier is another source of error in DC amplifier. This DC bias current will create additional offset errors depending on the DC output impedance. 22 Operational Amplifier DC amplifier with high gain in the form of integrated circuit. Can be used to performs an important functions like isolation, addition, inversion, multiplication, subtraction and division. Other mathematical operations can be perform such as integration and differentiation. Variety of operational amplifiers available commercially with different specifications. For the purpose of this course, the discussion is focus on the 741 op-amps. 23 Operational Amplifier… continued Operational amplifier typically have more than one cascaded differential amplifiers which led to achieving a very high overall gain. As in figure below, every stage give different value of gain and in the end the total gain is obtained by a multiplication of them all. Source: [Kalsi2005] pg. 464 24 Operational Amplifier… continued From figure, the first stage is dual input balanced output differential amplifier with constant current source that can have gain up to 60. This stage contribute huge voltage gain to the overall gain and also the input impedance is resolve here. Intermediate stage is where the overall gain is increased furthermore with the dual input unbalanced output differential amplifier. Intermediate stage can have gain up to 30. Level shifter is used to stabilize the high DC output voltage from intermediate stage to the ground potential. Last stage is push-pull amplifier with low output impedance and minimum offset that have a gain up to 10. 25 Ideal Operational Amplifier Characteristics of ideal operational amplifier can be listed as follows: Infinite input impedance Zero output impedance Infinite open loop gain Infinite bandwidth, slew rate and CMRR For ideal operational amplifier the output voltage is zero whenever there is equal voltage is applied to both of its inputs. 26 Common Mode Rejection Ratio 27 Equivalent circuit of operational amplifier Output voltage can be expressed as follows; Vo = A(V1 – V2) where A is large signal voltage gain, V1 is voltage at inverting terminal and V2 is voltage at non-inverting terminal, Vid in the figure is differential input voltage, take note that V1 and V2 are both with respect to ground Source: [Kalsi2005] pg. 465 28 741 Operational Amplifier For this actual operational amplifiers, the characteristics is contrary to the ideal operational amplifier as it does not have infinite input impedance, non-zero output impedance and input current. Other important factor is offset voltage. The µA741 operational amplifier is manufactured by Fairchild Semiconductor (LM741) and its specifications are discussed in detail here. 29 741 Operational Amplifier… continued µA741 features (adapted from LM741 datasheet by Fairchild Semiconductor) Internal frequency compensation Excellent temperature stability Offset voltage null capability High input voltage range Short circuit protection 30 Parameters of 741 Operational Amplifier There are typical parameters that associated with µA741 (or any other) operational amplifier that need to be taken into account when using it the design. Absolute maximum ratings Input parameters Output parameters Dynamic and other parameters These parameters generally has been presented in the operational amplifier datasheet by the manufacturer. 31 Absolute Maximum Ratings #find example diff input/input Supply Voltage Differential Input Voltage This value indicated the maximum voltage that can be applied across the + and – negative inputs of the operational amplifier. Input Voltage This is the value of the maximum positive or negative voltage that can be supply to the operational amplifier safely. Maximum voltage that can be applied simultaneously between both input with reference to ground. Output Short Circuit Duration The amount of time that the operational amplifier’s output can be shorted to supply voltage. 32 Absolute Maximum Ratings… continued Power Dissipation Operating Temperature Range Maximum power that operational amplifier can dissipate with respect to certain temperature. Ambient temperature range that the operation of operational amplifier meet the manufacturer's specifications. Storage Temperature Range The range of temperature that the operational amplifier can be stored into. 33 Absolute Maximum Ratings… continued Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001. 34 Input Parameters Input Offset Voltage Input Offset Current Average of both current flowing into both of the inputs. Input Resistance Differences of two input bias current when the output voltage is zero. Input Bias Current Voltage that must be applied to one of the input pins in order to give zero output voltage. Resistance of the operational amplifier at either input when the other grounded. Input Voltage Range Voltage that common to both inputs and ground. 35 Input Parameters… continued Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001. 36 Output Parameters Output Short Circuit Current Output Voltage Swing Maximum output current that the operational amplifier can deliver to the load. Maximum output voltage (peak) that the operational amplifier can give without distortion or clipping. This depends on the load resistance. Output Resistance Resistance at the operational amplifier’s output. 37 Output Parameters… continued Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001. 38 Dynamic Parameters Large Signal Voltage Gain Slew Rate The ratio of the maximum voltage swing to the change in the input voltage required to drive the output from zero to a specified voltage. Rate of change of the output voltage with the operational amplifier having a unity gain. Open-Loop Voltage Gain Output voltage to input voltage ratio of the operational amplifier without feedback. 39 Other Parameters Supply Current Common Mode Rejection Ratio A quantify of the ability of the operational amplifier to reject signals that are simultaneously present at both inputs Bandwidth Current that the operational amplifier will draw from the supply. Usually specified as unity gain bandwidth. Power Consumption The power consume by the operational amplifiers when operated. 40 Dynamic and Other Parameters Source: LM741 Single Operational Amplifier datasheet, Fairchild Semiconductor Corporation, 2001. 41 Comparison of Operational Amplifiers Source: [Rangan2004] pg. 243 42 Type of amplifier circuits Several amplifier circuits can be constructed using the operational amplifier (such as µA741). These are: Non-Inverting Amplifier Inverting Amplifier Differential Amplifier Instrumentation Amplifier 43 Non-Inverting Amplifier A close loop gain (feedback is used) of the non-inverting amplifier can be expressed as follow: vo R f AF 1 vi R1 Generally AF is made very large. 44 Inverting Amplifier The input signal for inverting amplifier is applied to its negative input terminal. The close loop gain of this amplifier can be computed as follows: AF Rf R1 45 Inverting & Non-inverting 46 Differential Amplifier The output voltage of the differential amplifier is relative to the difference between the two input voltages. Output voltage, vo can be expressed as follows: vo = Ad (v+ - v-) Where Ad is a differential gain that is designed to be at very high value. Ad also implies that both of the input voltages is almost equal. i. e. v+≈ v-. 47 Differential Amplifier… continued Input impedance for differential amplifier is significantly large. Ad also known as Difference Mode Gain and (v+-v-) is Difference Signal. Equally applied input voltage is common mode signals as the differential amplifier will output zero voltage theoretically. Practically, the output of differential amplifier is not equal to zero when both inputs are equal. 48 Gain = R2/R1 Input impedance = R1+R2 49 CMRR in Differential Amplifier The distinction between the gain desired for difference signals and the gain for the common mode signals can be made available. Common mode gain Ac vo vc Where vc is common mode input signal. The CMRR is defined as follows: CMRR Ad Ac CMRR is the measure of the desired signal to the undesired signal. The larger the CMRR the better the amplifier. 50 Common Mode Rejection Ratio 51 Advantages of Differential Amplifier Noise Immunity Can be used in the situations where the operations of single ended operational amplifiers is impractical due to the ground potential differences and interferences from pick-up. Differential amplifier only responds to the differences between its input signals not the noise pick-up or ground voltage that came in phase on both signals. Drift Immunity Changes in voltage gain and level due to ageing and variations of temperature can be eliminate. Two inputs and outputs of one differential amplifier can be imagine as it is made up of two amplifier. Therefore any changes in voltage and variations of temperature affect both of its inputs. 52 Instrumentation Amplifier Dedicated differential amplifier with very high input impedance. High common mode rejection features make instrumentation amplifier useful in recovering small signals hidden in large common mode offsets and noise. Instrumentation amplifier is a close loop devices with certain value of gain. Can be optimised as signal conditioner for low level signals (DC) in high noise environments. 53 Instrumentation Amplifier… continued Instrumentation amplifier is divided into two stages; first stage give a very high input impedance to both of input signals and with single resistor gain setting, second stage is a differential amplifier with the negative feedback, ground connections and output are all taken out. Input stage consists of two matched operational amplifiers. 54 Instrumentation Amplifier… continued Both of input signals is applied to the noninverting input terminal of the operational amplifiers. The operational amplifier are configured as voltage follower which give the instrumentation amplifier a very high input impedance. 55 Instrumentation Amplifier… continued Rg is a gain setting resistor in the following formulae for computing vo: 2R vo v 2 v1 1 Rg From the equation, the smaller the value of Rg, the larger the output voltage vo. It is clear that Rg can be used in setting the gain of this first stage. 56 Instrumentation Amplifier… continued The second stage of the instrumentation amplifier is a differential amplifier with unity gain. The full instrumentation amplifier circuit uses three operational amplifiers hence it is called three amplifier configuration. 57 Instrumentation Amplifier… continued Important features of instrumentation amplifier: Differential input capability with high gain common mode rejection. Selectable gain with high gain accuracy and linearity. High stability of gain with low temperature coefficient. Low DC offset and drift errors referred to input. Low output impedance. 58 Contents Introduction Amplifiers Filters 59 Filters Filter is the network used to attenuate certain frequencies but allow others without attenuation. Consist at least one pass band, which is a band of frequencies that the output is approximately equal to input and attenuation band that the output is equal to zero. Cut-off frequencies is the frequencies that separate the various pass and attenuation bands. Important characteristic of filter networks is its construction make use of purely reactive elements. Two types of filter: Passive Filters Active Filters 60 Types of Filters Passive filters only use passive circuit component such as resistors, capacitors and inductors. Active filters use active elements like operational amplifiers in addition to passive elements like resistance, capacitance and inductance. Both of passive and active filters can be classified as follows: Low Pass Filter High Pass Filter Band Pass Filter Band Stop Filter All Pass Filter 61 Types of Filters… continued Source: [Kalsi2005] pg. 505 62 Passive Filters Low Pass Filters (LPF) A RC network. At low frequencies, the capacitive reactance is very high, therefore the capacitor circuit acts like an open circuit. These condition gives Vo = Vi. At very high frequencies, the capacitive reactance is very low therefore Vo is very small compared to Vi. 1 fc 2RC 63 Passive Filters… continued High Pass Filters (HPF) A RC network. At low frequencies, the gain is small, therefore Vo is small compared to Vi. As the frequencies goes high the gain approaches unity. 1 fc 2RC 64 Passive Filters… continued Band Pass Filters (BPF) Can be constructed by cascading LPF and HPF. At frequencies below the pass band, BPF behave like HPF while above the pass band frequencies the BPF acts like LPF. In pass band, the BPF circuit is almost as a resistive network. ABPF R2 R1 R2 f clower 1 1 , f cupper 2R2C2 2R1C1 65 Passive Filters… continued Band Stop Filters (BSF) Simple RC filters. Twin T BSF; At the very low and high frequencies the gain is almost unity, but between the two there is a frequency where the gain become zero. The frequency is known as Notch Frequency, f0. 1 f0 2RC 66 67 Active Filters Generally the impedances are used in the inverting amplifiers using operational amplifiers. Basic relationship can be used to obtain the desired filter sections is as follows (in the case of inverting amplifiers). V0 Z f V1 Z1 The voltage can also be amplified. 68 Active Filters… continued Low Pass Filters (LPF) V0 R fH R1H 1 jR fH C fH 1 H 2f c R fH C fH H refers to characteristic high frequency V1 1 fc 2R f C 1 R 2fC 69 Active Filters… continued High Pass Filters (HPF) V0 jR fLC1L 1 jR1LC1L 1 L 2f c R1LC1L L refers to characteristic low frequency V1 1 fc 2R1C 1 R 2fC 70 Active Filters… continued Low Pass Filters (LPF)… non-inverting configuration V0 R f R1 R1 1 jRH CH V1 1 H RH C H H refers to characteristic high frequency 71 Active Filters… continued High Pass Filters (HPF)… non-inverting configuration. V0 R f R1 jRLCL R1 1 jRLCL V1 1 L RLC L L refers to characteristic low frequency 72 Active Filters… continued Band Pass Filters (BPF) V0 R f2 R2 R f 1 R1 jRLCL R2 R1 1 jRLCL 1 jRH CH V1 73 Band stop filter 74