Secton 5 Sgnal Condtonng and Data Analyss 6/27/2017 Engneerng Measurements 5 1 Common Input Sgnals 6/27/2017 Engneerng Measurements 5 2 1
Analog vs. Dgtal Sgnals 6/27/2017 Engneerng Measurements 5 3 Current Sgnals 6/27/2017 Engneerng Measurements 5 4 2
General Speccatons or A/D Converters Many speccatons are quoted by the hardware manuacturers. ange esoluton Gan Lnearty Throughput 6/27/2017 Engneerng Measurements 5 5 General Speccatons or A/D Converters ange eers to the maxmum acceptable ampltude o the nput sgnal: Unpolar 0 to +10 v bpolar -10 to +10 v sotware selectable onboard amps 6/27/2017 Engneerng Measurements 5 6 3
esoluton o A/D Converters 6/27/2017 Engneerng Measurements 5 7 Gan and esoluton o A/D Converters Gan On board amplers may permt you to reduce the range & thus ncrease resoluton. Gan voltage put voltage nput and esoluton ange Gan 2 N 6/27/2017 Engneerng Measurements 5 8 4
Samplng ate & Alasng The A/D samplng rate o a sgnal MUST be greater than twce the hghest requency present n the sgnal, otherwse alasng wll take place. ALIASING s the msnterpretaton o a hgh requency sgnal as beng a low requency sgnal. 6/27/2017 Engneerng Measurements 5 9 Samplng ate & Alasng Data Acquston Example: 4 cycles o a 10 Hz reerence sgnal. To capture the requency o ths sgnal the mnmum samplng rate must be greater than 20 Hz. 6/27/2017 Engneerng Measurements 5 10 5
Samplng ate & Alasng Data Acquston Example: 4 cycles o a 10 Hz reerence sgnal. Usng a samplng rate10 tmes greater than the sgnal requency reconstructon o the wave ampltude and requency s possble. 6/27/2017 Engneerng Measurements 5 11 Samplng ate & Alasng Data Acquston Example: Usng a samplng rate 5 tmes the sgnal requency also allows or good reconstructon o the wave ampltude and requency. Yet a samplng rate o 2.5 tmes the sgnal requency s margnal. 6/27/2017 Engneerng Measurements 5 12 6
Samplng ate & Alasng Data Acquston Example: A samplng rate twce the sgnal requency only allows or capturng the requency o the event, but not the ampltude. However a samplng rate less than twce the sgnal requency ntroduces alasng and mples an apparent requency o 5 Hz. 6/27/2017 Engneerng Measurements 5 13 Samplng ate & Alasng Data Acquston Example: educng the samplng rate urther to only 7.5 Hz the sgnal s urther alased and an apparent requency o only 2.5 Hz s now mpled. 6/27/2017 Engneerng Measurements 5 14 7
Amplers Ground eerence Commercal Operatonal Amplers Comparators Sngle ended Amplers Derental Amplers 6/27/2017 Engneerng Measurements 5 15 Ground eerence Earth Ground s what you call ground. elatve to Earth ground +30v elatve to Sgnal ground +15v +15v +0v Sgnal or User ground +0v -15v 6/27/2017 Engneerng Measurements 5 16 8
Analog Devces Ampler 6/27/2017 Engneerng Measurements 5 17 Analog Devces Ampler (Internal Schematc 6/27/2017 Engneerng Measurements 5 18 9
Operatonal Amplers (Non-Invertng ex- n+ n- (Invertng + - ex+ ery Hgh Impedance Input (very low current draw Low Impedance Output (current drver Less than 100 = ex+ - 1.4v - + n = ex- + 0.7v 6/27/2017 Engneerng Measurements 5 19 Comparator Crcut = ex+ - 1.4v ex+ s + - n = n+ - n- ex- = ex- + 0.7v eerence Level 6/27/2017 Engneerng Measurements 5 20 10
Sngle Ended Amplers (Non-Invertng type n + - ex+ + - I I n n I = 0 ex- ( I I +I z I n ( G( ( G n 6/27/2017 Engneerng Measurements 5 21 Sngle Ended Amplers (Invertng type z I = 0 + ex+ + - ex- - n I I ( I I +I n I n ( G( n G 6/27/2017 Engneerng Measurements 5 22 11
12 Derental Amplers 6/27/2017 Engneerng Measurements 5 23 + - ex- ex+ + - I = 0 I 1 - + 2 1 z I 2 I 2 2 1 I 2 2 ( I and earrange and substtute I I 1 1 I 1 1 and earrange and substtute 2 1 6/27/2017 Engneerng Measurements 5 24 Derental Amplers I 2 1 ( (1 ( ( (
13 6/27/2017 Engneerng Measurements 5 25 Derental Amplers ( (1 ( G ( ( ( ( ( --- The End --- ero Adjustment 6/27/2017 Engneerng Measurements 5 26 z - + p 1 2 mn max I = 0 Ip P P P P I I I mn max mn max 1 2 max & mn are the lmts set or. P s selected. 1 and 2 are calculated.
Sgnal-to-Nose ato Over a gven bandwdth B: P S sgnal power P N nose power S sgnal voltage N nose voltage SN Sgnal to Nose ato ecall; 2 2 P E * I E / I * SN P S 10 log 20 PN log S N 6/27/2017 Engneerng Measurements 5 27 Sgnal Contamnated wth Nose Ampltude [a. u.] Ampltude [a. u.] Tme [a. u.] Tme [a. u.] Common Sources o Nose Internal: External: Shot nose Induced voltage rom electrcal lnes Thermoelectrc potental Fluorescent lghtng xtures Electrochemcal acton AC & DC swtchng crcuts Equpment @ audo & rado requences 6/27/2017 Engneerng Measurements 5 28 14
Electro-Magnetc Intererence (EMI Sources Impedance Sheldng Early Amplcaton Early Dgtzng 6/27/2017 Engneerng Measurements 5 29 EMI Sources Devce or wre Emttng an Electromagnetc Feld Swtch operatons Current varatons ado transmsson 60 hz power lnes Induced voltage Sensng wre Induces a voltage varaton n the sensor wrng 6/27/2017 Engneerng Measurements 5 30 15
Impedance ( The lower g and s values are, the lower the. Power source or Ampler Output Sensor wre s g ground The hgher g and s are, the easer EMI can nluence wre voltage. 6/27/2017 Engneerng Measurements 5 31 EMI Sheldng A sheld conducts the nduced voltage to ground. Sheld s Power source or Ampler Output g ground Induced voltage Electromagnetc eld 6/27/2017 Engneerng Measurements 5 32 16
EMI Sheldng Induced Intererence (Ground loop Power source or Ampler Output Sensor wre nduced s g I Ground1 Ground2 nduced Between grounds Electromagnetc eld 6/27/2017 Engneerng Measurements 5 33 Spectral Analyss An analytcal technque used to determne the requences present wthn a complex waveorm. Fourer Transorm (FT - a computatonal analyss appled to contnuous, perodc analog sgnals wth a known undamental requency. Fast Fourer Transorm (FFT - a smple & ecent way to calculate the Dscrete Fourer Transorm, - can be appled to non-perodc sgnals - no a pror sgnal requency normaton requred 6/27/2017 Engneerng Measurements 5 34 17
Spectral Analyss Tme [a. u.] Spectrum [a. u.] Spectrum [a. u.] Ampltude [a. u.] Ampltude [a. u.] Tme [a. u.] Frequency [a. u.] Frequency [a. u.] 6/27/2017 Engneerng Measurements 5 35 Fourer Transorm Any harmonc sgnal can be represented by an nnte seres o sne & cosne waves o varyng ampltudes & requences (.e., a Fourer seres 2 nt B sn nt A A cos 2 2 n o n n1 o y( t where A o /2 = DC component A n = harmonc coecents o the cosne terms B n = harmonc coecents o the sne terms o = undamental requency o 6/27/2017 Engneerng Measurements 5 36 18
Fourer Transorm y ( y ( t e 2 t dt A Fourer Transorm (FT provdes a means o transormng a sgnal dened n a tme doman nto a sgnal dened n a requency doman (.e., breaks down the sgnal nto ts requency components Many sotware packages (ncludng EXCEL are capable o computng the ast FT (reerred to as a FFT o an nput sgnal. 6/27/2017 Engneerng Measurements 5 37 FFT o an Unknown Sgnal 6/27/2017 Engneerng Measurements 5 38 19
Sample FFT Problem: Determne the Frequency and Source o Acoustc nose wthn a 2D cavty. KOmegaSST_InletPro 100,000.00 P vs Tme at varous X locatons (X=x/L 95,000.00 P (Pa 90,000.00 85,000.00 X = 0.25 X = 0.5 80,000.00 20 120 220 320tme (mllsecs 420 520 620 720 820 6/27/2017 Engneerng Measurements 5 39 Unknown Sgnal? How to Proceed? Sample the sgnal o nterest at several derent requences and compare resultng spectra. I ampltude spkes appear at derent requences, alasng s occurrng. Increase the samplng requency untl spkes wthn the spectra do not change. Avod alasng errors by lterng beore computng the FFT. A low pass lter s used to elmnate alasng. 6/27/2017 Engneerng Measurements 5 40 20
Common DAQ Hardware 6/27/2017 Engneerng Measurements 5 41 Interacng between DAQ and PC 6/27/2017 Engneerng Measurements 5 42 21
Summary o Communcaton Optons Internal S-232 S-485 GPIB Ethernet Max Dstance 0 m 50 m 1000 m 2 m 185 m/cable segment Max Speed 250,000 samples/s 700 samples/s 700 samples/s 200,000 bytes/s 36,000 samples/s Number o devces Computer Adapter Needed PC dependent 1/cable 31/cable 14/cable 29/cable segment No No Yes Yes Yes 6/27/2017 Engneerng Measurements 5 43 Flterng Optons Low Pass Hgh Pass Band Pass 6/27/2017 Engneerng Measurements 5 44 22
Analog Sgnal Flterng 0 Gan [db] -10-20 1st-order (-20dB/D -30 2nd-order (-40dB/D 4th-order (-80dB/D -40 0.1 1 10 100 Normalzed Frequency 0 low-pass lter Gan [db] -10-20 1st-order (-20dB/D -30 2nd-order (-40dB/D 4th-order (-80dB/D -40 0.1 1 10 100 Normalzed Frequency hgh-pass lter 6/27/2017 Engneerng Measurements 5 45 Analog Sgnal Flterng 0 Band-pass lter Gan [db] -10-20 1st-order (-20dB/D -30 2nd-order (-40dB/D 4th-order (-80dB/D -40 0.1 1 10 100 Normalzed Frequency 6/27/2017 Engneerng Measurements 5 46 23