Kev Tshawb Fawb Txog Kev Txheeb Ze Bio (BIA) Nrog AD5933: 9 Cov Kauj Ruam

2024 Tus sau: John Day | [email protected]. Kawg hloov kho: 2024-01-30 09:27

Kuv tau txaus siab ua tus Bio Impedance Analyzer rau kev ntsuas lub cev sib xyaw thiab kuv qhov kev tshawb nrhiav tsis raug khaws cia nrhiav qhov tsim los ntawm 2015 Kev Kawm Kho Mob Biomedical ntawm Vanderbilt University. Kuv tau ua haujlwm dhau los ntawm kev tsim qauv thiab txhim kho nws me ntsis. Kuv xav qhia kuv qhov kev tshawb pom nrog koj. Coj yam uas koj tuaj yeem siv los ntawm qhov "taug kev-dhau" yog tias qee yam tsis meej thov qhia kev txhim kho. Qee zaum kuv yuav sau kuv qhov kev xav hauv daim ntawv sib koom ua ke, tab sis tam sim no kuv vam tias koj tuaj yeem siv txhua yam uas koj pom ntawm no. (Yog tias koj xav tias koj tuaj yeem sau qhov no thiab txhim kho, koj zoo siab tos txais)

Teddy

Kev tsim qauv no suav nrog AD5933 nti thiab kev cai sib piv ua ntej-kawg (AFE) los cuam tshuam AD5933 nrog lub cev. AD5933 tom qab ua qhov ntsuas thiab cov txiaj ntsig tuaj yeem ua tiav los ntawm lub microcontroller (piv txwv li Arduino).

Yog tias koj npaj yuav siv Arduino ua lub hwj chim ua kom ntseeg tau tias kev ua haujlwm thiab ntsuas cov cuab yeej siv hluav taws xob (op-amps thiab in-amps) txhawb kev hu ua "ib leeg" cov khoom hluav taws xob thiab muaj cov kev qhia tshwj xeeb rau kev tsheb nqaj hlau.

(Hauv qab no kuv yuav siv lub hwj chim muab (los ntawm Arduino) ntawm 5V thiab Range 1 teeb tsa ntawm AD5933.)

Kauj Ruam 1: The Re-bias Stage

Thawj ntu ntawm AFE yog theem rov ua dua tshiab. Cov teeb liab hluav taws xob tso tawm tsis yog nyob nruab nrab ntawm cov khoom siv hluav taws xob ntau (VDD/2). Qhov no raug kho los ntawm kev siv lub tshuab hluav taws xob los thaiv DC ib feem ntawm lub teeb liab thiab xa nws los ntawm qhov hluav taws xob sib cais kom ntxiv DC offset rov qab rau hauv lub teeb liab.

Ob qhov kev rov ua dua tus neeg tawm tsam tuaj yeem yog tus nqi tsuav yog lawv zoo ib yam. Tus nqi tshwj xeeb ntawm lub hau tseem tsis tseem ceeb.

Lub sijhawm rov ua dua kev ua haujlwm zoo li lub siab lim lim thiab yog li ntawd muaj qhov txiav tawm zaus:

f_c = 1 / (2 * pi * (0.5 * R) * C)

Nco ntsoov tias kev txiav tawm zaus yog ob peb xyoos qis dua qhov tsawg zaus uas koj npaj yuav siv. Yog tias koj npaj yuav siv 1kHz hauv koj daim ntawv thov koj yuav tsum mus rau lub kaus mom thiab tus tiv thaiv qhov muaj txiaj ntsig uas yuav ua rau koj txiav tawm zaus ntawm qhov kev txiav txim ntawm 1-10 Hz.

Qhov kawg ntawm theem no yog op-amp teeb tsa los ua qhov ntsuas hluav taws xob. Qhov no yog kom ntseeg tau tias qhov txiaj ntsig ntawm tus tiv thaiv tsis cuam tshuam nrog qib tom ntej

Kauj ruam 2: Tam sim no Sensing Resistor

Thawj ntu ntawm theem tom ntej yog qhov ntsuas pom tam sim no. Qhov tam sim no los ntawm tus tiv thaiv no yuav yog tib yam tam sim no uas lub tshuab ua kom nrov nrov yuav sim tswj hwm los ntawm lub cev. Nco ntsoov tias tam sim no ua raws IEC6060-1 tus qauv kev nyab xeeb*:

Hauv qab zaus ntawm 1 kHz qhov siab tshaj ntawm 10 microAmps (RMS) raug tso cai los ntawm lub cev. Ntawm cov zaus ntau dua 1kHz qhov sib npaug hauv qab no muab qhov siab tshaj plaws tso cai tam sim no:

Max AC tam sim no <(yam tsawg zaus hauv kHz) * 10 microAmps (RMS)

Qhov sib piv ntawm qhov siab tshaj plaws ntawm qhov teeb liab AC thiab nws tus nqi RMS yog: Peak = sqrt (2) * RMS. (10 microAmps RMS sib haum rau 14 microAmps ncov siab tshaj plaws)

Siv Txoj Cai Ohms ntawm tus tiv thaiv peb tuaj yeem suav tus nqi tiv thaiv uas yuav ua raws li tus qauv kev nyab xeeb. Peb siv qhov ntsuas hluav taws xob los ntawm AD5933 thiab tus nqi siab tshaj tam sim no:

U = R * I => R = U / I

Piv txwv li siv Range 1 teeb tsa Upeak = 3V / 2 = 1.5V (lossis 1V @3.3V)

Siv 14 microAmp cov txiaj ntsig siab tshaj los ntawm saum toj no kuv tau txais tus nqi tiv thaiv tsawg kawg 107kOhms

Cov ntaub ntawv:

* Cov khoom siv sib piv: "Bio-Impedance Circuit Tsim rau Lub Cev Lub Cev Lub Cev"

Kauj Ruam 3: Lub Trans-conductance Amplifier

Tom qab qhov kev pom zoo tam sim no muaj op-amp hauv qhov kev tawm tswv yim tsis zoo. Nov yog qhov hu ua teeb tsa hauv-lub-Loop teeb. Lub davhlau ya nyob twg zoo ntawm op-amp txuas nrog VDD/2 qhov hluav taws xob. Tam sim no op-amp yuav sim kho nws cov zis hauv qhov kev coj rov qab rau lub teeb liab zoo li qhov hluav taws xob ntawm lub davhlau ya nyob twg tsis zoo yuav sib npaug rau VDD/2. Qhov no yuav tsim kom pom lub peev xwm thawb thiab rub tam sim no hla lub cev.

Tam sim no kos los ntawm lub davhlau ya nyob twg tsis zoo ntawm op-amp yog qhov zoo. Txhua qhov tam sim no los ntawm qhov kev paub tam sim no tus tiv thaiv yog li ntawd yuav tsum ntws los ntawm lub cev. Nov yog cov txheej txheem uas ua rau teeb tsa no ua kom muaj kev hloov pauv hloov pauv (tseem hu ua qhov ntsuas hluav taws xob tswj tam sim no, VCCS).

Op-amp tsuas tuaj yeem tswj hwm qhov tam sim no yog tias qhov cuam tshuam ntawm lub cev tsis siab dhau. Txwv tsis pub cov op-amp tso tawm tsuas yog tawm ntawm qhov muab hluav taws xob (0 lossis 5 V). Qhov siab tshaj qhov hluav taws xob uas tuaj yeem tswj tau yog VDD/2 + Upeak (2.5 + 1.5V = 4V @ 5V mov). Qhov ntsuas hluav taws xob ntawm op-amp yuav tsum raug txiav tawm ntawm tus nqi no, tab sis yog tias op-amp muaj cov tsheb nqaj hlau-rau-kev qhia tshwj xeeb uas yuav tsuas yog me me xwb. Qhov siab tshaj plaws impedance uas op-amp tuaj yeem tsav tau yog li ntawd:

Z <(VDD / 2 + Upeak) / Imax

(Hauv kuv teeb tsa Z <4V / 14 microAmps = 285 kOhms, xav kom txaus los npog qhov tsis cuam tshuam ntawm lub cev)

Tus tiv thaiv tus tiv thaiv muaj tus nqi loj heev (1-1.5 MOhms) piv rau lub cev (kwv yees li 100kOhms) thiab rau txhua qhov haujlwm ib txwm ua no yuav tsis kos ib qho uas pom tau tam sim no thiab qhov cuam tshuam ntawm kev sib txuas sib txuas tau los ntawm kev cuam tshuam ntawm lub cev. Yog tias qhov cuam tshuam ntawm lub cev yuav tsum nce (piv txwv li cov ntaub qhwv los xoob) qhov tam sim no tuaj yeem dhau los ntawm tus tiv thaiv thiab qhov siab tshaj ntawm op-amp yuav tsis tsim qhov tsis txaus siab hauv cov pawm.

Kauj Ruam 4: Cov Khoom Siv Ua Kom Tau Zoo

Cov theem tom ntej yog ntsuas qhov ntsuas hluav taws xob (hauv-amp) uas ntsuas qhov hluav taws xob hla lub cev. Qhov hluav taws xob hla lub cev yog oscillating nyob ib puag ncig 0V, tab sis AD5933 xav tau qhov ntsuas hluav taws xob kom nyob hauv qhov zoo. Yog li hauv-amp ntxiv rau DC offset ntawm VDD/2 rau qhov ntsuas qhov ntsuas hluav taws xob.

Qhov VDD/2 siv yog tsim los ntawm kev faib hluav taws xob. Txhua tus nqi tiv thaiv tuaj yeem siv tau tsuav yog lawv zoo ib yam. Qhov faib hluav taws xob tau sib cais los ntawm qhov cuam tshuam ntawm qhov seem ntawm cov khoom siv hluav taws xob los ntawm cov neeg siv hluav taws xob. Cov txiaj ntsig ntawm qhov ntsuas hluav taws xob tuaj yeem xa mus rau ob qho tib si hauv-amp thiab trans-conductance amplifier.

Kauj Ruam 5: The Input Stage and Calibration

Cov theem nkag ntawm AD5933 muaj cov op-amp hauv kev tawm tswv yim tsis zoo. Muaj ob lub resistors: ib qho hauv series (Rin) thiab ib qho nyob rau hauv parallel (RFB). Qhov nce ntawm op-amp yog muab los ntawm

A = - RFB / Rin

Cov txiaj ntsig ntawm cov tswv yim op-amp thiab hauv-amp (thiab PGA) xav tau kom ntseeg tau tias lub teeb liab nkag mus rau ADC ntawm AD5933 yog ib txwm nyob hauv 0V thiab VDD.

(Kuv siv kev sib koom ua ke nce hauv-amp thiab tus tiv thaiv qhov tseem ceeb uas yuav muab kwv yees li A = 0.5)

Sab hauv AD5933 ADC yuav hloov cov teeb liab hluav taws xob mus rau hauv rau hauv cov teeb liab digital. Qhov voltage ntau ntawm 0V rau VDD tau hloov pauv mus rau digital ntau 0-128 (2^7). (Cov ntaub ntawv tsis meej ntawm qhov no tab sis kev tshuaj xyuas ze ntawm cov phiaj hauv [1] thiab som kev sim ntawm kuv ib feem lees paub qhov no.)

Sab hauv DFT tus qauv muaj lwm qhov ntsuas ntawm 256 (1024/4, saib [1]) ua ntej qhov txiaj ntsig tau txais kev cawmdim hauv qhov tiag thiab kev xav.

Los ntawm kev ua raws cov teeb liab hluav taws xob hla AFE, rau hauv ADC thiab siv cov ntsuas ntsuas uas tau hais ua ntej nws tuaj yeem kwv yees qhov txiaj ntsig tau los:

g = (VDD * Rcurrent * Rin) / (256 * PGA * Upeak * RFB * 2^7)

qee qhov kev ntsuas yuav tseem tsim nyog yog li ntawd suav nrog qee qhov cuam tshuam tsis yog ib feem ntawm tus qauv lej no, yog li thov ntsuas qhov txiaj ntsig tau txais txiaj ntsig tiag tiag los ntawm kev ntsuas cov khoom ntawm kev paub tsis meej, zoo li cov neeg tawm tsam. (g = Z / mag, saib hauv qab)

Tam sim no tus impedance tuaj yeem suav los ntawm

Z = g * mas

mag = sqrt (tiag^2 + xav txog^2)

PA = arctan2 (tiag tiag, kev xav) - deltaPA

PA tej zaum yuav tsum tau ntsuas ntxiv nrog rau muaj cov txheej txheem hloov pauv raws li kev ua haujlwm ntawm zaus hauv AD5933. deltaPA yuav yog qee qhov ua haujlwm ntawm cov zaus.

Tam sim no qhov tsis kam thiab reactance tuaj yeem suav los ntawm

R = Z * cos (PA)

X = Z * sin (PA)

Cov ntaub ntawv ua pov thawj: [1] Leonid Matsiev, "Txhim Kho Kev Ua Tau Zoo thiab Kev Muaj Peev Xwm ntawm Cov Txheej Txheem Raws Li Ib Zaug Zaus DFT Cov Ntes Zoo Li AD5933", Electronics 2015, 4, 1-34; doi: 10.3390/electronics4010001

Kauj Ruam 6: Cov Khoom Ua Tau Zoo: Spectral Leakage (DC)

Lub teeb liab uas peb muab tso rau hauv AD5933 yog qhov hluav taws xob/tam sim no ua lub luag haujlwm ntawm lub sijhawm, tab sis peb qhov kev txaus siab tseem ceeb yog qhov cuam tshuam raws li kev ua haujlwm ntawm zaus. Txhawm rau hloov pauv ntawm lub sijhawm-tus lej thiab zaus-tus lej peb yuav tsum tau siv Fourier hloov pauv ntawm lub sijhawm-npe teeb liab. AD5933 muaj qhov tsim-hauv kev txiav txim siab Fourier hloov pauv (DFT) qauv. Ntawm qhov tsawg zaus (qis dua kwv yees li 10 kHz) kev tsim hauv DFT yog cuam tshuam los ntawm cov npe tsis zoo thiab cov duab xau. Hauv [1] nws dhau los ntawm kev ua lej ntawm yuav ua li cas thiaj kho tau qhov pom kev zoo. Lub hauv paus ntawm qhov no yog xam tsib (ntxiv rau ob) tus lej rau txhua zaus zaus hauv kev cheb. Qhov no tuaj yeem ua tau yooj yim xws li los ntawm Arduino hauv software.

Cov dej xau tuaj nyob hauv ob daim ntawv: DC tso tawm uas yog ib qho ntxiv hauv qhov thiab AC tso tawm uas yog qhov sib ntxiv hauv qhov.

DC qhov xau tawm los ntawm qhov tseeb tias lub teeb liab hluav taws xob ntawm ADC tsis nyob ib puag ncig 0V tab sis nyob ib puag ncig VDD/2. Ib qib DC ntawm VDD/2 yuav tsum sib haum rau digital DC nyeem ntawm kwv yees li 64 (xaiv qhov delta hauv [1]).

Cov kauj ruam los kho DC spectral to:

1) Xam Cov Ntawv Hnab Ntawv-E rau qhov zaus tam sim no.

2) Xam ob qhov tau txais txiaj ntsig GI (tiag) thiab GQ (kev xav)

3) Rho tawm delta * GI los ntawm tus nqi ntawm kev sau npe tiag thiab delta * GQ los ntawm tus nqi ntawm kev xav sau npe

Cov ntaub ntawv:

[1] Leonid Matsiev, "Txhim Kho Kev Ua Tau Zoo thiab Kev Muaj Peev Xwm ntawm Cov Txheej Txheem Raws Li

Ib-Zaus DFT Cov Ntes Xws li AD5933 , Electronics 2015, 4, 1-34; doi: 10.3390/electronics4010001

[2] Konrad Chabowski, Tomasz Piasecki, Andrzej Dzierka, Karol Nitsch, "Yooj Yim Dav Dav Zaus Ntau Yam Ntau Yam Tsis Tau Zoo Raws Li AD5933 Kev Sib Koom Ua Ke", Metrol. Ntsuas. Sib., Vol. XXII (2015), Tsis yog 1, phab 13-24.

Kauj Ruam 7: Cov Khoom Ua Tau Zoo: Spectral Leakage (AC)

Ib yam li DC qhov xau AC tawm tuaj yeem kho ua lej. Hauv [1] qhov tsis kam thiab reactance hu ua A*cos (phi) thiab A*sin (phi) feem, qhov twg A sib raug rau qhov loj ntawm qhov tsis kam thiab phi sib raug rau theem kaum (PA).

Cov kauj ruam los kho AC spectral to:

1) Xeem Lub Hnab Ntawv-yam E (tsis zoo ib yam rau DC) rau qhov tam sim no.

2) Xam peb yam a, b, thiab d. (kwv yees qhov tseem ceeb ntawm ntau zaus: a = d = 256 thiab b = 0)

3) Kuj (Acos (phi)) thiab reactance (Asin (phi)) tam sim no tuaj yeem suav hauv cov lej digital

Cov ntaub ntawv ua pov thawj: [1] Leonid Matsiev, "Txhim Kho Kev Ua Tau Zoo thiab Kev Muaj Peev Xwm ntawm Cov Txheej Txheem Raws Li Ib Zaug Zaus DFT Cov Ntes Zoo Li AD5933", Electronics 2015, 4, 1-34; doi: 10.3390/electronics4010001

[2] Konrad Chabowski, Tomasz Piasecki, Andrzej Dzierka, Karol Nitsch, "Yooj Yim Dav Dav Zaus Ntau Yam Ntau Yam Tsis Tau Zoo Raws Li AD5933 Kev Sib Koom Ua Ke", Metrol. Ntsuas. Sib., Vol. XXII (2015), Tsis yog 1, phab 13-24.

Kauj Ruam 8: Cov Khoom Ua Tau Zoo: Theoretical Gain-factor

Muab cov lej ua lej ntawm DFT nws kuj tseem tuaj yeem ua qauv tag nrho AFE kev ua lej. Kev ua lej ntawm qhov ntsuas hluav taws xob tuaj yeem piav qhia los ntawm kev ua haujlwm sine nrog qhov muab zaus tas li, DC offset thiab AC oscillation nrog qhov siab tshaj plaws. Qhov zaus tsis hloov pauv thaum ua ntu zaus. Raws li qhov nce-qhov tseem ceeb tsuas yog hloov pauv qhov loj ntawm qhov cuam tshuam thiab tsis yog PA peb yuav nyob ntawm no tsis muaj kev txhawj xeeb nrog ib theem hloov pauv ntawm lub teeb liab.

Nov yog cov ntsiab lus luv luv ntawm cov teeb liab hluav taws xob thaum nws nthuav tawm los ntawm AFE:

1) Tom qab rov ua qhov tsis ncaj ncees AC qhov dav dav tseem yog Upeak = 1.5V (1V @ VDD = 3.3V) thiab DC offcet tau hloov mus rau VDD/2.

2) Hauv qhov kev ntsuas tam sim no tus tiv thaiv qhov hluav taws xob yog stille tib yam li qib dhau los …

3) … tab sis vim yog qhov pom-voltage ntawm op-amp AC oscillations muaj qhov loj ntawm Z*Upeak/Rcurrent. (Lub DC offset raug tshem tawm los ntawm op -amps qhov ntsuas hluav taws xob ntawm VDD/2 - lub ntsiab lus pivot ntawm lub sawaw - thiab dhau los ua qhov av zoo nyob hauv ib feem ntawm Circuit Court)

4) Kev sib koom hauv-amp ntxiv DC offset ntawm VDD/2 rov qab rau hauv thiab xa cov teeb liab mus rau theem nkag ntawm AD5933

5) op-amp nyob rau theem nkag tau nce ntawm A = -RFB/Rin thiab AC amplitude yog li ua (Z*Upeak/Rcurrent)*(RFB/Rin)

6) Tsuas yog ua ntej ADC muaj qhov programmable amplifier (PGA) nrog ob qhov teeb tsa tau nce ntawm 1 lossis 5. Cov teeb liab hluav taws xob ntawm ADC yog li ntawd ua: PGA*(Z*Upeak/Rcurrent)*(RFB/Rin)

ADC hloov v (t) teeb liab mus rau digital teeb liab x (t) = u (t) / VDD * 2^7 nrog 12 qhov tseeb.

Qhov loj A txuas nrog tus impedance Z los ntawm qhov tau txais txiaj ntsig, k, zoo li A = k * Z thiab muaj tus nqi kwv yees k = PGA * Upeak * RFB * 2^7 / (VDD * Rcurrent * Rin).

Yog tias koj nyiam ua haujlwm nrog nce-faktor hloov pauv g = 1 / k thiab Z = g * A.

Kauj Ruam 9: Cov Khoom Ua Tau Zoo: PA Ua Haujlwm

Hauv [2] lawv pom kev hloov pauv hloov pauv hauv PA raws li kev ua haujlwm ntawm zaus. Qhov no yog vim lub sijhawm ncua ntawm DAC qhov chaw ua kom muaj lub suab paj nruag zoo siab thiab DFT qhov twg cov teeb liab nkag mus yuav tsum tau sib sau ua ke nrog cov teeb liab tawm.

Kev hloov pauv yog tus naj npawb ntawm cov moos-ncig lub teeb liab ncua ntawm DAC thiab DFT sab hauv hauv AD5933.

Cov ntaub ntawv ua pov thawj: [1] Leonid Matsiev, "Txhim Kho Kev Ua Tau Zoo thiab Kev Muaj Peev Xwm ntawm Cov Txheej Txheem Raws Li Ib Zaug Zaus DFT Cov Ntes Zoo Li AD5933", Electronics 2015, 4, 1-34; doi: 10.3390/electronics4010001

[2] Konrad Chabowski, Tomasz Piasecki, Andrzej Dzierka, Karol Nitsch, "Yooj Yim Dav Dav Zaus Ntau Yam Ntau Yam Tsis Tau Zoo Raws Li AD5933 Kev Sib Koom Ua Ke", Metrol. Ntsuas. Sib., Vol. XXII (2015), Tsis yog 1, phab 13-24.