1 / 21

Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays

Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays Guillermo Bedoya Advanced Hardware Architectures ( UPC ) Laboratoire des Images et des Signaux ( INPG ). UNIVERSITAT POLITÉCNICA DE CATALUNYA. UPC. INP Grenoble. Outline. Introduction

zeki
Download Presentation

Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays Guillermo Bedoya Advanced Hardware Architectures (UPC) Laboratoire des Images et des Signaux (INPG) UNIVERSITAT POLITÉCNICA DE CATALUNYA UPC INPGrenoble

  2. Outline Introduction - Overview - What is Blind Signal Separation (briefly) Identifiability & Algorithmic solutions -When can it be done -How do we do it (briefly) Applications to semiconductor-based sensor arrays -The ISFET/CHEMFET devices -How does BSS apply to semiconductor-based chemical sensing

  3. Overview • Potential advantages of integrated circuit technology applied to the field of physiological data acquisition and water monitoring systems : • small size • reliability • low cost & rapid time response • multi sensor chip • on chip signal processing • Our objective is to design a low cost/high performance smart sensor system for Biomedical and environmental monitoring applications, using ISFET/CHEMFET sensor arrays.

  4. Introduction Blind Source Separation deals with the « separation » of a mixture of sources, with a little prior information about the mixing process and the sources signals x = As Ŝ = Wx Environment x1 Ŝ 1 S1 S2 SN Source Separation Algorithm W x2 Ŝ2 . . . . . . . . . . . . xN ŜN Sources Sensors Observations

  5. What is Blind Source Separation x1[n]=a11s1[n]+…+a1psp[n] . . xm[n]=am1s1[n]+…+ampsp[n] x[n]=(x1[n]…xm[n])=As[n] y[n]=(y1[n]…ym[n])=Wx[n]=s[n] s x y A W

  6. Identifiability & Algorithmic solutions Principles of information theory can be applied to the BSS problem. We suppose that source signals are independents (realistic assumption). s x= As y= Wx Then, we minimize a measure of the independence (e.g., the mutual information (MI) I(.)) of the outputs I(y), where y=Wx.

  7. x y How do we do it? W g(x,W) We consider a processing function g, which operates on a scalar X using a function Y=g(X;w) in order to maximize the MI between X and Y. Parameter w is chosen to maximize I(X;Y). I(X;Y)=H(Y) – H(Y|X) The MI is maximized when H(Y) is maximized (for g deterministic). H(y)=sum[H(yi)] - I(y1,…, yN) In order to maximize H(y) (we can maximize each H(yi) or minimize I(y1,…, yN)). g must be the CDF of x. The mutual information is minimized when all the outputs are independent !

  8. How do we do it? Assuming a particular functional form: y = g(x) = 1/(1+e-(wx)) ∂H(y) ∂w An adaptive scheme is to take: Δw α Δw = ; ∂ (ln |∂y/∂x|) ∂w ∂y/∂x = wy (1-y) then, Δw α And we have a weight update rule : w[k+1]=w[k]+ stepsizeΔw 1 w + x(1-2y)

  9. Applications to semiconductor-based sensor arrays Ion-Sensitive Field Effect Transistors (ISFETS and CHEMFETs) are basically metal oxide semiconductor field-effect devices. The construction of an ISFET differs from the conventional MOSFET devices, in that the gate metal is omitted and replaced by a membrane sensitive to the ions of interest. Potentiometric sensors!

  10. ISFET/CHEMFET sensors (1) Reference Electrode (Vref) (2) Solution (Electrolyte) (3) Membrane (MOSFET gate metal) (4) Gate Insulator ID S D VG VD Silicon Substrate

  11. ISFET/CHEMFET sensors (Short view) ID for the conventional MOSFET is: ID = α[(VG –VT)-0.5VD]VD (1) where α = μCoW VD/L We have to establish new expressions for VG and VT to adapt equ. (1) to the ISFET. MOSFET VGground-to-gate metal potential ISFET VG*ground-to-membrane potential VG* = VG + (Electrode-electrolyte potential)+Nernst Potential

  12. ISFET/CHEMFET sensors (Short view) Qss+QB Co VT = Φms + 2 ΦF - Qss+QB Co VT*= Φcs + 2 ΦF - +Eref – E0 zi/zj VG* = Vref + RT ln (ai + Kij aj ) nF ID*= α[(VG* –VT*)-0.5VD]VD (2)

  13. Main characteristics • Sensitivity: • Lowest level of chemical concentration that can be detected. • The smallest increment of concentration that can be detected in the sensing environment. • Selectivity: • The sensor’s ability to detect what is of interest and to separate that from interferents

  14. Comparison of Chemical sensors The use of different type of sensors is recomended to obtain more robust semiconductor-based chemical sensor arrays

  15. Current problems • Miniaturized chemical sensors have yet to achieve their full potential. • They must accomodate: • High noise levels in chemical composition of the field environment. • Highly variable environmental conditions (temperature, humidity)

  16. Solutions: Sensor fabrication Application of signal processing techniques • 1. Classical methods : • calibration (LUTs, polygonal interpolation, progressive calibration) • compensation (Structural, adjust, etc) • 2. ISFET SOURCE SEPARATION techniques • Advantages: • Cancel crosstalk (added noise caused by interferent ions • Cancel cross-sensitivity (when the response varies with the temperature – drift). • Low cost/high performance implementation • New materials. • New devices. • . • . High cost !

  17. How does BSS apply to semiconductor-based chemical sensing? Non-linear BSS We have the observations matrix: Spatial diversity ID1* = α[(Vref + RT ln (ai + K1 aj ) - VT*)- 0.5VD] zi/zj nF ID2* = α[(Vref + RT ln (ai + K2 aj ) - VT*)- 0.5VD] zi/zj nF IDN* = α[(Vref + RT ln (ai + KN aj ) - VT*)- 0.5VD] zi/zj nF

  18. Adaptive algorithm for Non-Linear BSS gID1 ai ID1 âi f1 g1 A W aj gID2 âj ID2 f2 g2 Linear mixing stage Non –linear distortion Non –linear compensation Linear de-mixing stage (ID –Φa)/ Φb zi/zj ID=Φa + Φb*ln(ai + Kijaj ) ; gID = e

  19. Adaptive algorithm for Non-Linear BSS • Initialization W = I, gID = ID, Φa andΦb. • Loop • Compute outputs by: y =Wx; • Estimation of parameters • Φa[k+1]= Φa[k]+ stepsizeΔΦa • Φb[k+1]= Φb[k]+ stepsizeΔΦb • 3. Normalization • 4. Linear BSS algorithm: w[k+1]=w[k]+ stepsizeΔw • 5. Repeat until convergence

  20. Preliminary Results • The algorithm recovers the wave forms of the main ion activity (ai) and the interferent ion activity (aj). aj is considered as noise in other approaches. Selectivity is improved. • The algorithm works well for sensor arrays with poor selectivity coefficients. Sensor arrays with poor performance bring to the algorithm spatial diversity and more statistical information. We can use low cost sensors. • The adaptive scheme allows the separation when environment characteristics varies (e.g., temperature). We hope to study the algorithm behaviour as a function of the device drift. • The scheme can be adjusted to build a multi-parametric system, adding more sensors (sensitive to other ions) and adjusting the algorithm parameters. • Hardware implementation using a DSP card is currently being developed. Previous implementations had shown good performance.

  21. Non-linear Blind Source Separation Applied to Ion-sensitive Field Effect Transistor Sensor Arrays UNIVERSITAT POLITÉCNICA DE CATALUNYA UPC INPGrenoble

More Related